germania-based sol-gel organic-inorganic hybrid coatings
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University of South Florida University of South Florida
Scholar Commons Scholar Commons
Graduate Theses and Dissertations Graduate School
4-1-2009
Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings for Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings for
Capillary Microextraction Capillary Microextraction
Li Fang University of South Florida
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Germania-Based Sol-Gel Organic-Inorganic Hybrid Coatings for Capillary
Microextraction
by
Li Fang
A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy
Department of Chemistry
College of Arts and Sciences
University of South Florida
Major Professor: Abdul Malik, Ph.D.
Milton D. Johnston, Ph.D.
Jennifer Lewis, Ph.D.
Robert Potter, Ph.D.
Date of Approval:
April 1, 2009
Keywords: Sol-Gel Germania Coating, CME, SPME, In-Tube SPME, Gas
Chromatography, Stationary Phase
© Copyright 2009 , Li Fang
DEDICATION
To
my parents Duohui Li and Fengshan Fang
my sister Ying Fang
who made this possible, for their endless love and support.
ACKNOWLEDGEMENT
I would like to express my gratitude to many people who gave me the possibility
to complete this thesis. I am deeply grateful to my major professor, Dr. Abdul Malik, for
his supervision, patience and encouragement in the past five years. I also want to present
my sincere thanks to my dissertation committee members: Dr. Milton D. Johnston, Dr.
Jennifer Lewis, and Dr. Robert Potter for their valuable advice, thoughtful comments and
support.
I would like to thank all my former and current colleagues, Dr. Khalid Alhooshani,
Dr. Abuzar Kabir, Dr. Tae-Young Kim, Dr. Sameer M. Kulkarni, Dr. Wen Li, Anne M.
Shearrow, Erica Turner and Scott Segro for their continuous assistance, encouragement,
and friendship. I also want to extend thanks to the Department of Chemistry for financial
support over the past five years.
i
TABLE OF CONTENTS
LIST OF TABLES ............................................................................................................ vii
LIST OF FIGURES ........................................................................................................... ix
LIST OF SCHEMES......................................................................................................... xii
LIST OF SYMBOLS AND ABBREVIATIONS ............................................................ xiii
ABSTRACT .......................................................................................................................xv
CHAPTER ONE: BACKGROUND OF SOLID PHASE MICROEXTRACTION
(SPME) .......................................................................................................................1
1.1 Introduction to sample preparation .......................................................................1
1.2 Introduction to classical sample preparation approaches ......................................2
1.2.1 Liquid-liquid extraction (LLE) ....................................................................2
1.2.2 Soxhlet extraction ........................................................................................3
1.3 Solvent-free extraction ..........................................................................................3
1.3.1 Gas-phase extraction ....................................................................................5
1.3.2 Supercritical fluid extraction (SFE) .............................................................6
1.3.3 Membrane extraction ...................................................................................7
1.3.4 Solid phase extraction (SPE) .......................................................................8
1.4 Introduction of exhaustive-extraction and non-exhausive extraction
techniques .............................................................................................................8
1.5 Introduction to solid phase microextraction .......................................................10
ii
1.5.1 SPME device and application ....................................................................11
1.5.2 Theoretical aspect of SPME.......................................................................13
1.6 Conventional SPME coating ...............................................................................16
1.6.1 Homegeneous polymers .............................................................................19
1.6.2 Composite extraction media ......................................................................20
1.6.3 Emerging extraction media ........................................................................20
1.7 Different formats of SPME .................................................................................21
1.7.1 Fiber SPME ................................................................................................21
1.7.1.1 Direct immersion SPME (DI) .........................................................22
1.7.1.2 Headspace fiber SPME (HS-SPME) ...............................................22
1.7.2 In-tube SPME (also termed capillary microextraction (CME)) .................23
1.7.2.1 Comparison of fiber SPME and in-tube SPME ..............................24
1.7.2.2 Theoretical aspect of in-tube SPME ...............................................30
1.7.2.3 Parameters influencing in-tube SPME ............................................32
1.7.3 Stir bar sorptive extraction (SBSE)............................................................32
1.8 References for chapter one..................................................................................34
CHAPTER TWO: SOLID PHASE MICROEXTRACTION AND SOL-GEL
TECHNOLOGY .......................................................................................................39
2.1 Introduction to sol-gel process and some basic definitions ................................39
2.2 Historical background of sol-gel technology ......................................................42
2.3 The potential of sol-gel chemistry in coating technology for analytical
microextraction .................................................................................................43
2.3.1 Problems with traditional SPME coating technology ................................43
iii
2.3.2 The advantages sol-gel technology for use in analytical
microextraction ..........................................................................................44
2.4 Fundamentals of sol-gel technology ...................................................................46
2.5 Creation of sol-gel CME sorbents.......................................................................49
2.5.1 Pre-treatment of the fused-silica capillary .................................................49
2.5.2 Preparation of the sol solution ...................................................................52
2.5.3 Sol-gel coating process ..............................................................................55
2.5.4 Further treatment of sol-gel-coated CME capillay ....................................55
2.6 Characterization of sol-gel extracting phase and its morphology .......................56
2.7 Reported sol-gel coating materials in SPME and CME......................................57
2.8 Drawbacks of silica-based sol-gel materials .......................................................62
2.9 Sol-gel process using transition metal alkoxide..................................................62
2.10 Non-silica sol-gel materials in analytical microextraction and
chromatographic separation ..............................................................................67
2.11 References for chapter two ...............................................................................71
CHAPTER THREE: GERMANIA-BASED SOL-GEL HYBRID
ORGANIC-INORGANIC COATINGS FOR CAPILLARY
MICROEXTRACTION AND GAS CHROMATOGRAPHY .................................80
3.1 Introduction of germanosilicate glasses material ................................................80
3.2 Preparation of sol-gel GeO2/ormosil organic-inorganic hybrid material ............82
3.3 Ormosil precursors at low treatment temperature ...............................................83
3.4 Problems of germanosilicate material during sol-gel process ............................85
iv
3.5 Introduction of Germania –based sol-gel organic-inorganic hybrid
coatings for capillary microextraction and gas chromatography ........................86
3.6 Experimental .......................................................................................................91
3.6.1 Equipment ..................................................................................................91
3.6.2 Chemicals and materials ............................................................................92
3.6.3 Preparation of sol-gel germanis-PDMS coated capillaries for CME .........92
3.6.4 Preparation of silica-based sol-gel PDMS columns for GC analysis ........94
3.6.5 Preparation of aqueous samples for CME-GC...........................................94
3.6.6 Gravity-fed sample dispenser for capillary microextraction .....................94
3.6.7 Sol-gel capillary microextraction ...............................................................95
3.6.8 Safety precautions ......................................................................................96
3.7 Results and discussion ........................................................................................96
3.7.1 Chemical reactions involved in the preparation of sol-gel germania
coatings ......................................................................................................96
3.7.2 IR Characterization ..................................................................................103
3.8 CME profile for sol-gel Germania coated capillaries .......................................103
3.9 Extraction characteristics of sol-gel germania coatings in CME ......................106
3.9.1 Extraction of non-polar and moderately polar compounds by sol-gel
germania-PDMS coated capillary for CME-GC analysis ........................106
3.9.2 pH stability of sol-gel germania-PDMS coated capillary ........................113
3.9.3 Extraction of polar compounds by sol-gel germania CME-GC...............114
3.9.4 GC separation of sol-gel germania-PDMS coated column ......................119
3.10 Conclusion ......................................................................................................122
v
3.11 References for chapter four .............................................................................122
CHAPTER FOUR: MIXED GERMANIA-SILICA SOL-GEL
CYANOPROPYL-POLY(DIMETHYLSILOXANE) COATING FOR
CAPILLARY MICROEXTRACTION OF POLAR ORGANIC TRACE
CONTAMINANTS IN AQUEOUS MEDIA .........................................................128
4.1 Introduction .......................................................................................................128
4.2 Experimental .....................................................................................................132
4.2.1 Equipment ................................................................................................132
4.2.2 Chemicals and materials ..........................................................................155
4.2.3 Preparation of sol-gel germanis-CN/PDMS-coated capillaries for
CME .........................................................................................................133
4.2.4 Preparation of aqueous samples for CME-GC.........................................134
4.2.5 Sol-gel capillary microextraction by an in-lab gravity-fed sample
dispenser ..................................................................................................135
4.3 Results and discussion ......................................................................................136
4.3.1 Chemical reactions involved in the preparation of sol-gel
germania-CN/PDMS coatings .................................................................136
4.3.2 Extraction profiles for sol-gel germania-CN/PDMS capillaries ..............140
4.3.3 Extraction characteristics of sol-gel germania-CN/PDMS in CME ........144
4.3.4 Thermal stability of sol-gel germania coatings in CM ............................154
4.4 Conclusion ........................................................................................................156
4.5 References for chapter five ...............................................................................156
APPENDICES .................................................................................................................160
vi
Appendix A: Germia-based, sol-gel hybird organic-inorganic coatings for
capillary microextraction and gas chromatography ..........................................161
Appendix B: Sol-gel immobilized cyano-polydimethylsiloxane coatings for
capillary microextraction of aqueous trace analytes ranging from
polycyclic aromatic hydrocarbons to free fatty acids .......................................172
ABOUT THE AUTHOR ................................................................................... END PAGE
vii
LIST OF TABLES
Table 1.1 Commercial SPME coatings and their properties ..............................................18
Table 2.1 Organic sorbents and their applications .............................................................58
Table 3.1 Chemical ingredients of the coating solution used in preparing sol-gel
germania organic-inorganic hybrid coatings ............................................................98
Table 3.2 GC peak area and retention time repeatability data for PAHs, aldehydes,
and ketones extracted from aqueous samples by CME using a sol-gel
germania-PDMS coated microextraction capillary .................................................107
Table 3.3 Detection limit data for PAHs, aldehydes, and ketones extracted from
aqueous samples by CME-GC using a sol-gel germania-PDMS coated
microextraction capillary ........................................................................................108
Table 3.4 Capillary-to-capillary reproducibility (n=3) data for extracted amounts in
CME-GC experiments conducted on sol-gel germania-PDMS coated capillaries
using a PAH (phenanthrene), an aldehyde (undecanal), and a ketone
(hexanophenone) as test solutes ..............................................................................109
Table 4.1 Chemical ingredients of the coating solution used in preparing sol-gel
germania organic-inorganic hybrid coatings chemically anchored to the inner
surface of a fused silica capillary ............................................................................138
viii
Table 4.2 GC peak area and retention time repeatability data (n=4) for
chlorophenols,alcohols, free fatty acids and ketones extracted from aqueous
samples by CME using sol-gel germania-CN/PDMS coated microextraction
capillary...................................................................................................................145
Table 4.3 GC-FID detection limit data (n=4) for chlorophenols, alcohols, free fatty
acids and ketones extracted from aqueous samples by CME using sol-gel
germania-CN/PDMS coated microextraction capillary ..........................................146
Table 4.4 Capillary-to-capillary reproducibility (n=3) data for extracted amounts in
CME-GC experiments conducted on sol-gel germania-CN/PDMS coated
capillaries using a chlorophenol (pentachlorophenol), an alcohol (undecanol),
a free fatty acid (undecanoic acid) and a ketone (heptanophenone) as test
solutes .....................................................................................................................147
ix
LIST OF FIGURES
Figure 1.1 Classification of Solvent-free extraction techniques ..........................................5
Figure 1.2 Classification of the extraction techniques .......................................................10
Figure 1.3 Schematic of a SPME device ...........................................................................12
Figure 1.4 Extraction step for SPME .................................................................................13
Figure 1.5 Principle of SPME ............................................................................................14
Figure 1.6 Modes of SPME operation ...............................................................................23
Figure 1.7 Comparison SPME fiber with in-tube SPME ...................................................24
Figure 1.8 Comparison of extraction process for the transfer of analytes in fiber
SPME and in- tube SPME.........................................................................................25
Figure 1.9 Illustration of extraction procedure of extraction by fiber SPME and
analyte desorption for HPLC analysis ......................................................................28
Figure 1.10 Schematic diagrams of in-tube SPME-LC-MS systems ................................29
Figure 1.11 Distribution of analyte between sample matrix and the extraction phase ......30
Figure 1.12 Schematic of a stir bar of SBSE .....................................................................33
Figure 2.1 Overview of the Sol-gel process.......................................................................41
Figure 2.2 Homemade capillary filling/purging device .....................................................51
Figure 3.1-A FTIR spectra representing: pure PDMS .....................................................104
Figure 3.1-B FTIR spectra representing: sol-gel germania PDMS hybrid material
used as a sorbent in capillary microextraction ........................................................104
x
Figure 3.2 Capillary microextraction profiles of pyrene and heptanophenone on a
sol-gel germania coated capillary ...........................................................................105
Figure 3.3 CME-GC trace analysis of a mixture of PAHs using a sol-gel
germania-PDMS coated microextraction capillary .................................................111
Figure 3.4 CME-GC trace analysis of a mixture of aldehydes and ketones using a
sol-gel germania-PDMS coated microextraction capillary .....................................112
Figure 3.5 CME-GC trace analysis of alcohols using a sol-gel germania-APTMS
coated microextraction capillary .............................................................................115
Figure 3.6 Excellent stability of sol-gel germania-PDMS coating under highly basic
conditions demonstrated through CME-GC analysis of a mixture of PAHs
using a sol-gel germania-PDMS coated microextraction capillary before
(3.6-A) and after (3.6-B) continuously rinsing the capillary with a 0.1 M
NaOH solution (pH = 13) for 24h ...........................................................................116
Figure 3.7 Excellent stability of sol-gel germania-PDMS coating under highly
acidic conditions demonstrated through CME-GC analysis of aldehydes using
a germania-PDMS coated microextraction capillary before (3.7-A) and after
(3.7-B) continuously rinsing the capillary with a 0.05 M HCl solution
for 24 h ....................................................................................................................117
Figure 3.8 CME-GC trace analysis of 2, 4, 6 Trichlorophenol using a sol-gel
germania-PDMDPS coated microextraction capillary............................................118
Figure 3.9 CME-GC analysis of free fatty acids using a sol-gel germania-APTMS
coated microextraction capillary .............................................................................120
xi
Figure 3.10 Gas chromatogram of a natural gas sample separated on sol-gel
germania-PDMS stationary phase...........................................................................121
Figure 4.1 Capillary microextraction profiles of undecanol, heptanophenone,
pentachlorophenol on a sol-gel germania-CN/PDMS coated capillary ..................143
Figure 4.2 CME-GC analysis of a mixture of alcohols using a sol-gel
germania-CN/PDMS coated microextraction capillary ..........................................149
Figure 4.3 CME-GC analysis of a mixture of acids using a sol-gel
germania-CN/PDMS coated microextraction capillary ..........................................151
Figure 4.4 CME-GC analysis of a mixture of chlorophenols using a sol-gel
germania-CN/PDMS coated microextraction capillary ..........................................152
Figure 4.5 CME-GC analysis of a mixture of ketones using a sol-gel
germania-CN/PDMS coated microextraction capillary ..........................................153
Figure 4.6 Effect of conditioning temperature on the performance of sol–gel
germania-CN/PDMS microextraction capillary in the extraction of nonanol ........155
xii
LIST OF SCHEMES
Scheme 2.1 Illustration of hydrolysis and condensation of tetramethoxysilane
(TMOS) .....................................................................................................................49
Scheme 2.2 Deactivating reagents .....................................................................................53
Scheme 2.3 Deactivation of silica surface with HMDS ....................................................54
Scheme 2.4 General sol-gel process for transitional metal alkoxides ...............................64
Scheme 2.5 Mechanisms of sol-gel hydrolysis and condensation reactions of metal
alkoxides ...................................................................................................................65
Scheme 3.1 Typical hydrolytic ploycondensation of germanium alkoxide to form
germania ....................................................................................................................82
Scheme 3.2 Hydrolysis of sol-gel germania precursor, TMOG ........................................99
Scheme 3.3 Polycondensation of the hydrolyzed precursor and chemical bonding
of the sol-gel-active organic ligand (Y) to the evolving sol-gel network ...............100
Scheme 3.4 Chemical anchoring of the evolving sol-gel germania hybrid
organic-inorganic polymer to the inner walls of a fused silica capillary ................101
Scheme 4.1 Hydrolysis of sol-gel germania precursor (TEOG) ......................................140
Scheme 4.2 Polycondensation of the hydrolyzed precursor and chemical bonding
of the sol-gel-active organic ligand (Y) to the evolving sol-gel network ...............141
Scheme 4.3 Chemical anchoring of the evolving sol-gel germania hybrid
organic-inorganic polymer to the inner walls of a fused silica capillary ................142
xiii
LIST OF SYMBOLS AND ABBREVIATIONS
AN Nucleophilic Addition
AFM Atomic Force Microscopy
AMTEOS Anilinemethyltriethoxysilane
APTMS 3-Aminopropyltrimethoxysilane
BMA Butyl methacrylate
C8-TEOS n-Octyltriethoxysilane
CE Capillary Electrophoresis
CEC Capillary Electrochromatography
CME Capillary Microextraction
CME-GC Capillary Microextraction - Gas Chromatography
CPs Chlorophenols
CPTES 3-Cyanopropyltriethoxysilane
CVD Chemical Vapour Deposition
CW/DVB Carbowax/divinylbenzene
CW/TPR Carbowax/templated resin
CW Carbowax
DCCA Drying Control Chemical Additive
DEOS Diethylorthosilicate
DHS Dynamic Headspace
DVB Divinyl benzene
EPA Environmental Protection Agency
EVP-ICP-MS Environmental Vaporization - Inductively Coupled Plasma - Mass
Spectrometry
FA Formic Acid
FID Flame Ionization Detector
GC Gas Chromatography
GC-MS Gas Chromatography - Mass Spectrometry
GLYMO Glycidoxypropyltrimethoxysilane
GODC Germanium Oxygen Deficient Centers
HMDS 1,1,1,3,3,3-Hexamethyldisilazane
HPLC High - Performance Liquid Chromatography
HS-SPME Headspace - Solid Phase Microextraction
ICP-MS Inductively Coupled Plasma - Mass Spectrometry
INCAT Inside Needle Capillary Adsorption Trap
LC-MS Liquid Chromatography - Mass Spectrometry
LLE Liquid - Liquid Extraction
MA Methyl acrylate
MMA Methyl methacrylate
xiv
MS Mass Spectrometry
MSDS Material Safety Data Sheet
MTES Methyltrimethoxysilane
MTMOS Methyltrimethoxysilane
NMR Nuclear Magnetic Resonance
OH-TSO Hydroxyl-terminated polydimethylsiloxane
ppq Parts Per Quadrillion
ppt Parts Per Trillion
poly-THF Polytetrahydrofuran
PA Polyacrylate
PAHs Polycyclic Aromatic Hydrocarbons
PDMDPS Polydimethyldiphenylsiloxane
PDMS Polydimethylsiloxane
PDMS/DVB Polydimethylsiloxane/divinylbenzene
PEG Polyethylene glycol
PheDMS Phenyldimethylsiane
PMHS Poly(methylhydrosiloxane)
PMPS Polymethylphenylsiloxane
PMPVS Poly (methylphenylvinylsiloxane)
PTFPMS Poly(trifluoropropyl)methylsiloxane
PVA Poly (vinyl alcohol)
R.S.D Relative Standard Deviation
SN Nucleophilic Substitution
SBSE Stir Bar Sorptive Extraction
SCF Supercritical Fluids
SEM Scanning Electron Microscopy
SFE Supercritical Fluid Extraction
SHG Second Harmonic Generation
SHS Static Headspace
SPE Solid - Phase Extraction
SPME Solid - Phase Microextraction
SPME-GC Solid Phase Microextraction - Gas Chromatography
SPME-HPLC Solid Phase Microextraction - High Performance Liquid
Chromatography
SPME-HPLC-MS Solid Phase Microextraction - High Performance Liquid
Chromatography - Mass Spectrometry
TEOG Tetraethyoxygermane
TEOS Tetraethoxysilane
TFA Trifluoroacetic Acid
THF Tetrahydrofuran
TMOG Tetramethoxygermane
TP-i-OG Germanium iso-propoxide
TPOG Tetrapropyloxygermane
VOCs Volatile Organic Compounds
XPS X-ray Photoelectron Spectroscopy
xv
GERMANIA-BASED SOL-GEL ORGANIC-INORGANIC HYBRID COATINGS
FOR CAPILLARY MICROEXTRACTION
Li Fang
ABSTRACT
For the first time, germania-based hybrid organic-inorganic sol-gel materials were
developed for analytical sample preparation and chromatographic separation. Being an
isostructural analog of silica (SiO2), germania (GeO2) is compatible with silica network.
This structural similarity, which is reflected by the relative positions of germanium and
silicon in the periodic table, stimulated our investigation on the development of
germania-based sol-gel hybrid organic-inorganic coatings for analytical applications.
Sol-gel sorbents and stationary phases reported to date are predominantly
silica-based. Poor pH stability of silica-based materials is a major drawback. In this work,
this problem was addressed through development of germania-based organic-inorganic
hybrid sol-gel materials. For this, tetramethoxygermane (TMOG) and
tetraethyoxygermane (TEOG) were used as precursors to create a sol-gel network via
hydrolytic polycondensation reactions to provide the inorganic component (germania) of
the organic-inorganic hybrid coating. The growing sol-gel germania network was
simultaneously reacted with an organic ligand that contained sol-gel-active sites in its
chemical structure. Hydroxy-terminated polydimethylsiloxane (PDMS) and
xvi
3-cyanopropyltriethoxysilane (CPTES) served as sources of nonpolar and polar organic
components, respectively. The sol-gel reactions were performed within fused silica
capillaries. The prepared sol-gel germania coatings were found to provide excellent pH
and thermal stability. Their extraction characteristics remained practically unchanged
after continuous rinsing of the sol-gel germania-PDMS capillary for 24 hours with highly
basic (pH = 13) and/or acidic (pH = 1.3) solution. They were very efficient in extracting
non-polar and moderately polar analytes such as polycyclic aromatic hydrocarbons,
aldehydes, ketones. Possessing the polar cyanopropyl moiety, sol-gel germania
cyanopropyl-PDMS capillaries were found to effectively extract polar analytes such as
alcohols, fatty acids, and phenols. Besides, they also showed superior thermal stability
compared with commercial cyano-PDMS coatings thanks to the covalent attachment of
the coating to capillary surface achieved through sol-gel technology. Their extraction
characteristics remained unchanged up to 330 ºC which is significantly higher than the
maximum operation temperature (< 280 ºC) for commercial cyano-PDMS coatings.
Low ng/L detection limits were achieved for both non-polar and polar test solutes. Our
preliminary results also indicated that sol-gel hybrid germania coatings have the potential
to offer great analytical performance as GC stationary phases.
1
CHAPTER ONE
BACKGROUND OF SOLID PHASE MICROEXTRACTION (SPME)
1.1 Introduction to sample preparation
The analytical procedure for complex samples consists of several typical steps:
sampling, sample preparation, separation, quantification, statistical evaluation, and
decision making. These analytical steps follow one after another, and a subsequent step
cannot begin until the preceding one has been completed. Therefore, the slowest step
determines the overall speed of the analytical process, and each step is critical for
obtaining correct and informative results. To improve analysis speed, all the steps should
be considered.
Sample preparation as a part of the analysis takes about 80 % of analysis time for
complex samples [1]. One reason is the difficulty to couple sampling and sample
preparation steps with today’s sophisticated instruments such as Gas
Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography/Mass
Spectrometry (LC/MS). Those sophisticated instruments, can separate and quantify
complex mixtures and automatically apply chemometric methods to statistically evaluate
results. Despite the advances in separation and quantitation techniques, many sample
preparation techniques are based on classical technologies such as Soxhlet extraction
method are time consuming and involve multi-step procedures that are prone to lose
analyte loss use of the toxic organic solvents. These characteristics make it extremely
difficult to integrate such methods with today’s sophisticated separation and quantitation
2
instruments for the purpose of automation. Another reason is that often one has to deal
with real world complex or natural samples, therefore the fundamentals of extraction
techniques involving them are much less well developed and understood compared well
defined physicochemical principles governing instrument used in separation and
quantification steps. Lastly, a new awareness of the hazards has resulted in international
initiatives [2] to develop new sample preparation techniques aimed at complete
elimination of using organic solvents that we used in large volume by classical sample
preparation techniques.
1.2 Introduction to classical sample preparation approaches
It is not an exaggeration to say that choice of sample preparation method greatly
influences the analytical reliability, efficiency and accuracy. Soxhlet extraction for solid
samples [3] and liquid-liquid extraction (LLE) for liquid samples [4-6] are two most
commonly used classical sample preparation approaches.
1.2.1 Liquid-liquid extraction (LLE)
Liquid-liquid extraction involves the use of an organic solvent to extract analytes
from aqueous samples in reparatory funnels or other vessels. It is one of the oldest
techniques and still is widely used. Many of the US EPA standard methods for organic
trace analysis still utilize LLE. The main disadvantage of the technique is the use of large
quantity of organic solvents, which might be expensive to buy and dispose of, and cause
environmental problem and potential health concerns. It is also very difficult to couple
LLE technique with any other instrumental analysis systems. LLE is not easily automated.
3
Lastly, the enrichment factor is quite limited and for many sample types there are
problems with emulsion formation and precipitation.
1.2.2 Soxhlet extraction
Soxhlet extraction technique has been the most widely used method for
exhaustive extraction analytes from solid samples. Typically, a Soxhlet extraction is used
where the target analyte has a limited solubility in a solvent, and the matrixes are possible
impurities are insoluble in that solvent. Because a simple filtration can be used to
separate the compound easily from the insoluble substance it is desirable that the target
analytes has a high solubility in the solvent. Soxhlet extraction normally involves placing
the solid sample in the porous cellulous thimble placed in a holder. During operation the
thimble is filled with fresh warm organic solvent from a distillation flask. During the
process of extraction, the extracted analytes accumulate in the solvent and are
automatically siphoned into the distillation flask regularly. These steps are repeated until
exhaustive extraction of the target is achieved. Although the soxhlet technique uses
inexpensive equipment to operate, the processes involved are quite slow and may require
the use of large amounts of hazardous solvents causing concerns about the environment
and human health. At the same time, the used solvents are highly costly to be disposed of.
1.3 Solvent-free extraction
Classical sample preparation techniques consume large amounts of solvents.
During the volume reduction step, waste solvents are often disposed into the environment.
Solvent disposal into the atmosphere causes pollution and contributes to unwanted
4
atmospheric effects such as smog and ozone holes. In addition, solvent disposal adds
extra cost to the overall analytical process by complicating work of the regulatory
agencies, and it increases health hazards to laboratory personnel. The desire for efficient
extractions while consuming less solvent has led to a trend on increased research on new
and practical solvent-free alternatives. A few of these research areas include Gas-phase
extractions [7-8] supercritical fluid extraction (SFE) [9-11], membrane extraction [12-13]
and solid-phase extraction (SPE) [14-17].
Solvent-free extraction techniques [6] [18] have been an important direction to
address the problems associated with classical sample preparation methods. Solvent-free
sample preparation techniques use little or no organic solvent. This direction not only
addresses health and pollution prevention issues, but also in most cases provides an easier
way to implement on-site monitoring in field conditions. This direction has been creating
a lot of interests and research opportunities recently and it is expected to continue to be a
very active area in the near future. Solvent-free extraction techniques can be broadly
classified, as shown in the Figure 1.1, into gas phase, membrane, and sorbent extraction
techniques.
5
Figure 1.1 Classification of Solvent-free extraction techniques [19]
1.3.1 Gas-phase extraction
Gas-phase extractions can be carried out in two different modes static head space
(SHS) and dynamic head space (DHS) (also known as purge and trap) [7, 8]. In static
head space mode, the volatile analytes in the sample matrix diffuse into the headspace in
a vial, and the concentration of the analyte in the headspace reaches equilibrium with the
concentration in the sample matrix. After the equilibrium is established, a small volume
of the head space gas is injected into a GC. Using static headspace method has the
advantages of simplicity. It is more convenient for qualitative analysis since the sample
can be placed directly into the headspace vial and analyzed with no additional preparation.
However, this technique suffers from low sensitivity because there is no mechanism of
sample preconcentration. DHS potentially provides improved sensitivity when compared
with SHS, due to the constant depletion of the analytes from the sample. In dynamic head
Solvent-Free Sample Preparation Methods
Membrane
Extraction Gas-Phase extraction Sorbent Extraction
SPE SPME
Cartridge
Disk
Direct
Headspace
In-tube
Headspace
Dynamic
Static
Dynamic
Static
SFE
6
space, carrier gas is bubbled through solid or liquid samples containing volatile organic
compounds (VOCs), are subsequently in a sorbent trap. The carrier gas sweeps volatile
organic compounds into the sorbent trap. Desorption of trapped analytes for subsequent
analysis can be performed either with small volumes of appropriate solvents or using an
on-line automated thermal desorption device, the latter is suitable for routine procedures.
1.3.2 Supercritical fluid extraction (SFE)
Supercritical fluid extraction [9-11] is an attractive technique because it is fast and
selective. This extraction technique is based on the fact that near critical conditions of the
solvent, its properties change rapidly with only slightly variations of pressure. A
supercritical-fluid extractor typically consists of a tank of the extracting fluid, usually
liquid CO2, a pump to pressurize CO2, an oven containing the extraction vessel, a
restrictor to maintain a high pressure in the extraction line, and a trapping vessel.
Analytes are trapped by letting the solute-containing supercritical fluid decompress into
an empty vial, through a solvent, or onto a solid sorbent material. Extractions are done in
dynamic-, static-, or combination modes. In a dynamic SFE, the supercritical fluid
continuously flows through the sample in the extraction vessel and out the restrictor to
the trapping vessel. In static mode the supercritical fluid circulates in a loop containing
the extraction vessel for some period of time before being released through the restrictor
to the trapping vessel. In the combination mode, a static extraction is performed for some
period of time, followed by dynamic extraction. Several compounds have been examined
as SFE solvents. For example, hydrocarbons such as hexane, pentane and butane, nitrous
oxide, sulphur hexafluoride and fluorinated hydrocarbons [20]. However, carbon dioxide
7
(CO2) is the most popular SFE solvent because it is safe, readily available in highly pure
form and has a low cost. It allows supercritical operations at relatively low pressures and
at near-room temperatures. The main drawback of CO2 is its lack of polarity for the
extraction of polar analytes [21]. Compared with other traditional extraction techniques,
it is a flexible process due to the possibility of continuous modulation of the solvent
power/selectivity of the supercritical fluids (SCF), and it is also environmental friendly
than organic solvents and avoids the expensive post-processing of the extracts for solvent
elimination. Because of these advantages, carbon dioxide (CO2) is the most widely used
supercritical solvent. A serious drawback of SFE is the higher investment costs compared
to traditional atmospheric pressure extraction techniques. The interest in SFE has
dramatically declined because of the dependence of the extraction on sample condition.
This dependency makes the technique difficult for routine analysis due to the tedious
optimization procedure which is required to overcome this problem. Furthermore, SFE
requires liquid-like compressed CO2 as the extraction medium. The high cost of pure CO2
and overall design of SFE makes the technique incompatible with field analyses [22-24].
1.3.3 Membrane extraction
Membrane extraction [12, 13] can be divided into two categories, porous and
non-porous membrane techniques. The porous membrane techniques involve filtration
and dialysis in different formats. In these techniques the solutions on both sides of the
membrane are in physical contact through the membrane pores, so this is in fact a
one-phase system. Non-pores membrane techniques involve a membrane that is either a
polymeric material or a liquid separating the donor and acceptor. In these techniques, it
8
could be two-phase or three-phase system. Membrane extraction consists of two
simultaneous processes. In the first step, a membrane is used to extract analyte from the
matrix. This step is followed by extracting the analyte from the membrane surface with a
stripping phase. This method is only suitable for non-polar volatile and semivolatile
compounds. The major limitations of membrane extraction include slow response,
carryover, and difficulty in interfacing the technique with other instrumentation [17, 18,
25, 26].
1.3.4 Solid phase extraction (SPE)
Solid phase extraction [14-17] is based on the selective accumulation of target
analytes from a liquid matrix onto a sorbent bed. SPE has several formats including
cartridge tube, flat disk membrane, monolithic bed, and flow-through membrane
containing a sorbent on a particulate support. The major advantages of the technique are
the significant reduction in the use of organic solvent, ease in operation, and low cost.
However, in some cases the extraction solvent may not be compatible with an analytical
system. To overcome this, the solvent is evaporated and the remaining residue is
dissolved in a compatible solvent. Furthermore, SPE has poor reproducibility and high
carry over problem.
1.4 Introduction of exhaustive-extraction and non-exhaustive extraction techniques
The objective of exhaustive technique is to completely remove analytes from a
sample matrix and transfer them to the extracting phase. The fundamental advantage of
exhaustive methods is that, in principle, they do not require calibration since the vast
9
majority of analytes are transferred to the extracting phase. To reduce the amounts of
solvents and time required to accomplish exhaustive removal, batch equilibrium
techniques (for example, liquid-liquid extractions) are frequently replaced by
flow-through techniques. SPE is an example in this category. Alternatively, a sample
(typically a solid sample) can be packed in the bed and the extraction phase can be used
to remove and transport the analytes to the collection point. In SFE, supercritical fluid is
used to wash analytes from the sample matrix, and in Soxhlet, the solvent continuously
removes the analytes from the matrix at the boiling point of solvent.
On the other hand, non-exhaustive approaches can be designed based on
equilibrium, pre-equilibrium and permeation principles. Equilibrium non-exhaustive
techniques are fundamentally analogues to equilibrium exhaustive techniques; however,
the capacity of the extracting phase is smaller, and in most cases is not sufficient to
remove the majority of analytes from the sample matrix. This is caused by the use of a
small volume of the extracting phase relative to sample volume. Microextraction, such as
solvent microextraction or solid phase microextraction is in this category. Pre-equilibrium
conditions are accomplished by breaking the contact between the extracting phase and
sample matrix before the point of equilibrium with the extracting phase has been reached.
The devices frequently used are identical to the microextraction system, but extraction
times are shorter. The general classification of the extraction techniques based on these
fundamental principles is in Figure 1.2 [19].
10
Figure 1.2 Classification of the extraction techniques [19]
1.5 Introduction of solid-phase microextraction
Solid-phase microextraction (SPME) is an important and practical approach to
sampling and sample preparation. It was developed in 1989 by Belardi and Pawliszyn [27]
to overcome the limitations inherent in solid-phase extraction (SPE) and liquid-liquid
extraction (LLE). SPME facilitates rapid sample preparation for both laboratory and field
analyses. SPME has been successfully applied to a variety of analytes in environmental
[28-30], biological [31-33], food [34], drug [35-37], flavor [38], and other types of
samples in hyphenation with gas chromatography (GC) [39], high-performance liquid
chromatography (HPLC) [40], capillary electrophoresis (CE) [41], and Mass
Extraction techniques
Batch equilibrium
and Pre-equilibrium
Flow through Equilibrium
and Pre-equilibrium
Steady State Exhaustive
and non-Exhaustive
Exhaustive Non-Exhaustive
LLE
Soxhlet
Sorbents
Headspace
LLME
SPME
Exhaustive
Purge and Trap
Solvent
SPE
HSE
SFE
Non-Exhaustive
In-tube SPME
Membrane
11
spectrometry (MS) [42]. Solid-phase microextraction provides many advantages over
other sample preparation techniques including:
(1) It was a simple device which enables in situ sampling. Low set-up cost.
(2) SPME techniques eliminate the use of toxic organic solvents (green
chemistry).
(3) SPME is shorten the analysis time integrating sampling, extraction, sample
preconcentration, and sample introduction into a single step (no intermediate step
is necessary between extraction and analysis).
(4) High sensitivity (detection limit down to part per quadrillion (ppq) level) can
be easily achieved using SPME.
(5) SPME extraction is an equilibrium process that requires small (sample
volume require).
(6) The technique can be easily automated with analytical instruments (GC,
HPLC, and CE).
(7) SPME is portable which makes it attractive for field sampling.
1.5.1 SPME device and application
In SPME, the extracting phase is coated on the surface of a fused-silica fiber
(fiber format) or capillary (in-tube format). The extraction in SPME is an equilibrium
process of analyte distribution between the extracting phase and sample matrix. The
conventional SPME device (Figure 1.3) is very simple; it is based on a fiber which is
housed in a specially designed syringe like apparatus. The retractable metal rod, serving
as the syringe piston, is replaced by stainless steel microtubing to hold and protect the
12
SPME fiber. Typically, 1.5-cm long silica rod (fiber) is used. The protective external
coating of this SPME fiber is removed from about 0.5 cm length of the fiber of one end,
and this end is attached to the syringe plunger using epoxy glue. The microtubing (needle
of the syringe) protects the SPME fiber coating from mechanical damage and provides a
suitable way for SPME fiber insertion and withdrawing during extraction and analysis.
Sampling and sample injection occur in a manner which is identical to typical syringe
injections.
Figure 1.3 Schematic of a SPME device. Adapted from [43]
The fiber solid phase microextraction technique is best suited for GC (SPME-GC).
SPME-GC generally consists of two steps: (1) extraction: the needle of the SPME device
at first pierces the septum of the sample container. After adjusting the depth of needle to a
13
proper position, the fiber is exposed to the sample matrix for a certain time. During this
period the analytes are extracted by the SPME coating on the outer surface of the fiber
(Figure 1.4); (2) Injection: the analytes are thermally desorbed from the fiber and are
transferred by the carrier gas (mobile phase) to the GC separation column for further
separation and analysis. In liquid chromatography, the extracted analytes are desorbed by
the organic solvents (LC mobile phase) and passed into the separation solution.
Figure 1.4 Extraction step for SPME [44]
1.5.2 Theoretical aspects of SPME
The transport of analytes from the matrix into the coating begins as soon as the
coated fiber has been placed in contact with the sample (Figure 1.5). SPME is a non
exhaustive extraction technique, and it is based on an equilibrium distribution. Analytes
continue to sorb onto the SPME coating until the equilibrium between the sample matrix
and the SPME fiber coating is achieved. Typically, SPME extraction is considered to be
14
completed when the distribution equilibrium has been reached. In practice, this means
that once equilibrium is reached, the extracted amount is constant within the limits of
experimental error and it is independent of further increase of extraction time. The
thermodynamic and kinetic fundamentals of solid phase extraction have detailed by
Pawliszyn and co-workers [45-48]. SPME can be used for extraction from both gas and
liquid samples. In both cases, analyte molecules are distributed between the sample
matrix and the sorbent coating on the SPME fiber.
Figure 1.5 Principle of SPME [45]
The distribution constant represents the ratio of analyte concentrations in the
sorbent coating and in the sample. The analyte distribution equilibrium for liquid
polymeric sorbent extraction phase is represented as:
analytesample analytefiber
15
The distribution constant can be described mathematically as:
(1.1)
where,
Kfs is distribution constant between fiber coating and sample matrix,
Cf is analyte concentration in the sorbent coating,
Co is analyte concentration in the sample.
The amount of analyte in the sorbent coating (nf) at equilibrium condition is as
follows [45]:
(1.2)
Where,
Vs is the volume of the sample,
Vf is the volume of the sorbent coating on the fiber
Since Vf is very small (µL), generally (Kfs Vf << Vs) the amount of analyte in the sorbent
coating can be written as :
n = Kfs Vf Co
(1.3)
This equation (1.3) shows that the amount of the extracted analyte in the sorbent
coating for direct SPME (no head space) is independent of the sample volume. SPME is a
non-exhaustive extraction technique. SPME doesn’t exhaustively extract target analytes
existing in the sample matrix, and the extraction is based on the equilibrium. This is also
n =Kfs Vf Vs
Kfs Vf + Vs
Co
16
a theoretical basis for SPME to be used as a portable extraction technique in field
analysis. In practice, there is no need to collect a defined sample prior to analyses as the
fiber can be exposed directly to the ambient air, water, production steam, etc. The amount
of extracted analytes will correspond directly to its concentration in the matrix, without
being dependent on the sample volume. When the sampling step is eliminated, the whole
analytical process can be accelerated, and errors associated with analyte losses through
decomposition or adsorption on the sampling container walls will be prevented. This
advantage of SPME still needs to be explored practically, by developing portable field
device on a commercial scale. This equation also shows that there is a direct
proportionality between the sample concentration and the amount of analyte extracted,
and this is the basis for analyte quantification. Furthermore, it implies that the extraction
efficiency and sensitivity depend on the distribution constant. Thus, a high distribution
constant, Kfs , is desirable to enhance selectivity and sensitivity. This can be achieved by
careful selection of the fiber coating material, changing sample pH, and through the
derivatization of target analyte (s).
1.6 Conventional SPME coating
Equation (1.3) indicates that the efficiency of the extraction process is dependent
on the distribution constant. Distribution constant is an important parameter that
characterizes properties of a certain coating and its selectivity toward a class of analyte
with certain polarity over others. Coating volume is also related with method sensitivity,
for example, thicker coatings will increase the sample load and increase the extraction
17
sensitivity, however sometimes that will result in longer extraction times. Therefore, it is
important to use the appropriate coating for a given application. Coating selection and
design can be based on chromatographic experience, and specific coatings can be and
should be designed for specific applications.
Conventional sorbents for SPME can be divided into two groups [45] (a)
homogeneous polymers and (b) composite extraction media consisting of a partially
cross-linked polymers embedded with porous particles of a second component. However,
the newly emerged polymers such as sol-gel polymers and molecular imprinted polymers
etc can be classified as new categories of SPME sorbents. To achieve good selectivity for
the analytes of interest, the choice of most suitable coating can be based on the principle
of ―like dissolve like‖. According to the polarity of the analytes, a coating with similar
polarity will provide a better affinity. Therefore from the polarity point, it is also useful to
classify conventional SPME coatings into four categories: (1) non-polar, (2) moderately
polar, (3) polar, and (4) highly polar. Table 1.1 shows some commercial SPME coating
and their properties [49].
18
Table 1.1 Commercial SPME coatings and their properties
Fibre coating
Film
thickness
(µm) Polarity
Coating
method
Maximum
operating
temperature
(°C) Technique
Compounds
to be
analysed
PDMS 100 Non-polar Non-bonded 280 GC/HPLC Volatiles
PDMS 30 Non-polar Non-bonded 280 GC/HPLC Non-polar
semivolatiles
PDMS 7 Non-polar Bonded 340 GC/HPLC Medium- to
non-polar
semivolatiles
PDMS-DVB 65 Bipolar Cross-linked 270 GC Polar
volatiles
PDMS-DVB 60 Bipolar Cross-linked 270 HPLC General
purpose
PDMS-DVBa 65 Bipolar Cross-linked 270 GC Polar
volatiles
PA 85 Polar Cross-linked 320 GC/HPLC Polar
semivolatiles
(phenols)
Carboxen-PDMS 75 Bipolar Cross-linked 320 GC Gases and
volatiles
Carboxen-PDMSa 85 Bipolar Cross-linked 320 GC Gases and
volatiles
Carbowax-DVB 65 Polar Cross-linked 265 GC Polar
analytes
(alcohols)
Carbowax-DVBa 70 Polar Cross-linked 265 GC Polar
analytes
(alcohols)
Carbowax- TPR 50 Polar Cross-linked 240 HPLC Surfactants
DVB-PDMS-Carboxena 50/30 Bipolar Cross-linked 270 GC Odors and
flavors aStableflex type is 2 cm length fiber
19
1.6.1 Homogeneous polymers
PDMS coated SPME fibers are commercially available in three different
thicknesses (7, 30, and 100 µm), and commercial polyacrylate (PA) fibers have a coating
thickness of 85 µm [50, 51]. These coatings extract analytes through an absorption
mechanism, where the analytes dissolve and diffuse onto the coating material.
PDMS fibers are the most popular and frequently used polymer in SPME because
of its inherent versatility, ruggedness, and high thermal stability. The PDMS fiber with 7
µm coating thickness is cross-linked, while the other two are not. This is because it is
very difficult to stabilize thick coatings through cross-linking reactions. Compared with
maximum operating temperature (280 ºC) of 30 and 100 µm PDMS coatings, 7 µm
PDMS coating possesses higher operating temperature (340 ºC). Cross-linked SPME
fiber coatings are more rigid and are characterized by relatively high thermal stability
compared to non-cross-linked or partially cross-linked counterparts. That may explain the
different thermal stability of these three types of PDMS coatings. However, cross-linked
PDMS coating possesses smaller sample capacity due to the smaller coating thickness of
7 µm. Since PDMS is non-polar in nature, it can only extract non-polar analytes very well.
More polar compounds can be extracted by optimizing extraction conditions such as pH,
salt effect, and temperature however they are not efficient for extraction of polar analytes.
Polyacrylate (PA) based coatings [52, 53] are more suitable for the extraction of
polar analytes. Commercial PA is available in 85µm thickness, and it is a highly polar
sorbent, immobilized by partial crosslinking. Because of its high polarity, PA coated fiber
frequently recommended for extracting polar analytes from different matrices. Unlike
20
other sorbents, at room temperature, polyacrylate is a rigid, low density solid polymer.
Therefore diffusion of analytes into this coating requires longer time resulting in longer
extraction time. Moreover, higher temperatures are required for complete desorption of
the extracted analytes.
1.6.2 Composite extraction media
Composite extraction coatings are prepared using a blend of two components in
which the particulate component is embedded in the partially crosslinked polymeric
component. Among the mixed coatings are carbowax/divinylbenzene (CW/DVB) [54],
polydimethylsiloxane/divinylbenzene (PDMS/DVB) [55]
polydimethylsiloxane/carboxane (PDMS/carboxane) [56], and carbowax/templated resin
(CW/TPR) [56]. Such composite coatings usually extract via adsorption, which is a
competitive process. In mixed phases the analytes stay on the surface of SPME coatings.
The mixed phases provide better selectivity due to the existence of two sorbents in the
coating. However unlike the preparation of homogeneous polymeric coatings, the process
to make composite coatings is difficult to automate and it is usually done manually.
1.6.3 Emerging extraction media
Commercially available fibers are limited to the scale of polarity. Consequently,
this either limits the selectivity in structural analogues analysis or prevents the sensitivity
of trace level analysis in complex sample analysis [57]. In recent years homemade
coatings with different chemical and physical characteristics [58-62] have been reported.
These include fullerene [58, 59], crown ester [60, 61], and calixarene-based [62] coatings
that provide increased sensitivity and selectivity. Fiber coating procedures have been
21
recently reviewed [63], and include sol-gel technology [64], electrochemical methods [65,
66], and physical deposition [67]. Among them, sol-gel technology is gradually turning to
be an important coating technology to prepare all kinds of customized extraction media,
and the resultant sol-gel organic-inorganic polymeric materials have been widely used to
meet application selectivity and sensitivity. Molecularly imprinted polymers have also
been introduced as extracting phases for SPME. They have a great potential for selective
extraction of target analytes from various complex matrices.
1.7 Different formats of SPME
1.7.1 Fiber SPME
As the dominant format of Solid-phase microextraction, Fiber SPME possesses a
number of inherent deficiencies arising from the fiber construction and its syringe-based
mechanical operation. In fiber SPME, the protective polyimide coating is removed from
the external surface of a fused silica fiber end segment (~ 1 cm). Therefore the fiber and
the outside sorbent layer is vulnerable to mechanical damage during operation and
handling of the SPME device due to the lack of external protective coating. The short
length (~ 1 cm) of the coated segment results in low sorbent loading, which leads to
reduced extraction sensitivity. Moreover, fiber SPME is difficult to couple to
high-performance liquid chromatography (HPLC) and other liquid-phase separation
techniques. A number of SPME variants have been proposed to overcome these
difficulties such as membrane extraction, in-tube microextraction, and stir bar sorptive
extraction (SBSE), etc.
22
1.7.1.1 Direct immersion SPME (DI)
Fiber SPME can be carried out in two different modes: (1) direct immersion
SPME and (2) headspace SPME (Figure 1.6). In direct SPME, the coated segment of
SPME fiber is inserted into the sample matrix containing the target analytes directly.
Under agitation, the target analytes distribute between the sample matrix and solid phase
on the fiber. The equilibrium will be reached after a certain period of time. When
extraction is carried out in aqueous solution, different means of agitation such as stirring
or sonication of the solution, rapid fiber or vial shaking, and fast flow of the aqueous
solution may be employed to fasten the extraction. When the sample matrix is gaseous
sample, natural convection of the air or the use of a fan may be employed to speed up the
extraction.
1.7.1.2 Headspace fiber SPME (HS-SPME)
In headspace fiber SPME, the coated fiber stays in the headspace (the gaseous
phase) above the liquid sample in a sealed container. The analyte distribution equilibrium
is first reached between the sample matrix (either liquid or solid) and the headspace of
the closed container. After the coated fiber is introduced, the extraction will be carried out
from gaseous headspace to the extracting phase on the fiber. Headspace SPME can avoid
unwanted interferences from sample matrix such as compounds having high molecular
mass (e.g., humic materials, poteins, etc). During extraction, those high molecular mass
molecules, characterized by low vapor pressure, stay in the liquid or solid but the small
target anaytes and other small molecules characterized by big vapor pressure distribute
themselves between the sample matrix the headspace. Compared with direct SPME, a
23
certain temperature is often used to help the evaporation of target analytes into the
gaseous headspace of the container. Headspace SPME allows the modification of the
matrix such as pH change, addition of salts etc to improve the extraction efficiency of the
coatings without any harm to the coating on the fiber. However, for this extraction mode,
the target analytes cannot be non-volatile, only volatile or semi-volatile compounds can
be extracted from headspace with enough sensitivity.
Figure 1.6 Modes of SPME operation: (a) direct extraction, (b) headspace SPME
1.7.2 In-tube SPME (also termed capillary microextraction (CME))
In-tube SPME (also termed capillary microextraction (CME)) [68-70], is
practically free form the shortcomings inherent in conventional fiber SPME. Unlike fiber
SPME, where a sorbent coating on the outer surface of a small-diameter solid rod serves
as the extraction medium, in CME a piece of fused silica capillary with a stationary phase
24
coating on its inner surface (e.g., a short piece of GC column) is typically used to perform
extraction (Figure 1.7). In the capillary format, the protective polyimide coating on the
external surface of the capillary remains intact and provides reliable protection against
mechanical damage to the capillary. Moreover, in-tube SPME provides a simple, easy,
and convenient way to couple SPME to gas- as well as to liquid-phase separation
techniques.
Figure 1.7 Comparison SPME fiber with in-tube SPME
1.7.2.1 Comparison of fiber SPME and in-tube SPME
Although the theory of fiber and in-tube SPME methods are similar, the
significant difference between these methods is that in fiber SPME the extraction of
25
analytes is performed with the coating on the outer surface of the fiber using mechanical
agitation of the sample while in-tube SPME method uses a sorbent coating on the inner
surface of a capillary to perform extraction by a flow-though process that involves
agitation by sample flow in and out of the extraction capillary. Figure 1.8 illustrates the
comparison of extraction process for the transfer of the analytes in fiber SPME and
in-tube SPME. For in-tube SPME, it is necessary to prevent plugging of the extraction
capillary and flow lines during extracting process and the particles in the sample matrix
must be removed by filtration before extraction.
Figure 1.8 Comparison of extraction process for the transfer of the analytes in fiber
SPME and in-tube SPME [44]
26
Both SPME and in-tube SPME can be performed as: (a) active (dynamic) and (b)
passive (static) mode. To use in-tube SPME in active mode, the matrix containing the
analytes is passed through the extraction capillary and the analytes are extracted by
sorbent coating on the capillary inner walls as sample passes through. To use fiber SPME
in active mode, a stir bar is used to agitate the sample, thereby speeding up the extraction
of analytes from sample matrix by the extracting phase coating on the surface of the fiber.
In static in-tube SPME mode, the capillary is filled with sample matrix and the high
affinity of the analytes for the sorbent material serves as the driving force for their
extraction. Static fiber SPME is carried out mainly in the headspace mode. In this case,
the fiber and extracting phase resides inside the protective tubing (needle), is not exposed
directly to the matrix sample. The extraction occurs through diffusion of the gaseous
sample contained in the tubing. The static fiber SPME is suitable for field analysis of
gaseous samples.
The SPME fiber should be handled carefully, because it is fragile, and the fiber
coating can be damaged during insertion and agitation. In in-tube SPME, on the other
hand, the protective polyimide coating on the external surface of the capillary remains
intact and provides reliable protection against mechanical damage to the capillary.
Therefore in-tube SPME provides safe and convenient way to couple SPME to gas- as
well as to liquid-phase separation techniques.
The introduction of in-tube SPME was primarily for coupling SPME to high
performance liquid chromatography (HPLC) for automated applications. For fiber
SPME-HPLC, analysts are currently limited to performing manual extractions and
27
desorptions. Analytes extracted by fiber SPME must be desorbed into a suitable receiving
solvent prior to HPLC analysis with the help of a SPME-HPLC interface constructed
with a special desorption chamber and switching valve. Figure 1.9 illustrates the
procedure of extraction by fiber SPME and desorption for HPLC. Analytes are directly
extracted by the fiber coating and then introduced to the chromatography is system for
further analysis. The extraction is carried out by exposure of the SPME fiber in the
headspace or in the sample solution. An SPME-HPLC interface (Figure 1.9) equipped
with a special desorption chamber is utilized for solvent desorption prior to HPLC
analysis. The analytes extracted in the fiber are released in the desorption chamber by
external addition of solvent or mobile phase, and then introduced to the HPLC column.
SPME-HPLC (-MS) has been applied to the analysis of various polar compounds such as
drugs and pesticides, and has been reviewed [44].
However, for automated extraction and analysis, in-tube SPME is relatively
simple to implement, and it can continuously perform extraction, desorption, and
injection using a conventional autosampler without any special SPME-HPLC interface
for the desorption of analytes. A schematic diagram of the automated in-tube SPME-LC
system is illustrated in Figure 1.10. For in-tube SPME system, the extraction is completed
by injection syringe that repeatedly draws and ejects sample from the vial under
computer control. The analytes partition from the sample matrix into the extracting phase
coating until equilibrium is reached. Subsequently, the extracted analytes are directly
desorbed from the capillary coating by mobile phase flow or by aspirating desorption
solvent after switching the six-port valve. Therefore, in-tube SPME provides automated
28
and high precision extraction for HPLC analysis by setting sample to autosampler. The
application of in-tube SPME technique to the determination of pesticides, environmental
pollutants, drugs, and food contaminants by hyphenation with HPLC, LC-MS has been
reviewed [44].
Figure 1.9 Illustration of the extraction procedure of extraction by fiber SPME and
analyte desorption for HPLC analysis [44]
29
Figure 1.10 Schematic diagrams of in-tube SPME-LC-MS systems [44]
Both SPME and in-tube SPME are also compatible with GC. For fiber SPME-GC,
the extraction procedure includes [71]: (1) piecing the septum of sample container using
the needle of SPME device, (2) exposing the SPME fiber to the sample and perform
extraction either by direction immersion or in the headspace (3) retracting fiber back into
the needle and withdraw needle. The extracted analytes are thermally desorbed in the GC
injection port for further transport into the GC column by the carrier gas. For in-tube
SPME-GC, the capillary with the extracted analytes is connected to the inlet end of the
GC column using a two-way press-fit fused-silica connector housed inside the GC
injection port [69] or by placing inside needle capillary adsorption trap (INCAT) device
[72].
30
1.7.2.2 Theoretical aspect of in-tube SPME
The fundamental thermodynamic principle to all extraction techniques commonly
involves distribution of analyte between sample matrix and the extracting phase. When a
liquid is used as the extracting phase, then the distribution constant Kes defines the
equilibrium conditions and enrichment factors achievable in the technique. The
partitioning is between aqueous sample matrix and organic extraction phase, which is
illustrated in Figure 1.11.
Figure 1.11 Distribution of analyte between sample matrix and the extraction phase [19]
When a liquid is used as an extracting phase, the distribution constant Kes can be
expressed as:
Kes
ae
as
Ce
Cs (1.4)
Where,
Kes: analyte distribution constant between liquid extracting phase and sample
matrix,
31
ae: activity of an analyte in the liquid extracting phase,
as: activity of an analyte in the sample matrix,
Ce: analyte concentration in the liquid extracting phase,
Cs: analyte concentration in the sample matrix.
In-tube SPME uses a flow-through system where solid phase is located on the
inner surface of capillary and is used as extracting phase. When a solid phase is used as
the extracting phase, a similar equation will be expressed [19]:
Kses
Se
Cs (1.5)
Where,
Kses: analyte distribution constant between solid extracting phase and sample
matrix,
Se: surface concentration of adsorbed analytes in the solid extracting phase,
For the in-tube SPME using flow-through technique system, the minimum
extraction time at equilibrium condition can be expressed [19]:
(1.6)
Where,
L is the length of the capillary holding the extraction phase,
Ve is the volume of the extracting phase,
Vv is the void volume of the capillary,
32
u is linear flow rate of the sample.
From the above equation, it can be seen that the extracting time is proportional to
the length of the capillary and inversely proportional to the linear flow rate of the sample.
Extraction time increases with an increase in the distribution constant between extracting
phase and sample matrix. Extraction time also increases with the volume of the extracting
phase (Ve) but decreases with an increase of the void volume of the capillary. In another
word, under a certain distribution constant, thinner coating with bigger capillary void
volume, less length of capillary and higher linear flow rate will help to fasten the
extraction equilibrium, therefore the extraction time can be reduced. It should be
emphasized that the above equation can be used only for direct extraction when the
sample matrix passes through capillary [19].
1.7.2.3 Parameters influencing in-tube SPME
In-tube SPME is an extraction method based on the transfer of analyte from the
sample to sorbent coating, in accordance with the analyte distribution between the two
phases. To obtain rapid and high efficiency of extraction, the following parameters
influencing in-tube SPME should be considered: (1) the selection of capillary coating (2)
sample solution (3) volume of draw/eject cycles and, (4) desorption of compounds from
capillary.
1.7.3 Stir bar sorptive extraction (SBSE)
Stir bar sorptive extraction (SBSE) is another sorptive technique which can
overcomes the limitations of traditional SPME fibers. The theory of SBSE is rather
straightforward and similar to that of SPME [73], which is based on distribution between
33
the sample and the extracting phase and the phase ratio. Stir bar sorptive extraction was
developed and used for the determination of volatile and semivolatile organic compounds
[73-75]. In SBSE, the coating covers a magnetic stir bar, varying in length from 1 to 4 cm,
coated with a relatively thick layer of PDMS (03.-1 mm), resulting in PDMS volumes
varying from 55 to 220 uL [74] (Figure 1.12). This allows for an extracting phase volume
50-250 times greater than that of SPME fibers [76]. The coated stir bar is added to an
aqueous sample for stirring and extraction. After a certain stirring time, the bar is
removed from the aqueous sample, and the sample is thermally desorbed by loading the
stir bar into the injection port of a gas chromatography.
Figure 1.12 Schematic of a stir bar of SBSE [71].
The extracting phase on the stir bar in SBSE is critical for performances of both
extraction and thermal desorption. However, the only commercially available phase for
SBSE is PDMS, marketed by Gerstel. A stir bar has three essential parts [76]. The
innermost part is a magnetic stirring rod, which is necessary for transferring the rotating
movement of a stirring plate to the liquid sample. The second part is a thin glass jacket
34
that covers the magnetic stirring rod. The outermost part is the layer of PDMS sorbent
into which the analytes are extracted. The glass layer is essential for the construction of a
high-quality stir bar as it effectively prevents the decomposition of the PDMS layer due
to catalytic effects of the metals in the magnetic stirring rod.
1.8 References for chapter one
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38
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39
CHAPTER TWO
SOLID PHASE MICROEXTRACTION AND SOL-GEL TECHNOLOGY
2.1 Introduction to sol-gel process and some basic definitions
Generally, the sol-gel process involves the transition of a system from a liquid
"sol" (mostly colloidal) into a solid ―gel‖ phase. The starting materials used in the
preparation of the ―sol‖ are usually inorganic metal salts or metal organic compounds
such as metal alkoxides. Typically, in sol-gel process the precursor is subjected to a
series of hydrolysis and polymerization reactions to form a colloidal suspension, or a
―sol‖. When the ―sol‖ is cast into a mold, a wet ―gel‖ will form. Further drying and
heat-treatment convert the ―gel‖ into dense ceramic or glass material. If the liquid in a
wet ―gel‖ is removed under a supercritical condition, a highly porous and extremely low
density material called ―aerogel‖ is obtained. Here are some terms that need to be
defined [1]. A colloid is a suspension in which the dispersed phase is so small (~ 1-1000
nm) that the gravitational forces are negligible and interactions are dominated by
short-range forces, such as van der Waals attraction and surface charges. The inertia of
the disperse phase is small enough to exhibit Brownian motion, a random walk drive by
momentum imparted by collision with molecules of the suspending medium. A sol is a
colloidal suspension of solid particles in a liquid. An aerosol is a colloidal suspension of
particles in a gas. In sol-gel process, the precursors (starting compounds) for preparation
of a colloid consist of a metal or metalloid element surrounded by various ligands
40
(appendage not including another metal or metalloid atom). Metal alkoxides are popular
precursors because they react readily with water. The reaction is called hydrolysis. Two
partially hydrolyzed molecules can link together, which is called condensation reaction.
Condensation liberates a small molecule, such as water or alcohol. This type of reaction
can continue to build larger and larger silicon containing molecules by the process of
polymerization.
Applying the sol-gel process, it is possible to fabricate ceramic or glass materials
in a wide variety of forms: ultra-fine or spherical particles in the form of powders [2],
thin film coatings [3], ceramic fibers [4], microporous inorganic membranes [5],
monolithic ceramics and glasses, or extremely porous aerogel materials etc. Sol-gel
technology has found interesting applications in development of new materials for
catalysis [6, 7], chemical sensors [8, 9], membrane [10], fibers [11], optical gain media
[12], photochromic and nonlinear applications [13, 14], solid state electrochemical
devices [15], nuclear industry [16], and electronic industry [17] in various scientific and
engineering fields. A simple overview of the sol-gel process is graphically presented in
Figure 2.1.
41
Figure 2.1 Overview of the sol-gel process. Adapted from [1, 18]
42
2.2 Historical background of sol-gel technology
Sol-gel research on ceramic and glass materials can be traced back to the mid
1800s when Ebelmen [19] found that solutions of certain compounds gelled on exposure
to atmosphere. However, these materials presented interest only to chemists. It was
finally in the 1930s, that systematic improvement of sol-gel technology was carried out
by Geffcken and Dislich of Schott Glass Company in Germany [20]. The network
structure of silica gels was widely accepted in the 1930s when Hurd [21] showed that
they must consist of a polymeric skeleton of silicic acid enclosing a continuous liquid
phase. In the 1950s, the method for the preparation of homogeneous powder was
popularized by Roy in ceramics research [22, 23]. However, that work was not directed
toward an understanding of sol-gel reaction mechanism. In 1970s, controlled hydrolysis
and condensation of alkoxides for preparation of multicomponent glasses was
independently developed by Levene and Thomas [24] and Dislich [25] due to an
increased interest from ceramics industry. Tremendous attentions in sol-gel research have
been obtained ever since Yoldas [26] and Yamane et al. [27] showed that large monoliths
could be made by the sol-gel method through the careful drying of gels in the 1980s.
In 1987, Cortes et al. [28] prepared sol–gel monolithic ceramic beds within
small-diameter capillaries as separation columns in liquid chromatography (LC). In 1993,
Crego et al. [29] prepared a thin layer of silica gel with chemically bonded C18 moieties
on the inner walls of fused-silica capillaries for use in reversed-phase HPLC. Guo and
Colon [30] used a similar sol-gel technology to prepare stationary phase coatings for
open-tubular LC and electrochromatography. Malik and co-workers introduced sol-gel
43
coated columns for capillary GC [31], sol-gel coated fibers for solid-phase
microextraction (SPME) [32] and in tube SPME [33]. Gbatu [34] also prepared SPME
fiber with sol-gel technology. These preceding works led to sol-gel research aiming at
developing novel sorbent materials for solid-phase microextraction [35-39] and
solid-phase extraction [40-43].
2.3 The potential of sol-gel chemistry in coating technology for analytical
microextraction
2.3.1 Problems with traditional SPME coating technology
In chapter one, some drawbacks of fiber SPME inherited from physical
construction of SPME device such as the breakage of the fiber and bending of needles
were pointed out. The introduction of an alternative format of SPME seems to be a
promising direction in solving those problems. The other drawbacks arise from the
traditional coatings and coating technology [44]. First, the traditional SPME coatings are
designed to extract either polar or non-polar analytes for a given matrix. It is important to
have a sorbent that can extract both polar and non-polar compounds with high extraction
sensitivity needed for trace analysis. Second, in traditional SPME only a short length of
the fiber is coated with sorbent. The short length of the fiber coated with sorbent
translates into low sorbent loading which in turn leads to low sample capacity, which
significantly limits the sensitivity of traditional SPME coating. This is particularly
important for analyzing ultra-trace contaminants. Increasing the coating thickness [45, 46]
seems one possible way of improving extraction sensitivity. However, the increased
coating thickness requires longer equilibrium time, which translates into a long analysis
44
time. Moreover, immobilization of thicker coating on fused silica surface is difficult to
achieve by conventional approaches [47, 48]. Therefore, developing an effective
alternative approach to immobilize thick coatings is important for reliable and effective
SPME performance. Third, low thermal and solvent stability of traditional SPME
coatings represents a major drawback of conventional SPME technology. In traditional
approaches, relatively thin coatings can be immobilized on the capillary inner surface
though free-radical cross-linking reactions [49, 50]. Because of the absence of direct
chemical bonding between the coating and the substrate, the thermal and solvent
stabilities of such coatings are typically poor. The problem is more severe in the case of
thicker coatings. When coupled to GC, such extracting coatings may cause reduced
thermal stability of conventional coatings leads to incomplete sample desorption and
sample carryover problems. Low solvent stability of conventionally prepared extraction
coatings presents an obstacle to the hyphenation of in-tube SPME with liquid-phase
separation techniques because HPLC employs organic mobile-phase systems for
desorption of analytes. Evidently, all above problems are related to coating materials and
coating technology. Novel approaches to sorbent chemistry and coating technology are
very important for solving these problems. Development of new methods for the
preparing of chemically immobilized coatings form advanced material systems to achieve
desired selectivity and performance in SPME is an important direction in SPME research.
2.3.2 The advantages sol–gel technology for use in analytical microextraction
Sol-gel technology is one approach to address most of the above mentioned
problems through the creation of chemically bonded sorbent coatings [31-33, 51].
45
Sol–gel chemistry provides a simple and convenient pathway to synthesize advanced
material systems that can be used to prepare surface coatings. The advantages sol–gel
technology in the area of fiber based SPME or capillary-based SPME (CME) are as
follows [44, 52]:
(1) it combines surface treatment, deactivation, coating, and sorbent immobilization into
a single-step procedure making the whole SPME fiber/capillary manufacturing process
very efficient and cost-effective; (2) it creates chemical bonds between the fused silica
surface and the created sorbent coating, therefore it can achieve enhanced extracting
phase stability (solvent stability and thermal stability) in sample preparation and
chromatographic separations (3) it provides surface-coatings with high operational
stability ensuring reproducible performance of the sorbent coating under operation
conditions involving high temperature and/or organic solvents, and thereby it expands the
SPME application range towards both higher-boiling as well as thermally labile analytes;
(4) it provides the possibility to combine organic and inorganic material properties in
extraction sorbents providing tunable selectivity; (5) it offers the opportunity to create
sorbent coatings with a porous structure which significantly increases the surface area of
the extracting phase and provides acceptable stationary phase loading and sample
capacity using thinner coatings; (6) it reaches a better homogeneity from raw material on
the molecular level; (7) it provides mild reaction temperature; (8) it provides the
possibility to design the material structure and property through proper selection of
sol-gel precursor and other components; (9) it can easily provide different products such
as coatings, and monolithic bed, particles etc.
46
2.4 Fundamentals of sol-gel technology
To design and create proper sol-gel material, understanding the general chemical
reactions involved in the sol-gel process is very important. Chemical reagents for the
preparation of sol-gel sorbents normally include (1) sol-gel precursor(s); (2) a solvent for
dispersing and mixing all the reagents; (3) a polymer which provides selective functional
group; (4) deactivating reagents; (5) a catalyst, and (6) water.
At least one sol-gel precursor is used in sol-gel synthesis. Sol-gel precursors are
usually alkoxide-based materials [53]. However, alkoxides of various metals and
metalloids (Si, Al, Ti, Zr, Ge, etc.) [54-59] have been utilized to create sol-gel sorbents,
and they have been applied in analytical microextraction. Among them, silicon-based
alkoxides, especially tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMOS)
are the most commonly used precursors [60-64]. Conventionally, tetraalkoxysilanes are
used as sol-gel precursors [65]. However, the use of alkyl or aryl derivatives of
tetraalkoxysilanes as precursors may provide important advantages in coating technology
because sol-gels obtained by using these derivatives are reported to be less prone to
cracking and shrinking [31, 65].
An organic polymer is usually used to form a network and provide the pendent
functional groups for sol-gel materials for selective interaction with the target analytes.
The resultant microextraction performance of sol-gel stationary phase will mainly be
affected by the characteristics of the pendent functional group. They provide the sol-gel
material with different affinity to certain classes of analytes based on the structure and
polarity of pendent functional groups acquired from the polymer(s). Polysiloxanes are
47
important SPME coating materials due to its high thermal stability and good film forming
characteries. Poly(dimethlysiloxane) (PDMS) has been commonly used in sol-gel
microextraction sorbent either as a mono-polymer [54] or co-polymers [63].
Hydroxy-termanited, PDMS has been used to synthesize sol-gel sorbents that have been
used in a variety of microextraction applilcation. Other used polymers such as poly (vinyl
alcohol) (PVA) [63], polyethylene glycol (PEG) [66, 67], hydroxyfullerene [68], Ucon
[69] and poly (methylphenylvinylsiloxane) (PMPVS) [70] etc. have been used to produce
sorbent coatings.
All the above mentioned precursors, polymers, and deactivating reagents (if
necessary) are dispersed and mixed well in solvent(s). Here the solvent can be a pure
organic solvent or a mixture of organic solvents. The purpose of the solvent is usually for
the dissolution of all components of sol solution to achieve a molecule level uniformity in
the mixed system. After mixing, water and a proper catalyst are added to the system to
initiate hydrolysis of the precursor(s).
The catalysts for sol-gel reactions usually include (I) acids [60, 71-72], (II) bases
[73] (III) or ions (e.g., F-), [74, 75]. A radical initiator is used in conjunction with
ultraviolet (UV) light induced polymerization. In sol-gel synthesis, trifluoroacetic acid is
the most popular catalyst since its initial use in 1997 [31]. It serves not only as a catalyst
but also an organic solvent and a source of water (1% water). As a strong organic acid of
pKa value of 0.3, it provides for relatively the short gelling times. Other than TFA, HCl is
the second most commonly used catalyst for sol-gel reactions [60]. Acid-catalyzed sol-gel
reactions are more popular in the preparation of microextraction sorbents. However,
48
NaOH, and F- are sometimes used for comparison purposes in sol-gel CME techniques
[76]. Generally, acid-catalyzed sol-gel processes are believed to produce linear polymers
while base-catalyzed processes produce highly condensed particulate structures.
Acid-catalyzed material also has a tendency to be highly porous and possesses higher
surface areas in comparison to base-catalyzed sol-gel materials. High surface area and
narrowly distributed pore structure of acid-catalyzed sol-gel material make them very
suitable for being used as sorbent coatings in analytical microextraction.
Sol-gel synthesis consists of two processes: (1) hydrolysis of the precursor(s) and
(2) water or alcohol polycondensation of the sol-gel-active species present in the sol
solution [1]. The sol-gel-active species include fully or partially hydrolyzed precursors
and polymers which include silica-based species, analogous non-silica species, silanol
group or any other species chemically reactive to them. Scheme 2.1 illustrates hydrolysis
and condensation of tetramethoxysilane (TMOS) as an example (Scheme 2.1). During the
process, a liquid like colloidal suspension (the sol) is transformed by hydrolysis. Then a
3-D network, a gel form via polycondensation reactions. In sol-gel process, hydrolysis
and condensation reactions occur simultaneously — not stepwise. A wet
three-dimensional sol-gel network of porous material is formed as the polycondensation
reaction progresses.
49
Si
OCH3
OCH3
OCH3
CH3O + H2Ohydrolysis
Si
OCH3
OCH3
OCH3
HO + CH3OH
Si
OCH3
OCH3
OCH3
CH3O + Si
OCH3
OCH3
OCH3
HOalcohol condensation
Si
OCH3
O
OCH3
CH3O Si
OCH3
OCH3
OCH3
CH3OH+
Si
OCH3
OH
OCH3
CH3O + Si
OCH3
OCH3
OCH3
HOwater condensation
Si
OCH3
O
OCH3
CH3O Si
OCH3
OCH3
OCH3
H2O+
Scheme 2.1 Illustration of hydrolysis and condensation of tetramethoxysilane (TMOS).
Adapted from [18]
2.5 Creation of sol-gel CME sorbents
Further advancements in the area of analytical microextraction are greatly
dependent on improvements and new breakthroughs in sorbent development and coating
technology to replace traditional coating technology. The development of sol-gel coatings
and/or monolithic beds has proved to be an important step in this direction. Generally,
preparation of the sol-gel stationary phase coatings for CME involves four typical steps:
(1) pre-treatment of the fused-silica capillary, (2) preparation of the sol solution, (3)
sol-gel coating process, and (4) treatment of coated capillary.
2.5.1 Pre-treatment of the fused-silica capillary
The purposes of capillary pretreatment are: (1) cleaning the possible contaminants
inside the fuse silica capillary, (2) increasing the concentration of surface silanol group.
50
Silanol group concentration on the capillary surface may be low due to the formation of
siloxane bridges due to condensation of neighboring silanol groups at high drawing
temperatures (about 2000 ºC), and may vary from one piece to another piece. Since
silanol groups on the capillary surface represent the binding sites for in situ created
sol-gel extracting phases, higher surface concentration of these binding sites is desirable
to facilitate the formation of chemical bonds between sol-gel extracting phase and the
capillary inner surface. Alkali solutions are traditional approaches for pretreatment of
fused silica capillaries [77-83]. The pretreatment usually starts with rinsing by 1 M of
NaOH, followed by diluted HCl to neutralize excessive NaOH, and rinsing with purified
water. After rinsing, the capillary is usually purged with helium or nitrogen. This
traditional pretreatment method has been used for prepare sol-gel CE preconcentration
columns [79], sol-gel SPME fibers [11, 35] and stir bars for sorptive extraction [77]. It
also was used in preparing CME [78, 81-83] sorbents.
Hayes and Malik [84, 85] reported the use of hydrothermal treatment of the inner
surface of fused silica capillary prior to in situ creation of surface coated and monolithic
capillaries. This hydrothermal treatment was performed for several reasons. First, the
water serves to clean the inner capillary surface, removing any contaminants originating
from the capillary drawing process or postdrawing manipulation and handling. Moreover,
this pretreatment with water enhances surface silanol concentrations, thereby providing a
higher percentage of bonding sites for anchoring the sol-gel coating. A homemade
gas-pressure-operated capillary filling/purging device [69] was used to realize this
(Figure 2.2). The fused silica capillary was sequentially rinsed with two organic solvents
51
of different polarities: CH2Cl2 was used first to clean the non-polar and moderately polar
contaminations inside the capillary. Then CH3OH was used to substitute CH2Cl2 and also
to clean the relatively polar contaminations on the inner surface. Deionized water was
then passed through the capillary under helium pressure. This was followed by purging of
the capillary with helium for 5 min. Both capillary ends were then sealed using an
oxyacetylene torch and the capillary was placed in a GC oven at 250 ºC for thermal
conditioning for 2 h. Following this, the capillary was removed from the GC oven, it ends
were cut open using an alumina wafer, and the capillary was further purged with helium
for an additional 30 min at ambient temperature.
Figure 2.2 Homemade capillary filling/purging device. Adapted from [69]
52
2.5.2 Preparation of the sol solution
Designing the sol solution is the most critical step in preparing sol-gel CME
capillary. Judicious selection of the sol solution ingredients is essential for a successful
creation of the desired sol-gel sorbent. Chemical reagents for the preparation of sol-gel
sorbents normally includes sol-gel precursor(s), a solvent for dispersing and mix all the
reagents, a polymer which provides the functional group deactivation reagent(s), a
catalyst (or inhibitor) and water. The role of each ingredient plays has been explained in
section 2.4.
In addition to typical ingredients in the sol solution, various additives are often
used, such as surface deactivating reagents or a drying control chemical additive (DCCA).
An important attribute of sol-gel for coating technology is the simplified deactivation step
compared with traditional coating technology [31]. In sol-gel chemistry, deactivation is
achieved by adding to the sol solution traditional deactivating reagents such as
poly(methylhydrosiloxane) (PMHS), Hexamethyldisilazane (HMDS), etc. that contain
chemically reactive hydrogen atoms. These chemicals are not sol-gel reactive but they get
physically incorporated in the coating and deactivate residual silanol groups on the
created sol-gel material during the thermal step. This is in contrast with traditional
coating approaches, where silica surface is deactivated prior to coating.
Malik and coworkers reported the use of various deactivation reagents for sol-gel
stationary phases, such as phenyldimethylsiane (PheDMS) [84, 86]
1,1,1,3,3,3-hexamethyldisilazane (HMDS) [50, 51, 57, 58], and polyethylhydeosiloxane
(PMHS) [31, 54]. Some deactivating reagents and deactivation reactions of HMDS are
53
shown in Scheme 2.2 and Scheme 2.3.
A drying control chemical additive (DCCA) such as formamide, glycerin,
dimethyl ether, and oxalic acid is usually used as a sol-solvent and it may play other
critical roles in the sol-gel reactions [87, 88]. During sol-gel process, stress may be
developed in the sol-gel network as the solvent escapes or as the pore size changes during
these processes. This stress may cause problems during the formation of the uniform
sol-gel coatings. To overcome this problem, DCCA is incorporated in the starting mixture
of sol solution, and they can release the capillary stress generated during of the coated
surface [1, 77, 89, 90]. Unfortunately, the exact way in which the DCCA improves the
drying process is still unclear [90].
Scheme 2.2 Deactivating reagents
H3C Si NH Si
CH3 CH3
CH3 CH3
CH3
Hexamethyldisilazane, HMDS
Si OH3C
CH3
CH3
Si O
CH3
CH3
Si O
CH3
Hn
Si O
CH3
CH3 m
CH3
Polymethylhydrosiloxane, PMHS
54
Si O Si O Si OSi O
OHOH O
(Bare silica surface)
+ SiCH3
CH3
CH3
NH Si
CH3
CH3
CH3
Si O Si O Si OSi O
O
(Derivatized silica surface)
O
SiCH3
CH3
CH3
O
SiCH3
CH3
CH3
+ NH3
HMDS
Scheme 2.3 Deactivation of silica surface with HMDS
The homogeneity of the sol is very important. Therefore careful selection of a
solvent system which is compatible with other ingredients is necessary. A mixture of
solvents compatible with inorganic and organic components in the sol system needs to be
used to achieve the homogeneity. The viscosity of the system is another important
parameter for in-tube SPME coatings. The water/precursors ratio in the sol solution also
affects the speed of reaction as well as the physical properties of the obtained sol-gel
materials. The ratio should be carefully controlled to adjust the reaction speed and to fine
tune structures of sol-gel materials.
55
2.5.3 Sol gel coating process
After preparing sol solution, the sol solution was filled into the capillaries. Sol
solution was kept inside of the capillary for a controlled period of time to facilitate the
formation of sol-gel reactions inside the capillary. During this process, the evolving
sol-gel organic-inorganic hybrid material gets chemically bonded to the inner walls of the
capillary via condensation with the surface silanol groups. After this, the unbonded
portion of the sol solution is expelled from the capillaries under inert gas leaving behind a
surface-bonded sol-gel thin coating within the capillary. In the end, this thin coating is
further purged with inert gas for an additional period to facilitate the evaporation of the
remaining volatile organic solvents. The in-capillary residence time of the sol solution is
various and it can be adjusted for application purpose. It can be adjusted to prepare
sol-gel coatings with various thicknesses.
A sol-gel in-tube SPME coating technology was first introduced by Malik and
co-workers [69]. A homemade capillary filling/purging device (Figure 2.2) was used to
fill the capillary with the coating sol solution under helium. Then an additional period of
purging of helium was applied to facilitate the evaporation of the remaining organic
solvents. Hu and coworkers [78, 81] used a syringe to introduce sol solutions into the
capillary to prepare CME sorbents followed by purging nitrogen to remove the sol
solution.
2.5.4 Further treatment of sol-gel-coated CME capillary
After coating, the capillary is further thermally treated for aging, drying,
conditioning and cleaning. Different temperature programs have be been developed to
56
achieve the desired performance. Malik and coworkers [31, 57] installed the capillary in a
GC oven using a temperature program of 40 to 300 ºC or even higher at a rate of 1
ºC/min under the purging of helium. Purging with inert gas helped to remove the solvent
and unbonded materials from the coating at high temperature. The prepared CME
capillary was coupled with both GC and HPLC [44, 47]. Hu and coworkers [81]
connected CME capillary in a GC oven under nitrogen, the solvent was removed through
the capillary at 1 bar for 10 min and then at 0.2 bar for 60 min to prepare a zirconia
coating, then heated at 573 K for 8 h in a muffle furnace. The prepared zirconia coating
was used for on-line hyphenation with ICP-MS. In prepare another
N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS)-silica coating for on-line
hyphenation with Inductively Coupled Plasma Mass Spectrometry (ICP-MS), they also
placed the capillary in a muffle furnace at 120 ºC for 8 h with the heating program of
1 °C min−1
rising from the ambient temperature for the formation of the coating. When
making a monolithic column for CME, Hu and coworkers [78] sealed both ends of the
capillary with silicon rubber followed by further reaction at 40 °C for 12 h. The prepared
monolithic column was for on-line CME coupled to HPLC. In the end, the CME capillary
was rinsed with ethanol in order to remove the surfactant [78] or template materials [91].
2.6 Characterization of sol-gel extracting phase and its morphology
Various characterization methods are employed to understand sol-gel coatings.
Scanning electron microscopy (SEM) is one of the most popular techniques to evaluate
the morphology of sol-gel coatings thanks to its combination of higher magnification,
57
larger depth of focus, greater resolution, and ease of sample observation [92]. SEM can
reveal the uniformity of the coating thickness and structural details or defects through
cross sectional and surface views of sol-gel coatings.
In addition to SEM, Infrared (IR) spectroscopy is also an important and simple
tool to follow the evolution of the sol-gel material and microstructure of sol–gel silica
films [93, 94]. FTIR spectroscopy is most commonly used for identifying specific
chemical bonds on the sol-gel sorbent coating. Besides IR, various techniques such as
mass spectrometry (MS), fluorescence microscopy and nuclear magnetic resonance
(NMR) have been used to study chemical bonds in sol-gel structure [95, 96]. NMR is
another important and powerful analytical technique to investigate the structural features
for sol-gel material. NMR, SEM and FTIR are the most frequently used techniques to
study gel formation and for surface coating characterization. Other techniques atomic
force microscopy (AFM) [97], small angel X-ray photoelectron spectroscopy (XPS) [98],
etc. are also used to study the morphology of sol-gel materials.
2.7 Reported sol-gel coating materials in SPME and CME
Sol-gel chemistry provides a simple and convenient pathway to synthesis
advanced coating materials for fiber-based SPME or capillary SPME (CME). Various
types of polymers and functionalized organic compounds have been used. Table 2.1
summarized polymer and organic components that have been used to prepare sorbents in
the area of SPME and CME in recent years.
58
Table 2.1 Organic sorbents and their applications. Adapted from ref [99]
Name of the stationary
phase
Functional Group or Repeating
Functional Group structure
SPME
Techniques Ref.
Polyvinyl chloride
(PVC)
CH2 CH
Cl
n
HP-SPME [100-102]
Poly(ethylene glycol)
(PEG)
CH2 CH2n
O
SPME
CME
[103-105]
Poly(ethylenepropylene
glycol) monobutyl ether
(Ucon)
CH2 CH2 O CH2 CH
CH3
On
SPME [104]
3-Aminopropyltrimetho
xysilane (APTMS)
Si
OCH3
OCH3
H2NCH2CH2CH2 OCH
SPME,
CME
[59, 106]
N-(2-aminoethyl)-3-ami
nopropyltrimethoxysilan
e (AAPTS)
HN2
N Si OCH33H
CME,
Monolithic
- CME
[91,107]
Hydroxy-terminated
polydimethylsiloxane
(PDMS)
Si
CH3
CH3
On
HS-SPME
SPME
CME
[32,59,
105, 108]
Hydroxy-terminated
polymethylphenylsiloxa
ne
(PMPS)
Si
CH3
CH3
O Si
CH3
CH3
O Si O
x y
SPME
CME
[109 -111]
59
Table 2.1 (Continued)
Polyacrylate (PA)
CH CH2
n
COOH
SPME
[112]
Divinyl benzene
(DVB)
CH2 CH CH CH2
HS-SPME [113]
Calixarenes
HS-SPME
[114-118]
β-cyclodextrins
HS-SPME
[119-123]
Poly(trifluoropropyl)me
thylsiloxane (PTFPMS)
Si
CH3
CH2
O
CH2CF3
n
HS-SPME [124]
Poly(butylacrylate)
CH2 CH
C O
n
SPME [125]
60
Table 2.1 (Continued)
Methyl acrylate (MA)
CHCH2C O CH3
O
HS-SPME [126]
Methyl methacrylate
(MMA)
CCH2 C O CH3
O
CH3
HS-SPME [126]
Butyl methacrylate
(BMA)
CCH2 C O C4H9
O
CH3
HS-SPME
[126 -128]
Anilinemethyltriethoxys
ilane (AMTEOS)
NH Si C2H5O3
SPME [129]
Crown Ethers
SPME
[11,
130-132]
Calixcrowns
SPME [35, 133]
61
Table 2.1 (Continued)
Poly(methylphenylvinyl
siloxane)
O Si O
CH3
Si O
CH3
CH3
Si
CH3
HC CH2
x y z
CME,
HS-SPME [37, 134]
Methyl groups
CME [135]
Cyano-Polydimethylsilo
xane
CNCH2CH2CH2
CME [67]
Polytetrahydrofuran
(poly-THF)
CME [44]
n-Octyltriethoxysilane
(C8-TEOS)
Si C2H5O3
C8H17
Monolithic
- CME
[78]
Dendrimers
CME [51]
Hydroxyfullerene
(fullerol) -OH-TSO
C60 OH n
HS-SPME [68]
Poly(vinyl alcohol)
(CH2 CH)n
OH
HS-SPME [63, 136]
62
2.8 Drawbacks of silica-based sol-gel materials
Sol-gel microextraction sorbents reported so far are predominantly silica-based.
Despite many attractive material properties such as mechanical strength, surface
characteristics, catalytic inertness, surface derivatization possibilities, etc., silica-based
sol gel materials have some inherent shortcomings. The main shortcoming of silica-based
sorbents is the narrow range of pH stability. Siloxane bond (Si-O-Si) on the silica surface
hydrolyzes at pH > 8 [137] and it happens rapidly under elevated temperatures [138].
Under acidic conditions, siloxane bond is also unstable [139], and it gets worse with
elevated temperatures [140, 141]. A number of methods such as multiple covalent bonds
[142], multi-dentate synthetic approach [143], and end-capping [144] have been used to
solve this problem. None of them, however, seems to work effectively. Therefore,
developing sorbents with a wide range of pH stability is fundamentally important for
chromatographic separation and sample preparation technology. From this point of view,
metal oxides such as zirconia, titania, and alumina are interesting materials to offer better
chemical and thermal stabilities [138]. Due to these advantages, they have attracted
attention of separation scientists and they have been used in chromatography for many
years.
2.9 Sol-gel process using transition metal alkoxide
Transition metal alkoxides, M (OR)z, (M is Ti, Zr, and Hf etc.) are used sol-gel
precursors to glasses and ceramics. These transitional metal alkoxides are distinguished
from silicon alkoxides (Si(OR)4) in two aspects [1]: (1) they have greater chemical
63
reactivity resulting from the lower electronegativity of transition elements, which causes
them to be more electrophilic and thus less stable toward hydrolysis, (2) they have the
ability to exhibit several coordination states, so that coordination expansion occurs
spontaneously when water or other nucleophilic reagents are added, and they are able to
expand their coordination via olation, oxolation, alkoxy bridging, or other nucleophilic
association mechanisms, (3) the greater reactivity of transition metal alkoxides requires
that they be processed with stricter control of moisture and conditions of hydrolysis in
order to prepare homogeneous gels rather than precipitates, (4) the generally rapid
kinetics of nucleophilic reactions cause fundamental studies of hydrolysis and
condensation of transition metal alkoxides to be much more difficult than for Si(OR)4.
Similar to silicon alkoxides (Si(OR)4), two main reactions typically are involved
in the sol-gel process including transition metal alkoxides: (1) hydrolysis and (2) water or
alcohol condensation reactions. The general scheme for sol-gel hydrolysis and
condensation reactions of metal alkoxide precursors are illustrated in Scheme 2.4.
For coordinatively saturated metals in the absence of catalysts, hydrolysis and
condensation both occur by nucleophilic substitution (SN) mechanism involving
nucleophilic addition (AN) followed by proton transfer from the attacking molecule to an
alkoxide or hydroxo-ligand within the transition state and removal of protonated species
as either alcohol or water. The condensation process by removal of alcohol molecule is
called alcoxolation, and that by removal of water molecule is called oxolation. Scheme
2.5 illustrates the mechanism for hydrolysis, alcoxolation, oxolation or olation [1].
64
(A) Hydrolysis
M
OR
OR
OR
RO + H2O + ROHM
OR
OH
OR
RO
(B) Water condensation
M
OR
OH
OR
RO M
OR
OR
OR
HO+ M
OR
O
OR
RO M
OR
OR
OR
+ H2O
(C) Alcohol condensation
M
OR
OR
OR
HO+M
OR
OR
OR
RO M
OR
O
OR
RO M
OR
OR
OR
+ ROH
Scheme 2.4 General sol-gel process for transitional metal alkoxides. Adapted from [1,
145]
65
(A) Hydrolysis
O
H
H + M OR O
H
H
M OR O
H
R
MHO
M OH + ROH
a b c
d
(B) Oxolation
+ M OHOM
H
M O
H
M OH O
H
H
MOM
a b c
M O +M
d
H2O
(C) Alcoxolation
+ M OROM
H
M O
H
M OR O
H
R
MOM
a b c
M O + ROHM
d
(D) Olation
+ ROHM O M + O
R
H
M M O M
H
+ H2OM O M + O
H
H
M M O M
H
Scheme 2.5 Mechanisms of sol-gel hydrolysis and condensation reactions of metal
alkoxides [1]
66
In hydrolysis, the first step (a) for hydrolysis is a nucleophilic addition, which
leads to a transition state (b), where the coordination number of metal atom has increased
by one. The second step involves a proton transfer within (b) leading to the intermediate
(c). The third step is the departure of the leaving group which should be the most
positively charged species within the transition state. The entire process, (a) to (d) follows
a nucleophilic substitution mechanism.
The oxolation is a reaction by which an oxo bridge (―O―) is formed between
metal atoms through the elimination of a water molecule. Basically, oxolation follows the
same mechanism as that for condensation process: the metal atom replacing hydrogen
atom in the entering group. When the metal is unsaturated in terms of coordination
number, oxolation occurs by AN with rapid kinetics. Alcoxolation follows pretty much the
same mechanism as oxolation except the leaving group is an alcohol molecule.
Alcoxolation and oxolation are two competitive processes. The thermodynamics of
hydrolysis, alcoxolation, and oxolation are governed by the strength of the entering
nucleophile, the electrophilicity of the metal, and the partial charge and the stability of
the leaving group. In addition, condensation can also occur by olation when the transfer
ability of the proton is large. The thermodynamics of olation depend on the strength of
the entering nucleophile and the electrophilicity of the metal. The kinetics of the olation
are systematically fast because there is no proton transfer in the transition state.
Acid or base catalyst can influence both the hydrolysis and condensation rates and
the structure of the condensed product. Acids serve to protonate alkoxide groups,
enhancing the reaction kinetics by producing good leaving group, and eliminating the
67
requirement for proton transfer within the transition sate. Hydrolysis goes to completion
when sufficient water is added.
2.10 Non-silica sol-gel materials in analytical microextraction and chromatographic
separation
Metal oxides such as zirconia, titania and alumina offer much better chemical
stability than silica which has attracted a great deal of interest in the separation science
community [138]. They have been used in chromatography for many years.
The literature shows that zirconia has been the most systematically studied
transition metal oxide. Zirconia appears possess thermal exceptionally high stability. It
has been extensively for capturing and storing radiochemical wastes. Zirconia possesses
superior alkali resistance than other metal oxides, such as alumina, silica and titania [2]
[138]. It is practically insoluble within a wide pH range (1-14) [146- 148]. Zirconia also
shows outstanding resistance to dissolution at high temperatures [149]. Besides the
extraordinary pH stability, excellent chemical inertness and high mechanical strength are
two other attractive features that add value to zirconia as a support material in
chromatography and membrane-based separations. Extensive research has been done on
zirconia particles and their surface modifications for use as HPLC stationary phase [150,
151]. A number of reports have also recently appeared in the literature on the use of
zirconia-modified fused silica capillary electrophoresis (CE) [152, 153]. A zirconia-based
hybrid organic-inorgnic sol-gel polydimethyldiphenylsiloxane (PDMDPS) coating was
developed for CME [58]. The sol-gel zirconia-PDMDPS coating was immobilized on the
inner surface of a fused silica microextraction capillary. The resultant CME coating
68
efficiently extracted polycyclic aromatic hydrocarbons, ketones, and aldehydes from
aqueous samples, and it performance remained practically unchanged even after
continuous rinsing with a 0.1 M NaOH solution for 24 h. Recently, a zirconia hollow
fiber was prepared by sol-gel technology and applied in microextraction [56]. For this, a
polypropylene hollow fiber was employed as the template, and it was immersed in the
proper zirconia sol precursor for specify followed by a drying process. After burning the
template off, the zirconia hollow fiber was obtained. The zirconia hollow fiber is similar
to its propylene hollow fiber template in terms of morphology. However, it exhibits a
bimodal porous structure consisting of: through- pores and skeleton mesopores. The pore
structure and wall thickness can be easily controlled during the coating process and heat
treatment. The zirconia hollow fiber was a highly selective for phosphonic
acid-containing compounds. Pinacolyl methylphosphonic acid is a nerve agent
degradation product and it was used as a model analyte.
Titania has also attracted interest recently in separation science due to its superior
pH stability and mechanical strength compared with silica [154-157]. Several studies
have been conducted on the application of titania in chromatographic separations
[156-161]. A sol-gel titania-poly(dimethylsiloxane) (TiO2-PDMS) coating was
successfully developed for capillary microextraction (CME) by Malik and coworkers in
2004 [57]. This is the first report of the use of sol-gel titania-based organic-inorganic
material as a sorbent in capillary microextraction. The sol-gel titania-PDMS coating was
immobilized on the inner surface of a fused silica capillary. It showed good affinity for
many non-polar and moderately polar compounds, and it was used for on-line extraction
69
in hyphenation with HPLC analysis of polycyclic aromatic hydrocarbons, ketones, and
alkylbenzenes. This newly developed coating demonstrated excellent pH stability.
Extraction characteristics remained unchanged after continuous rinsing with a 0.2 M
NaOH solution for 12 hours. Besides those neutral compounds, Zeng and coworkers
utilized sol-gel titania in conjunction with hydroxy-terminated silicone oil as a sorptive
coating for SPME fibers [162]. The sol-gel coating provided significant extraction and
had sensitivity to polar compounds in SPME-GC analysis [162]. The application
demonstrated extraction and analysis of six aromatic amines in dye process wastewater.
The developed sol-gel hybrid titania coating was extremely pH stable, and its extraction
characteristic remained unchanged after rinsing with a 3 M HCl or NaOH solution for 12
hours. The thermal stability of this coating was as high as 320 ºC. An ordered,
mesoporous sol-gel titania film was developed for preconcentration/separation of trace V,
Cr, and Cu by CME. The sorbed analytes were eluted for electrothermal
vaporization-inductively coupled plasma mass spectrometry (EVT-ICP-MS)
determination of the trace element ions in environmental and biological samples [82]. In
this work, the properties, stability, and other factors of the sol-gel titania coating affecting
the sorption behaviors of V, Cr, and Cu were investigated.
Alumina, due to its high surface area, mechanical strength, thermal and chemical
stability, has been widely used in the area of sample purification [163] and analytical
separations [164, 165]. The surface chemistry of alumina is quite different from that of
silica. Silica has a weak Brønsted acidity due to the silanol groups, while alumina
provides well Lewis acidity and basicity as well as a low concentration of Brønsted acid
70
sites [138]. Both Brønsted and Lewis acid–base characteristics are responsible for the
chromatography performance of alumina. The surface chemistry of alumina is more
sililar to that of zirconia than to silica. Thus, it is reasonable to expect rather more
complex surface properties. As compared to silica, alumina is capable of both ion and
ligand exchange [166]. The ligand exchange ability of alumina originates from the
presence of Lewis acid sites on the surface, i.e., coordinatively unsaturated Al3+
, and
water molecules or other easily displaced ligands coordinatively bond to the sites [138,
166]. Lewis basic analytes containing polar functional groups can substitute for the
surface hydroxyl groups or coordinated water molecules and form complexes with the
metal ions of the oxide surface [154, 167] and so ligand exchange interaction can play an
important role for selective extraction of these compounds by alumina-based sorbents.
Alumina-based sol-gel sorbents were prepared from a highly reactive precursor, alumina
sec-butoxide, and a sol-gel active organic polymer (hydroxyl-terminated
polydimethylsiloxane) [167]. The resultant sol-gel alumina-OH-TSO coating had
superior ligand exchange ability, thus it extracted polar compounds such as fatty acids,
phenols, alcohols, aldehydes, and amines. It also demonstrated outstanding pH usage
range, good thermal stability, and good coating preparation reproducibility. When applied
in analysis of volatile alcohols and fatty acids in beer, the alumina-OH-TSO coating
provided recoveries ranging from 85.7% to 104% and the relative standard deviation
values of less than 9% for all the analytes [167].
71
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80
CHAPTER THREE
GERMANIA-BASED SOL-GEL HYBRID ORGANIC-INORGANIC COATINGS
FOR CAPILLARY MICROEXTRACTION AND GAS CHROMATOGRAPHY
3.1 Introduction of germanosilicate glasses material
In recent years silica glasses containing germanium dioxide (germanosilicate
glasses) have attracted considerable interests due to their various appealing properties.
Most of them are related to the composition and to the structure of the glass [1] . As an
isostructural analogue of SiO2, GeO2 is well compatible with the silica network; it is
employed as a refractive index modifier in binary xSiO2-(1-x) GeO2 glasses for optics.
The presence of Ge atoms modifies the atomic structure and causes both permanent and
photo-induced variation of the refractive index [2-3]. Ge-doped silica glasses have
attracted much attention for giving rise to phenomena such as photosensitive grating
effects and second harmonic generation (SHG) [4, 5]. The low phonon energy of
germania is also another interesting property because it can be used to produce low losses
optical waveguides and amplifiers [6]. For their intrinsic characteristics such as insulating
behavior, low phonon energy with respect to the silicate glasses, transparency and
homogeneity, SiO2-GeO2 glasses are good candidates as host matrixes for metal or
semiconductor clusters for optical applications. Significant attention has been further
devoted to these remarkable optical properties of these binary systems manifested in
different optical and opto-electronic phenomena.
81
The photosensitivity exhibited by these materials is ascribable to the presence of
germanium dioxide and to the occurrence of germania-related defect center. From a
structural point of view, each Ge atom could be substitutional to a Si atom and could be
bonded to four O atoms, or it could be under coordinated giving rise to oxygen deficient
centers (GODC) [7]. It is well known that there is an UV absorption band near 5 eV for
germanosilicate material and that this absorption is associated with an oxygen-deficient
defect [8, 9]. A 5 eV absorption band of germania-related oxygen-deficient defects is
responsible for the photosensitivity of germanium oxide based materials [10, 11].
Both silica and germania are prototypes of simple glasses consisting of
corner-shared TO4 (T = Si, Ge) tetrahedra linked in a random network structure. T-O-T
bond angles are estimated by X-ray diffraction to be 144 º in SiO2 glass and 133 º in
GeO2 glass. Besides the structural similarities, a striking difference between these two
systems is the concentration of defects. In pure silica, they are of order of 10 -9
of the total
Si atoms [12], while in pure germania or in binary SiO2-GeO2 glasses the concentration
is around 10 -3
-10 -4
of the total germanium sites. This difference has been ascribed to
different lead of thermodynamic stability for SiO2 and GeO2. From a thermodynamic
point of view, at room temperature, formation of SiO2 quartz is favored with respect to
GeO2 in the hexagonal form. Moreover, the latter is much less stable than SiO2. GeO2
may occur in either the 4-fold coordination of the α-quartz structure, or the 6-fold
coordination of the rutile structure. However, although both GeO2 polymorphs exist at
ambient conditions, rutile is the stable one, whereas the α-quartz structure is metastable
[13].
82
3.2 Preparation of sol-gel GeO2/ormosil organic-inorganic hybrid material
Various methods including melting, flame hydrolysis, vapour axial deposition
(VAD), sputtering deposition, chemical vapour deposition (CVD), ion implantation and
sol-gel technology have been employed to fabricated GeO2 based materials [14]. Among
the synthetic methods, the sol-gel route has gained great interest during recent years for
making advanced materials and in particular for preparation oxide-based coatings and it
has gained great interest in preparing Ge-doped SiO2 glasses. Germanium alkoxide
typically undergo polymerization via hydrolysis and condensation reactions similar to
those seen in the case of alkoxysilanes (Scheme 3.1). When GeO2 is mixed with vitreous
silica at a molecular level, Ge atoms would randomly substitute Si atoms in SiO4
tetrahedra so that the microstructure of the glass would remain a continuous
three-dimensional network consisting of interconnected SiO4 and GeO4 tetrahedea [15].
Scheme 3.1 Typical hydrolytic ploycondensation of germanium alkoxide to form
germania [16]
Ge OR4
+ x H2O GeRO4-x
OHx
+ x ROH
Ge OR GeHO+ Ge GeO ROH+
Ge OH GeHO+ Ge GeO + H2O
83
Sol-gel method is a solution-based chemical process from which
inorganic-organic hybrid materials can be synthesized through hydrolysis and
condenstation reactions. The components involved are well mixed at the molecular level,
providing good control of chemical compositions film homogeneity. Interests in using the
sol-gel method to fabricate germanosilicate coatings originate from the advantages that it
offers: homogeneity, ease of composition control, ability to fabricate large area coatings,
and low equipment coat. Materials for photonics require a very high level of purity and
homogeneity. Sol-gel processing is therefore an important route to fabricating
GeO2-based materials with high degree of purity, and homogeneity as well as advanced
and controlled properties [7]. Compared with the conventional glasses and glass-ceramic
processes, the sol-gel technique has the notable advantages of providing chemical
homogeneity at relatively low process temperatures. In the case of multicomponent
systems it provides a more uniform phase distribution and the possibility of preparing
new crystalline and non-crystalline materials [14]. In addition, because sol-gel based
glass can be prepared on the basis of chemical reactions involving a solution, this process
is very suitable to fabricate glassy and crystalline films or coatings. It has been widely
used to produce various kinds of functional films, coatings, hollow-core fibers,
concavo-convex cavity waveguides, etc. and organic or inorganic materials [14].
3.3 Ormosil precursors at low treatment temperature
Sol-gel germanosilicate synthesis usually starts from solutions of silicon and
germanium alkoxides at a low germania content. In reported GeO2-based material system,
84
tetraethylorthosilicate (TEOS) is generally employed as a SiO2 source, and TEOG is
generally used as GeO2 source. However, the heat treatment temperature for TEOS is
quite high (around 1000 ºC). Therefore, it is difficult to produce a film thicker than 0.2
μm by a single-coating process in a single purely inorganic sol-gel layer [17]. This is
because high curing and sintering temperatures necessary for densification of the final
materials causes a large shrinkage of the film during the thermal densification process
which leads to the cracking in a thicker layer and then leads to possible stress fracture [18,
19]. The use of organically modified alkoxysilane precursors helps overcome this
problem. Since silica precursors can produce a relatively thick single-coating layer at a
low temperature or even room temperature [20-12] without cracking.
Methyltrimethoxysilane (MTES) has been used as precursors for the hybrid material to
provide SiO2 source [23] A GeO2/Methyltrimethoxysilane sol-gel processed hybrid thin
film has been fabricated by acid catalyzed solutions of methyltrimethoxysilane (MTES)
mixed with germanium isopropoxide. A single layer coating with a thickness of about 1.3
μm film has been obtained on silica-on-silicon substrate by single spin-coating process at
a heat treatment temperature of 100 ºC. The result indicated that a heat treatment
temperature below 300 ºC is expected to produce a dense, low absorption, high
transparent hybrid waveguide film for photonic applications. The introduction of MTES
provides the advantage of producing thicker and denser films at low temperature.
Germania/γ-glycidoxypropyltrimethoxysilane orgnic-inorganic hybrid has also been
prepared by a sol-gel technique and a spin coating process. The result indicated that a
heat treatment temperature below 300 ºC is expected to produce a dense, low absorption,
85
highly transparent hybrid film. The introduction of ormosinl GLYMO, provides the
advantage of producing thick and dense films at low temperature [24-28].
Ormosil-based organic-inorganic hybrid materials have been studied as a
promising system for photonic applications in recent years. This is because organic
groups are integrated in the glass and the bulky organic components fill the pores
between the inorganic oxide chains.
3.4 Problems of germanosilicate material during sol-gel process
There are disadvantages to fabricate germanosilicate using sol-gel process. It is
well known that cracks and crystallization could be easily occor in the sol-gel materials.
Since germanium ethoxide hydrolyzes extremely fast and the resulting sol-gel materials
are likely to crack, the production of thick, crack-free GeO2-SiO2 sol-gel films with high
GeO2 content is very difficult. It has been reported by Kozuka et al. that the maximum
thickness achievable without crack formation via non-repetitive deposition is often below
0.1 μm for non-silicate oxide films [29]. It was also reported that although
germanosilicate films containing up to 45 mol % germanium oxide were fabricated, the
film thickness was usually less than 1 μm [30]. Thus, cycles of dip-coating are widely
used to deposit thick sol-gel films to overcome this problem. However, generally the
lifetime of GeO2-SiO2 sol with a high TEOG content is too short to perform cycles of
dip-coating. Therefore, a coating sol solution with a long gelation time is needed. To
reach this goal, techniques capable of suppressing the increase in sol viscosity and the
crack formation in sol-gel films with a high GeO2 content are in great demand. Jing et al.
86
developed a new sol-gel route to fabricate germansilicate film by using
diethylorthosilicate (DEOS) and TEOG as the precursors for SiO2 and GeO2 [30]. A thick
(3.6 μm), crack-free SiO2-GeO2 film containing up to 70 mol % germanium oxide were
obtained. By comparing TEOS and DEOS, they pointed out different precursors differed
greatly during ageing although they had the same molar ratio of SiO2 to GeO2. The sol
synthesized using TEOS and TEOG as raw material exhibited very poor stability and the
use of DEOS as precursor for silica is of great help in stabilizing the sol by maintaining
low viscosity for 280 h. In the work, they also explained the reason from the structural
point of view of the colloidal particles. On the other hand, sol-gel processing of oxide
materials, bulk and thin films, requires a high firing temperature to achieve a full
densification. During this process some oxides show a tendency to crystallize. This is
actually the case of several oxides such as titania [31]. The crystallization of germania
generally took place at temperatures higher than 500 ºC, and a full densification cannot
be achieved without avoiding a partial crystallization of the system. Therefore it is a
difficult problem to solve when sol-gel processing is employed for fabrication process.
The limitation in a wider application of the hybrid materials is the lack of a deeper
knowledge and control of their structure. It was observed that control of the hybrid
structure through an appropriate and reproducible synthesis is a difficult task.
3.5 Introduction of germania-based sol-gel organic-inorganic hybrid coatings for
capillary microextraction and gas chromatography
Solid-phase microextraction (SPME) [32] is an effective approach to solvent-free
sampling and sample preparation for laboratory and field analyses. Developed in 1989 by
87
Belardi and Pawliszyn, [33] SPME integrates sampling, sample preparation, and analyte
preconcentration into a single-step procedure characterized by ease and simplicity. By
eliminating the use of organic solvents, SPME overcomes the limitations inherent in
conventional sample preparation techniques like liquid-liquid extraction (LLE), Soxhlet
extraction, solid-phase extraction (SPE), etc. As a simple, sensitive, time-effective,
easy-to-automate, and portable sample preparation technique, SPME has been
successfully applied to a variety of analytes in environmental [43-36], biological [37-39],
food [40], drug [41-43], flavor [44] and other types of samples in hyphenation with gas
chromatography (GC) [45], high-performance liquid chromatography (HPLC) [46],
capillary electrophoresis (CE) [47], and Mass spectrometry (MS) [48].
In spite of its popularity and wide acceptance, fiber SPME (the dominant format
of SPME) possesses a number of inherent deficiencies arising from the fiber design and
its syringe-based mechanical operation. In fiber SPME, the protective polyimide coating
is removed from the external surface of a fused silica fiber end segment (~ 1 cm). A
sorbent layer created and on this end segment, serves as the SPME extracting phase. The
lack of external protective coating makes the end segment as well as the sorbent phase
coated on it vulnerable to mechanical damage during operation and handling of the
SPME device. The short length (~ 1 cm) of the coated segment results in low sorbent
loading, which leads to reduced extraction sensitivity. Moreover, fiber SPME is difficult
to couple to high-performance liquid chromatography (HPLC) and other liquid-phase
separation techniques. A number of SPME variants [49-53] have been proposed to
overcome these difficulties. Among them, in-tube SPME [54-56] holds great promise,
88
especially for effective hyphenation with liquid-phase separation techniques.
In-tube SPME (also termed capillary microextraction (CME)) [57], is practically
free form the shortcomings inherent in conventional fiber SPME. Unlike fiber SPME,
where a sorbent coating on the outer surface of a small-diameter solid rod serves as the
extraction medium, CME typically uses a piece of fused silica capillary with a stationary
phase coating on its inner surface (e.g., a short piece of GC column) to perform extraction.
In CME, the protective polyimide coating on the external surface of the capillary remains
intact and provides reliable protection against mechanical damage to the capillary.
Moreover, in-tube SPME provides a simple, easy, and convenient way to couple SPME to
gas- as well as to liquid-phase separation techniques.
Despite all these advantageous features, in-tube SPME still suffers from other
shortcomings that originate from the stationary phase coating on the inner surface of a
short piece of conventionally coated GC capillary column typically used as in in-tube
SPME device. First, the thin (usually < 1 μm) coating in such a capillary often results in
low stationary phase loading which leads to low sample capacity and reduced extraction
sensitivity of the technique. Increasing the coating thickness is a possible way to solve
this problem [58, 59]. However, it is extremely difficult to reliably immobilize thicker
coatings using conventional approaches [60]. Second, conventionally prepared GC
coatings are not chemically bonded to the fused silica capillary inner surface. This lack of
chemical bonds is mainly responsible for the low thermal and solvent stability of
conventionally prepared coatings [61]. When used in hyphenation with GC, low thermal
stability of the SPME coatings forces one to use low desorption temperatures to preserve
89
coating integrity, which in turn, results in incomplete sample desorption and sample
carry-over problems. Low solvent stability of the sorbent coating, on the other hand,
prevents effective hyphenation of in-tube SPME with liquid-phase separation techniques
that employ organic or organo-aqueous mobile phases [62]. Obviously, sorbent coating
stability plays a fundamentally important role in solid-phase microextraction — both
fiber-based and capillary-based. Further advancements in these areas will greatly depend
on improvements and new breakthroughs in sorbent development and coating technology.
In recent years, sol-gel technology has attracted considerable interest among
analytical chemists. Sol-gel chemistry provides an effective pathway for the synthesis of
advanced organic-inorganic hybrid materials [63] in a wide variety of forms. The mild
thermal conditions typical of sol-gel reactions not only allow chemical incorporation of a
wide range of chemical species in the created material systems but also ease the
requirements for laboratory operation and safety. Sol-gel technology allows the use of a
wide range of chemicals (both organic and inorganic) to integrate desirable properties in a
single material system. Thanks to all these advantages, sol-gel chemistry is receiving
increased attention as an effective tool for the synthesis of advanced stationary phases
and extraction media for modern analytical techniques. In recent years, sol-gel chemistry
has been the used to prepare highly stable surface-bonded coatings or monolithic beds
within capillaries for use as chromatographic stationary phases and sorbents for
microextraction techniques [57, 63, 64-70].
Silica-based stationary phases and sorbents are predominantly used in modern
separation and sample preparation techniques. Silica surface chemistry and surface
90
modification process are well understood. However, the limitations of siliceous materials
are also well known. The pH stability problem is a major drawback of silica-based
stationary phases and extraction media. Siloxane bond (Si-O-Si) on the silica surface
hydrolyzes at pH > 8 [71] and it happens rapidly under elevated temperatures [72].
Under acidic conditions, siloxane bond is also unstable [73], and it gets worse with
elevated temperatures [74]. Transitional metal oxides such as zirconia, titania, and
alumina are interesting materials to offer better chemical and thermal stabilities [72], and
have attracted attention of separation scientists.
Although titania-, alumina and zirconia-based sol-gel systems [75-77] that
demonstrated high chemical and pH stabilities in applications, surface modification of
these systems is rather hard. The precursor selections for transitional metal systems are
limited. Therefore, we are interested in investigating novel non-silica sol-gel system and
their application in separation material. Germania as an isostructural analog of silica, can
be expected to offer advanced material properties for analytical separation and sample
preparation. However, the potentials and promises of these novel materials in analytical
chemistry remained practically unexplored. In this work, for the first time, the
preparation of germania-based hybrid organic-inorganic sol-gel coatings for capillary
microextraction was explored. The great performance of sol-gel germania-based coatings
in CME, and the preliminary GC results that point to the analytical potential of such
coatings as GC stationary phases was presented.
91
3.6 Experimental
3.6.1 Equipment
Capillary microextraction - gas chromatography (CME-GC) experiments with
sol-gel germania coated capillaries were carried out on a Shimadzu Model 14A GC
system equipped with a flame ionization detector (FID) and a split/splitless injector. An
in-lab designed liquid sample dispenser [57] was used to perform capillary
microextraction via gravity flow of aqueous samples through the sol-gel microextraction
capillary. A Fisher Model G-560 Genie 2 vortex (Fisher Scientific, Pittsburgh, PA) was
used for thorough mixing of sol solution ingredients. A Micromax model 3590F
microcentrifuge (Thermo IEC, Needham Heights, MA) was used for centrifugation at
14,000 rpm (15,682 x g) to separate out the precipitate from the sol solution. A
home-built gas pressure-operated filling/purging device [78] was used to perform a
number of operations: (a) filling the extraction capillary with the sol solution, (b)
expelling the solution from the capillary after predetermined period of in-capillary
residence, (c) rinsing the capillary with a suitable solvent to clean the coated surface, and
(d) purging the microextraction capillary with helium. Ultra pure water (17.0 MΩ) was
obtained from a Barnstead Model 04741 Nanopure deionized water system
(Barnstead/Thermodyne, Dubuque, IA). Online data collection and processing were
accomplished using ChromPerfect (version 3.0) computer software (Justice Laboratory
Software, Denville, NJ).
92
3.6.2 Chemicals and materials
Fused-silica capillary (250 μm, i.d.) with a protective polyimide external coating
was purchased from Polymicro Technologies Inc. (Phoenix, AZ). HPLC-grade solvents
(methylene chloride, methanol, and tetrahydrofuran (THF)) were purchased from Fisher
Scientific (Pittsburgh, PA). Fluorene, phenanthrene, naphthalene, pyrene, chrysene,
butyrophenone, heptanophenone, hexanophenone, decanophenone, trans-chalcone,
nonanal, decanal, undecanal, dodecanal, tridecanal and trifluoroacetic acid (TFA, 99%)
were purchased from Acros Organics (Pittsburgh, PA). Hydroxy-terminated
polydimethylsiloxane (PDMS) and hydroxy-terminated polydimethyldiphenylsiloxane
(PDMDPS) were purchased from United Chemical Technologies Inc. (Bristol, PA).
Tetramethoxygermane (TMOG) and 3-aminopropyltrimethoxysilane (APTMS) were
obtained from Gelest, Inc (Morrisville, PA).
3.6.3 Preparation of sol-gel germania-PDMS coated capillaries for CME
The coating sol solution was prepared in a clean polypropylene centrifuge tube
using 100 mg of hydroxy-terminated PDMS, 20 μL of TMOG, and 30μL of TFA (with
5% of water) in 100 μL of methylene chloride. The mixture was then vortexed for 3 min
followed by centrifugation for 5 min to remove possible precipitates. The clean
supernatant was transferred to another clean vial, discarding the precipitate. A
hydrothermally pretreated [79] fused silica capillary (1 m) was filled with the clear sol
solution using pressurized helium (50 psi) in a filling/purging device. The sol solution
was allowed to stay inside the capillary for 15 min to facilitate the formation of a sol-gel
coating. During this in-capillary residence period, a 3-dimensional sol-gel network was
93
formed in the sol solution due to hydrolytic polycondensation reactions. In this process,
patches of the sol-gel network evolving in the vicinity of the capillary inner surface had
the favorable conditions to get chemically bonded to the capillary walls due to
condensation reactions between the sol-gel active groups on the sol-gel network
fragments and the silanol groups on the capillary surface. Following this, the unbonded
portion of the sol solution in the central part of the capillary was expelled under helium
pressure (50 psi) and the coated surface was purged with helium (50 psi) for an hour,
leaving behind a dry surface-bonded sol-gel coating on the capillary inner walls. The
capillary was further conditioned under helium purge by programming the temperature
from 40 ºC to 300 ºC at 1 ºC/min and held at the final temperature for 5 hours. The
sol-gel coated microextraction capillary was then rinsed with a 1:1 (v/v) mixture of
methylene chloride and methanol (2 mL), and then the capillary was conditioned again
from 40 ºC - 300 ºC at 5 ºC/min. The conditioned capillary was then cut into 10-cm long
pieces that were ready for use in capillary microextraction. Sol-gel germania-PDMDPS
and sol-gel germania-APTMS coated capillaries were prepared in an analogous way by
replacing hydroxyl-terminated PDMS in the sol solution either by hydroxyl-terminated
PDMDPS or APTMS.
A sol-gel germania-PDMS coated GC column was prepared following a similar
procedure carried out within a 5-m piece of hydrothermally treated fused silica capillary
using a more dilute coating sol solution containing PDMS (200 mg), TEOG (60 μL), TFA
(50 μL containing 5% H2O), and CH2Cl2 (600 μL).
94
3.6.4 Preparation of silica-based sol-gel PDMS columns for GC analysis
The extracted analytes were separated by GC using a silica-based sol-gel capillary
column that was also prepared in-house following a modified version of the original
procedure described by us elsewhere [63]. Instead of using 100 μL each of methylene
chloride and TFA (with 5% of water) [63], in the present work we used 300 μL methylene
chloride and 200 μL TFA (with 5% of water) to prepare the sol solutions used in coating
the GC columns.
3.6.5 Preparation of aqueous samples for CME-GC
Sample solutions were prepared in several stages. Solute chemical standards were
first dissolved in methanol or THF to obtain the stock solutions (10 mg/mL), which was
further diluted to 0.1 mg/mL in the same solvent. The final aqueous solution was
prepared by additionally diluting this solution in water to achieve ng/mL level
concentrations. The aqueous solutions were freshly prepared prior to performing the
microextraction experiments. The used glassware was properly deactivated to avoid
analyte loss due to adsorption on the glassware surface [57].
3.6.6 Gravity-fed sample dispenser for capillary microextraction
An in-lab designed gravity-fed sample dispenser [57] was used for capillary
microextraction. It offered a consistent means to extract analytes by passing the liquid
sample through the microextraction capillary under gravity, as well as a convenient way
to clean the capillary via rising and purging. It was built by modifying a Chromaflex AQ
column (Kontes Glass Co., Vineland, NJ) consisting of a thick-walled Pyrex glass
cylinder concentrically placed in an acrylic jacket. The inner surface of the glass cylinder
95
was deactivated by treating with HMDS solution as described previously [57].
3.6.7 Sol-gel capillary microextraction.
A 10 cm long segment of previously conditioned hybrid sol-gel germania coated
fused silica capillary (250 μm i.d.) was vertically connected to the bottom end of an
empty sample dispenser. A 50 mL volume of the aqueous sample solution was then
poured into the dispenser from the top. The sample solution was allowed to pass through
the microextraction capillary under gravity. During this process, the analyte molecules
were extracted by the sol-gel germania hybrid coating residing on the inner walls of the
capillary. With the progression of this microextraction process, concentration equilibrium
was established between the sol-gel sorbent coating and the sample solution (typically
within 30-40 min). The capillary was then removed from the sample dispenser, briefly
purged with helium, and connected to the inlet end of the GC column using a two-way
press-fit connector. The free end of the microextraction capillary was introduced into the
GC injection port (from the bottom end of the port previously cooled down to 30 ºC) so
that ~8 cm of the capillary resided inside the quartz liner in the injection port. A graphite
ferrule was used to secure a leak-free connection between the capillary and the lower end
of injection port. The extracted analytes were then thermally desorbed from the
capillary in the splitless injection mode by rapidly raising the temperature of the injector
(from 30ºC to 300ºC in five min) and the desorbed analytes were transported to the GC
column by the helium flow through the injector. The desorbed analytes were focused at
the inlet of the GC column held at 30 ºC. Analyte desorption and focusing was performed
over a 5 min period. The GC column temperature was then raised @ 20 ºC/min to
96
facilitate transportation of the focused solutes through the column providing their GC
separation. A flame ionization detector (FID), maintained at 350 ºC, was used for analyte
detection.
3.6.8 Safety precautions
The presented work involved the use of various chemicals (organic and inorganic)
that may be environmentally hazardous with adverse health effects. In accordance with
material safety data sheet (MSDS) proper safety measures were taken in handling strong
acids, bases and organic solvents such as methanol, methylene chloride, and acetonitrile.
All used chemicals were disposed in the proper waste container to ensure personnel and
environmental safety.
3.7 Results and discussion
3.7.1 Chemical reactions involved in the preparation of sol-gel germania coatings
Sol-gel chemistry provides a powerful tool for the design and synthesis of novel
materials in desirable formats including thin films, monolithic beds, spherical particles,
etc. It provides the ease and flexibility in synthesis thanks to mild reaction conditions.
In general, a sol-gel process involves chemical reactions that convert simple sol-gel
active precursor molecules into an intermediate colloid state called the sol and then into a
three dimensional polymeric network structure (with solvent-filled pores) called the gel.
The sol-gel active components in the sol solution include one or more sol-gel precursors
(typically alkoxides), various hydrolyzed forms of the precursors and certain hydroxy- or
alkoxy-terminated polymers. Two key chemical reactions take place in the sol-gel process:
97
(a) hydrolysis of the sol-gel precursor, and (b) polycondensation of the hydrolyzed
precursors among themselves and/or with other sol-gel active components of the sol
solution. Besides silica-based precursors, metal alkoxide–based sol-gel precursors are
often used together with other sol-gel active components in the coating solution to create
a wide variety of material systems with desired properties. Zirconia-, titania- and
alumina- based sol-gel materials have already been prepared, and their various appealing
properties (e.g., excellent pH stability) attracted considerable interest [72]. Since GeO2 is
well established as an isostructural analog of SiO2 [80] and is compatible with the silica
network, it presents great potential in materials synthesis. In this work, our aim was to
use germanium alkoxide precursors in sol-gel reactions to create germania-based
surface-bonded organic-inorganic hybrid coatings within fused silica capillaries and
evaluate their performances as CME sorbents and GC stationary phases. Table 3.1 lists
the chemical ingredients used in the sol solution to in situ create germania-based sol-gel
hybrid organic-inorganic coatings within fused silica capillaries for use either in
solvent-free microextraction or in GC separations.
98
Table 3.1 Chemical ingredients of the coating solution used in preparing sol-gel germania
organic-inorganic hybrid coatings.
Chemical Name Function Structure
Tetramethoxygermane (TMOG)
Hydroxy-terminated
polydimethylsiloxane (PDMS)
Hydroxy-terminated
polydimethyldiphenylsiloxane
(PDMDPS)
3-Aminopropyltrimethoxysilane
(APTMS)
Methylene chloride
Trifluoroacetic acid (99%)
Water
Sol-gel
precursor
Sol-gel active
polymer
Sol-gel active
polymer
Sol-gel active
polymer
Solvent
Catalyst
Hydrolysis
reagent
Ge
OCH3
OCH3
OCH3
CH3O
Si
CH3
H
CH3
HO
n
O
Si
CH3
CH3
HO O Si
CH3
CH3
O Si O
x
Si
OCH3
OCH3
H2NCH2CH2CH2 OCH
CH2Cl2
CF3COOH
H2O
99
Here, tetramethoxygermane served as the sol-gel precursor and generated the
inorganic component of the resulting hybrid sol-gel organic-inorganic coating. The
hydroxy-terminated PDMS or PDMDPS served as the source of the organic component
of the hybrid coating, while APTMS played a dual role: it served as a sol-gel precursor
and also as the source of the organic (aminopropyl) ligand. An appropriate choice from
these sol-gel-active organic ligands was made for use in the sol solution to create a
sol-gel germania coating with the desired organic ligand. For example,
hydroxy-terminated PDMS was incorporated in the sol solution to create a sol-gel
germania PDMS coating. A general symbol ―Y‖ has been used in the reaction schemes
(3.2-3.3) to denote the residuals of these ligands incorporated into the sol-gel structure.
The following illustrates the creation of sol-gel hybrid germania coatings using
sol-gel GeO2-PDMS as an example, and involves the following chemical processes: (1)
hydrolysis of the alkoxide precursor (TMOG) (Scheme 3.2), (2) polycondensation of the
partially and/or fully hydrolyzed precursor molecules among themselves and with other
sol-gel active components of the solution (Scheme 3.3), (3) chemical bonding of PDMS
to the evolving sol-gel germania network, and (4) chemical anchoring of the evolving
hybrid organic-inorganic polymer to the inner walls of the capillary (Scheme 3.4).
Ge
OCH3
OCH3
OCH3
CH3O + 4H2O GeCH3O4-n
OHn
+ nCH3OH
TMOG
where : n=0,1,2,3 or 4
Scheme 3.2 Hydrolysis of germania precursor (TMOG)
100
+ +GeCH3O4-n
OHn
GeCH3O4-m
OHm Y
Where : Y =condensation residues from PDMS, PDMDPS or APTMS
Ge O
O
HO Ge
O
O
O q
O Ge
O
O
OGe
O
O
Y
OGe
O
Y
O
O Ge
O
OGe
O
O
YO
O
Y
Y
Scheme 3.3 Polycondensation of the hydrolyzed precursor and chemical bonding of the
sol-gel-active organic ligand (Y) to the evolving sol-gel network
101
Scheme 3.4 Chemical anchoring of the evolving sol-gel germania hybrid
organic-inorganic polymer to the inner walls of a fused silica capillary
102
The hydrolysis (partial or complete) of TMOG produces a variety of reactive
hydroxyl-containing species capable of undergoing condensation reactions with
neighboring sol-gel active chemical species in the sol solution including TMOG, APTMS
and its hydrolyzed forms, hydroxy-terminated polymers (PDMS and PDMDPS).
Condensation of these reactive species leads to a hybrid organic-inorganic material via
chemical integration of an inorganic component (germania, originating from TMOG) and
an organic residual (―Y‖, originating from one of the above mentioned sources of organic
ligands). The silanol groups residing on the inner walls of the fused silica capillary are
also sol-gel active, and can take part in the condensation reactions resulting in chemical
anchoring of the portion of the sol-gel material evolving in the vicinity of the capillary
walls. This surface bonded portion of the sol-gel material is left on the column surface in
the form of an organic-inorganic hybrid coating and serves as the sorbent for solvent-free
microextraction.
Sol-gel process is complex and proper control of the processing parameters (e.g.,
drying temperature, pressure, pH, precursor/water molar ratio, solvent, and the structure
of alkoxy groups, among others) is necessary during various stages of the process
(polymerization, gelation, drying) to prepare sol-gel materials with desired structural
characteristics [81, 82]. In this work, experimental conditions were carefully optimized to
obtain the germania-based hybrid gel within 2 hours. This was accomplished through
variation of relative proportions of various components of the solution. After coating and
thermal conditioning, the capillary was rinsed with a mixture of methylene chloride and
methanol to get rid of unreacted chemical species physically adhering to the coating.
103
3.7.2 IR Characterization
Figure 3.1 represents two FTIR spectra for: (A) pure PDMS, (B) sol-gel
Germania-PDMS Coating. According to literature data [83], Ge-O-Si vibration provides a
characteristic IR stretch at around 1000 cm-1
[84], which is determined by the vibrational
modes associated with O in Ge-O-Si. Based on FTIR measurements in germania-doped
SiO2 film sample, Mei et al. [83] assigned the peak at 987 cm-1
to the Ge-O-Si vibration.
However, they also noted that a shift in the position of this characteristic peak is possible
due to a difference in the proportions of Ge, Si, and O in the sample [85]. The FTIR
spectrum for our sol-gel germania-PDMS (Figure 3.1-B) coating contains a peak at
960.43 cm-1
. This peak is absent in the spectrum for pure PDMS (Figure 3.1-A) and, and
we attribute this to successful creation of sol-gel germania-PDMS hybrid material
containing Ge-O-Si bonds.
3.8 CME profile for sol-gel germania coated capillaries
In capillary microextraction, it is desirable that the extraction equilibrium is
reached in the shortest possible time. In this work, we investigated the relationship
between extraction time and the amount of analytes extracted in CME with sol-gel
germania-PDMS coating (Figure 3.2).
As can be seen in Figure 3.2, for both heptanophenone (a moderately polar solute)
and pyrene (a nonpolar analytes), the extraction equilibrium was reached within 40 min.
Based on this observation, 40 min extraction time was used in CME experiments
presented in this paper.
104
Figure 3.1-A FTIR spectra representing: pure PDMS
Figure 3.1-B FTIR spectra representing: sol-gel germania PDMS hybrid material used as
a sorbent in capillary microextraction
105
0
20000
40000
60000
80000
100000
120000
10 30 50 70
Extraction time (min.)
Peak a
rea (
arb
itra
ry u
nit
s)
pyrene
hepatanophenone
Figure 3.2 Capillary microextraction profiles of pyrene and heptanophenone on a sol-gel
germania coated capillary. Extraction conditions: 10 cm x 0.25 mm i.d. microextraction
capillary, gravity-fed sample dispenser for extraction at room temperature. Other
conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column; splitless
analyte desorption by rapidly increasing the GC injector temperature from 30 ºC to 300
ºC, 5 min; GC oven temperature programmed from 30 ºC to 300 ºC at a rate of 20 ºC/min;
helium carrier gas; FID 350 ºC
106
3.9 Extraction characteristics of sol-gel germania coatings in CME
By atomic structure, Ge and Si are closest neighbors located in Group 14 of the
periodical table, and they have similar properties. GeO2 is well known as the isostructural
analogue of SiO2 which makes it compatible with the silica network. In this work, we
prepared Germania-based sol-gel PDMS coatings and tested their performance in
capillary microextraction coupled to GC-FID. Our results suggest that sol-gel Germania
hybrid coatings have the ability to extract a wide range of analytes from aqueous media.
Such sol-gel coatings also provide outstanding solvent and pH stability which is difficult
to achieve on pure silica based extraction material.
3.9.1 Extraction of non-polar and moderately polar compounds by sol-gel
germania-PDMS coated capillary for CME-GC analysis
The consistency in the performance of sol-gel germania-PDMS capillaries was
characterized by the run-to-run and capillary-to-capillary RSD values for the extracted
amounts in CME-GC analysis. For this, aqueous samples of three different classes of
analytes (PAHs, aldehydes, and ketones) were used. GC peak areas obtained from the
extracted analytes were used as a measure of the extracted amounts in CME. As we can
see from Table 3.2, the peak area run-to-run R.S.D values for PAH samples were below
6%, and those for GC retention time were less than 0.33 %. For aldehydes, the run-to-run
peak area and GC retention time R.S.D values were under 6.5 % and 0.2 % respectively.
For ketones, the run-to-run peak area R.S.D value was under 7.2 % and that for retention
time was within 0.23. The detection limit value for different classes of analytes is shown
in Table 3.3.
107
Table 3.2 GC peak area and retention time repeatability data for PAHs, aldehydes, and
ketones extracted from aqueous samples by CME using a sol-gel germania-PDMS coated
microextraction capillary
Analyte
Peak area repeatability
(n=3)
tR Repeatability
(n=3)
Class Name
Mean peak
area
(aribitary
unit)
R.S.D. (%) Mean tR
(min)
R.S.D.
(%)
PAHs
Aldehyde
Ketones
Naphthalene
Fluorene
Phenanthrene
Pyrene
Nonanal
Decanal
Undecanal
Valerophenone
Heptanophenone
Hexanophenone
Decanophenone
5.2
19.9
28.2
15.7
1.5
3.3
2.5
3.2
11.0
11.8
10.1
2.84
4.82
5.05
2.33
4.91
6.46
1.32
2.69
3.97
7.18
2.38
7.07
9.75
10.94
12.87
6.31
7.08
7.80
8.40
9.02
9.12
9.79
0.12
0.33
0.17
0.26
0.18
0.01
0.21
0.16
0.23
0.13
0.17
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min extraction
time; Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS open tubular GC column;
splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5 min; GC
oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID
350 ºC.
108
Table 3.3 Detection limit data for PAHs, aldehydes, and ketones extracted from aqueous
samples by CME-GC using a sol-gel germania-PDMS coated microextraction capillary
Analyte Class Name Detection Limit,
(ng/L), (S/N =3)
PAHs
Aldehyde
Ketones
Naphthalene
Fluorene
Phenanthrene
Pyrene
Nonanal
Decanal
Undecanal
Valerophenone
Heptanophenone
Hexanophenone
Decanophenone
83.8
20.0
11.4
20.1
139.7
89.7
97.7
92.3
46.1
35.8
30.6
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min extraction
time; Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS open tubular GC column;
splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5 min; GC
oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID
350 ºC
109
Capillary-to-capillary reproducibility is another important characteristic which
shows the consistency of the method used to prepare the microextraction capillaries. In
this work, we used one ketone, one aldehyde and a polycyclic aromatic hydrocarbon as
test solutes for capillary-to-capillary reproducibility in CME performed on sol-gel
germania-PDMS coated microextraction capillaries (Table 3.4). For all the tested analytes,
the peak area R.S.D. values for the extracted amounts were below 6%, which means
germania-based sol-gel CME in conjunction with GC-FID shows good reproducibility,
and that the capillary preparation method possesses acceptable consistency.
Table 3.4 Capillary-to-capillary reproducibility (n=3) data for extracted amounts in
CME-GC experiments conducted on sol-gel germania-PDMS coated capillaries using a
PAH (phenanthrene), an aldehyde (undecanal), and a ketone (hexanophenone) as test
solutes
Analytes Mean peak area (arbitrary
unit)
R.S.D. (%)
Phenanthrene
Undecanal
Hexanophenone
27.9
3.5
13.2
5.4
5.5
5.9
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min extraction
time; Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS open tubular GC column;
splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5 min; GC
oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID
350 ºC
110
PAHs present potential health hazards because of their toxic, mutagenic, and
carcinogenic properties. A chromatogram of PAHs obtained in a CME-GC-FID
experiment using a sol-gel germania-PDMS coated CME capillary is presented in Figure
3.3. Parts per trillion (ppt) level detection limits were achieved. Mention should be made
here that, sol-gel coating technology can easily produce stable coating with different
thicknesses through manipulation of the in-capillary residence time of the sol solution
during preparation of the microextraction capillary. The use of microextraction capillaries
with thicker coatings should provide higher sensitivity in CME [57].
Aldehydes and ketones also present health and environmental concerns. Polar
nature of these carbonyl compounds often requires their derivatization prior to their
extraction or chromatographic analysis [86]. In CME-GC experiments, in which we used
sol-gel technology to prepare both the CME capillaries and GC columns, no
derivatization step was necessary either for extraction or for GC analysis [57, 87]. Sol-gel
germania-PDMS capillary provided highly efficient extraction for aldehydes and ketones.
As can be seen in Figure 3.4-A and 3.4-B, the GC peaks provided by the sol-gel column
are sharp and symmetrical, and this is indicative of effective focusing of the analytes at
the GC column inlet after their desorption form the CME capillary, as well as excellent
performance of the in-lab prepared sol-gel PDMS column used for GC separation. For
both solute types, parts per trillion level detection limits were achieved.
111
Figure 3.3 CME-GC trace analysis of a mixture of PAHs using a sol-gel germania-PDMS
coated microextraction capillary. Extraction conditions: 10 cm x 0.25 mm i.d.
microextraction capillary, extraction time, 40 min (Gravity fed at room temperature).
Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column;
splitless analyte desorption by rapidly increasing the GC injector temperature from 30 ºC
to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a rate of 20
ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Naphthalene (2) Chrysene (3) Fluorene
(4) Phenanthrene (5) Pyrene
112
Figure 3.4 -A Figure 3.4 -B
Figure 3.4 CME-GC trace analysis of mixture of aldehydes and ketones using a sol-gel
germania-PDMS coated microextraction capillary. Extraction Conditions: 10 cm x 0.25
mm i.d. microextraction capillary, extraction time, 40 min (gravity fed sample delivery at
room temperature). Other conditions: 5 m x 0.25 mm i.d. Sol-gel PDMS column; splitless
analyte desorption by rapidly increasing the GC injector temperature from 30 ºC to 300
ºC, 5 min; GC oven temperature programmed from 30 ºC to 300 ºC at a rate of 20 ºC/min;
helium carrier gas; FID 350 ºC; GC Peaks in 3.6 -A: (1) Nonanal (2) Decanal (3)
Undecanal (4) Dodecanal; GC Peaks in 3.6 -B: (1)Valerophenone (2) Heptanophenone (3)
Decanophenone (4) Anthraquinone
113
Being highly polar compounds, alcohols demonstrate higher affinity for water and
are usually difficult to extract from an aqueous matrix. In the present work, these highly
polar analytes were extracted from aqueous sample using sol-gel germania-APTMS
coated capillary (Figure 3.5) that did not require any analyte derivatization, pH
adjustment, or salting out effect. Our results demonstrate that sol-gel germania-APTMS
microextraction capillary has a pronounced affinity for these highly polar analytes.
3.9.2 pH stability of sol-gel germania-PDMS coated capillary
One serious shortcoming of conventional silica-based stationary phases and
sorbents is their poor pH stability under both acidic and basic conditions. In this work,
the pH stability of the prepared sol-gel germania coated capillaries was evaluated by
comparing the GC peak areas obtained in CME-GC-FID experiments carried out on
sol-gel germania-PDMS coated microextraction capillaries before and after prolonged
treatment of the coated capillary surface under extreme pH conditions.
Sol-gel germania-PDMS coating showed excellent pH stability. After coating and
conditioning, we conducted two sets of experiments in which we rinsed the sol-gel
germania-PDMS capillaries for 24 hours with (a) 0.05M HCl and (b) 0.1M NaOH
(pH=13). The results suggest that sol-gel germania-PDMS coating retain its excellent
extraction capability after being subjected to prolonged rinsing with a strong base (Figure
3.6) or a strong acid (Figure 3.7). For example, after continuously rinsing a sol-gel
germania-PDMS coated capillary for 24-hours with 0.1M NaOH, the GC peak areas for
extracted amounts of phenanthrene and nonanal were found to increase by 5.0 % and
7.1 %, respectively. Similarly, after a continuous 24-hours rinse of the microextraction
114
capillary with a 0.05 M HCl, the observed peak area changes for fluorene and
heptanophenone were +4.1 % and +3.3 %, respectively.
This data suggests that the new hybrid sol-gel germania coated capillary has
excellent pH stability. Rinsing with NaOH also led to improved extraction characteristics.
For most solutes, after rinsing the capillary with NaOH, the peak area increased. In some
instances, these increases were statistically significant and in other cases they were well
within experimental errors. A significant increase in the peak area for some solutes can be
explained by assuming that NaOH helps cleaning the sol-gel germania-PDMS coating by
removing from its pores and surfaces various sorbed species that hinder the extraction
process, and that getting rid of those sorbed species is often difficult via rinsing with
commonly used organic solvent.
3.9.3 Extraction of polar compounds by Sol-gel germania CME-GC
Chlorophenols are one of the major classes of environmental pollutants.
Analytical capability to preconcentrate traces of these compounds in aqueous samples,
therefore, presents significant interest in environmental science and technology. Figure
3.8 shows an example highlighting the ability of a sol-gel germania-PDMDPS coated
CME capillary to accomplish preconcentration of 2, 4, 6-triclorophenol—a typical
member of the chlorophenol family.
115
Figure 3.5 CME-GC trace analysis of alcohols using a sol-gel germania-APTMS coated
microextraction capillary. Extraction Conditions: 10 cm x 0.25 mm i.d. microextraction
capillary, extraction time, 40 min (Gravity fed at room temperature). Other conditions: 5
m x 0.25 mm i.d. Sol-gel PDMS GC capillary column; splitless analyte desorption by
rapidly increasing the GC injector temperature from 30 ºC to 300 ºC, 5 min; GC oven
temperature programmed from 30 ºC to 300 ºC at rate of 20 ºC/min; helium carrier gas;
FID 350 ºC. GC peaks: (1) decanol (2) undecanol
116
Figure 3.6-A Figure 3.6-B
Figure 3.6. Excellent stability of sol-gel germania-PDMS coating under highly basic
conditions demonstrated through CME-GC analysis of a mixture of PAHs using a sol-gel
germania-PDMS coated microextraction capillary before (3.6-A) and after (3.6-B)
continuously rinsing the capillary with a 0.1 M NaOH solution (pH = 13) for 24h.
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, extraction time, 30
min (Gravity fed at room temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel
PDMS GC capillary column; splitless analyte desorption by rapidly increasing the GC
injector temperature from 30 ºC to 300 ºC, 5 min; GC oven temperature rose from 30 to
300 ºC at rate of 20 ºC/min; helium carrier gas; FID 350 ºC. GC peaks: (1) naphthalene,
(2) fluorene, (3) phenanthrene, (4) pyrene
117
Figure 3.7-A Figure 3.7-B
Figure 3.7 Excellent stability of sol-gel germania-PDMS coating under highly acidic
conditions demonstrated through CME-GC analysis of aldehydes using a
germania-PDMS coated microextraction capillary before (3.7-A) and after (3.7-B)
continuously rinsing the capillary with a 0.05 M HCl solution for 24 h. Extraction
conditions: 10 cm x 0.25 mm i.d. microextraction capillary, extraction time, 40 min
(gravity-fed at room temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS
coated GC capillary column; splitless desorption splitless analyte desorption by rapidly
increasing the GC injector temperature from 30 ºC to 300 ºC, 5 min; GC oven
temperature programmed from 30 ºC to 300 ºC at a rate of 20 ºC/min; helium carrier gas;
FID 350 ºC. GC peaks: (1) nonanal, (2) decanal (3) undecanal (4) dodecanal
118
Figure 3.8 CME-GC trace analysis of 2, 4, 6 Trichlorophenol using a sol-gel
germania-PDMDPS coated microextraction capillary. Extraction Conditions: 10 cm x
0.25 mm i.d. microextraction capillary, extraction time, 40 min (gravity fed at room
temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary
column; splitless analyte desorption from the microextraction capillary by rapidly
increasing the GC injector temperature from 30 ºC to 300 ºC, 5 min; GC oven
temperature programmed from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID
350 ºC. GC peak: (1) 2, 4, 6 Trichlorophenol
119
In this example, a detection limit of 49.2 ppt was achieved for this chlorophenol
in CME-GC-FID. Extraction of free fatty acids and their analysis by GC are often met
with serious analytical difficulties which can be explained by the presence of highly polar
carboxyl group in the structure of these molecules and by the propensity of these
molecules toward adsorption. To overcome these difficulties, analytical chemists often
derivatize these molecules to methyl esters and analyze the generated esters. However,
quantitative derivatization of fatty acids especially when they are present in trace
concentrations may be problematic. Figure 3.9 presents a gas chromatogram highlighting
the possibility of extracting traces of free fatty acids from an aqueous medium using a
sol-gel germania-APTMS coated CME capillary and direct GC analysis of free fatty acids
using an in-lab prepared sol-gel PDMS open tubular GC column.
3.9.4 GC separation of sol-gel germania-PDMS coated column
Gas chromatographic performances of a sol-gel germania-PDMS coated columns
was investigated using a natural gas sample. Figure 3.10 represents a chromatogram
illustrating the possibility of analyzing a natural gas sample on a sol-gel germania-PDMS
coated open tubular GC column. The symmetrical and sharp peaks indicate the potential
of sol-gel germania-PDMS coating for use as a stationary phase in GC, especially for
low-boiling analytes.
120
Figure 3.9 CME-GC analysis of free fatty acids using a sol-gel germania-APTMS coated
microextraction capillary. Extraction Conditions: 10 cm x 0.25 mm i.d. microextraction
capillary, extraction time, 40 min (gravity fed at room temperature). Other conditions: 5
m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column; splitless analyte desorption
from the microextraction capillary by rapidly increasing the GC injector temperature
from 30 ºC to 300 ºC, 5 min; GC oven temperature programmed from 30 ºC to 300 ºC at
rate of 20 ºC/min; helium carrier gas; FID 350 ºC. GC peaks: (1) decanoic acid, (2)
undecanoic acid, and (3) dodecanoic acid
121
Figure 3.10 Gas chromatogram of a natural gas sample separated on sol-gel
germania-PDMS stationary phase. Conditions: 5 m x 0.25 mm i.d. sol-gel
germania-PDMS column; carrier gas, helium; injection, split (100:1, 300 ºC); FID 350 ºC,
constant column temperature, 30 ºC. GC peaks: (1) methane and (2) ethane
122
3.10 Conclusion
Germania-based sol-gel organic-inorganic hybrid coatings were developed for
high-performance capillary microextraction. Run-to-run and capillary-to-capillary
reproducibility (peak area RSD) were below 6.5 % and 7.2 %, respectively. The
developed method can be applied to chemically bind non-polar (e.g., PDMS), moderately
polar (e.g., PDMDPS) and highly polar (e.g., APTMS) ligands to the sol-gel germania
network in the course of its evolution from the sol solution. The sol-gel germania coated
capillaries are effective in extracting broad classes of analytes ranging from non-polar
PAHs to polar compounds such as alcohols and fatty acids. Parts per trillion level
detection limits were achieved in CME-GC-FID using sol-gel germania hybrid coatings
for microextraction. Sol-gel germania-PDMS coating also showed promising
chromatographic performance as a GC stationary phase. To our knowledge, this is the
first report on the use of sol-gel germania hybrid coatings as sorbents in microextraction
or as a GC stationary phase.
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128
CHAPTER FOUR
MIXED GERMANIA-SILICA SOL-GEL
CYANOPROPYL-POLY(DIMETHYLSILOXANE) COATING FOR CAPILLARY
MICROEXTRACTION OF POLAR ORGANIC TRACE CONTAMINANTS IN
AQUEOUS MEDIA
4.1 Introduction
Solid-phase microextraction (SPME) [1] is an effective approach to sampling and
sample preparation for both laboratory and field analyses. Developed in 1989 by Belardi
and pawliszyn [2], SPME integrates sampling, sample preparation, analyte preparation,
and sample introduction step to the analytical cycle providing a simple and efficient
solvent-free method for the extraction and preconcentration of analytes from various
sample matrices. A number of shortcomings inherent in traditional fiber SPME stem from
design and physical construction of the fiber and syringe like SPME device. As one of the
variants of SPME which has been proposed to overcome these difficulties [3-7], in-tube
SPME also called capillary microextraction, CME is promising, especially for effective
hyphenation with liquid-phase separation techniques [8-10].
The sorptive coating is the most important component fiber for both conventional
SPME and in-tube SPME, and its nature determines the fiber selectivity toward different
compound classes. Polydimethylsiloxane (PDMS) phase was one of the first polymers
used in SPME [11] over the years. SPME has served as the most commonly used
non-polar coating [12-13]. Other polymeric coatings such as polyacrylate (PA) and
129
carbowax (CW) have been used to extract polar compounds for SPME. However, all
these commercially available coatings present important drawbacks such as their
relatively low thermal stability (200 ºC - 270 ºC) and low organic solvent stability. The
lack of chemical bonding between the coating material and the substrate greatly
restrained the hyphenation of SPME with HPLC.
Sol-gel coating technology has been shown to overcome these problems by
synthesizing chemically bonded microextraction sorbents with different functional groups
[14-16]. Sol-gel chemistry offers an effective methodology to chemical incorporation of
organic components into evolving inorganic polymeric structures under extraordinarily
mild thermal conditions; therefore it provides a simple and efficient pathway to the
synthesis of advanced materials for use as extraction sorbents. An important aspect
direction of this coating technology is that the selectivity of a sol–gel coating can be
easily fine tuned by changing the composition of the used sol solution. Sol-gel chemistry
was applied to prepare a number of coatings including polyethylene glycol (PEG) [17],
hydroxyfullerene [18], and Crown ether [13] that provided extraction selectivity in SPME
for both polar and non-polar compounds.
Silica-based stationary phases and sorbents are dominantly used in modern
separation and sample preparation techniques. Silica surface chemistry and surface
modification process are well understood. However, the limitations of siliceous materials
are also well known. Under acidic pH conditions, coatings containing of siloxane bonds
(Si-O-Si) become unstable [19-20]. On the other hand, under a pH value of higher than 8,
siloxane bond on the silica surface hydrolyzes [21] and it happens rapidly under elevated
130
temperatures [22]. A number of methods such as multiple covalent bonds [23],
multi-dentate synthetic approach [24], and end-capping [25] have been used to solve this
problem. None of them, however, seems to work effectively. Therefore, developing
sorbents with a wide range of pH stability is a fundamentally important area of research
in chromatographic separation and sample preparation. Metal oxides such as zirconia,
titania, and alumina are alternative materials to offer better chemical and thermal
stabilities [22], and have attracted attention of separation scientists. The resultant metal
oxide-based sol-gel stationary phases have been applied in separation science due to their
superior pH stability and mechanical strength [22]. Besides, in recent years new research
accomplishments have been made in the development and application of metal
oxide-based sol-gel extraction sorbents. Titania- [26-28], zirconia- [29-30] and
alumina-based [31] sol-gel sorbents have been synthesized and applied in analytical
microextraction. Results revealed that those materials offer better thermal and pH
stability compared to silica-based systems [26, 27, 29, 31].
Our work focuses on developing organic-inorganic hybrid sol-gel coatings and
monolithic beds [32-38] for various substrates such as capillary, fiber or particle surface
to accomplish enhanced performance in analytical microextraction and separation. In past
several years, we worked on titania- and zirconia-based sol-gel systems that demonstrated
high chemical and pH stabilities in applications involving CME in hyphenation with both
GC and HPLC [26, 29]. As an isostructural analogue of SiO2 [39], GeO2 is compatible
with the silica network and they have similar properties. Tetramthyoxygermane (TMOG)
[39], tetraethyoxygermane (TEOG) [41], tetrapropyloxygermane (TPOG) [42],
131
germanium iso-propoxide (TP-i-OG) [41, 43], and ClGeCH2CH2COOH [44] etc have
been reported to introduce prepare mixed GeO2-SiO2 thin films [42, 43], monolithic
aerogel [40] and glasses [41] by sol-gel technology. From a structural point of view, each
Ge atom could be substitutional to a Si and be bonded to four O atoms, or it could be
under-coordinated giving rise to oxygen deficient centers (GODC) [45]. Those properties
attracted considerable attention on its optical properties and optoelectronic applications
[46-48]. Our preliminary research [32] has suggested that sol-gel germania hybrid
coatings not only have the ability to extract a wide range of analytes from aqueous media
but also showed outstanding solvent and pH stability which is difficult to achieve on pure
silica based extraction material. Because of high affinity of water for highly polar
compounds, direct extraction of such analytes is difficult. Solution of this problem
requires a highly polar extracting phase that can compete with water in term of polarity
and provide a favorable environment for the transfer of highly polar analytes from the
aqueous medium into the extracting phase. Cyanopropyl-based stationary phases provide
extremely high chromatographic polarity [49]. Cyanopropylsiloxanes exhibit both polar
and polarizable characteristics which are responsible for increased affinity of these phases
for alcohols, ketones, esters, and analytes bearing π-electrons. Cyano stationary phases
have been used in GC and HPLC [50-52], capillary electrochromatography (CEC) [53]
and as an extraction medium in solid-phase extraction (SPE) [54-56]. Sol–gel CN-PDMS
coating have been reported in our previous work [38]. The prepared cyano-based sol-gel
coating provided highly stable performance at elevated temperatures compared with
conventionally prepared cyano coatings (having no chemical bonds to the substrate).
132
Here we report on the development of cyanopropyl-based sol-gel germania hybrid
coatings for the extraction of highly polar analytes from aqueous media.
4.2 Experimental
4.2.1 Equipment
Sol-gel germania-based capillary microextraction-gas chromatography (CME-GC)
experiments were carried out on a Shimadzu Model 14A GC system equipped with a
flame ionization detector (FID) and a split/splitless injector. An in-lab designed liquid
sample dispenser was used to perform capillary microextraction via gravity flow of
aqueous samples through the sol-gel microextraction capillary connected at the bottom
end of the dispenser. A Fisher Model G-560 Genie 2 vortex (Fisher Scientific, Pittsburgh,
PA) was used for thorough mixing of sol solution ingredients. A Micromax model 3590F
microcentrifuge (Thermo IEC, Needham Heights, MA) was used for centrifugation at
14,000 rpm (15,682 x g) to separate out the precipitate from the sol solution. A
home-built gas pressure-operated filling/purging device was used to perform a number of
operations: (a) filling the extraction capillary with the sol solution, (b) expelling the
solution from the capillary after predetermined period of in-capillary residence, (c)
rinsing the capillary with a suitable solvent to clean the coated surface, and (d) purging
the microextraction capillary with helium. Ultra pure water (17.0 MΩ) was obtained from
a Barnstead Model 04741 Nanopure deionized water system (Barnstead/Thermodyne,
Dubuque, IA). Online data collection and processing were accomplished using
ChromPerfect (version 3.0) computer software (Justice Laboratory Software, Denville,
133
NJ).
4.2.2 Chemicals and materials
Fused-silica capillary (250 μm, i.d.) with a protective polyimide external coating was
purchased from Polymicro Technologies Inc. (Phoenix, AZ). HPLC-grade solvents
(methanol, and tetrahydrofuran (THF)), were purchased from Fisher Scientific
(Pittsburgh, PA). Valerophenone heptanophenone, hexanophenone, decanoic acid,
undecanoic acid, dodecanoic acid nonanal, decanal, undecanal, 2,4,6-trichlorophenol and
Pentachlorophenol, 1,1,1,3,3,3-hexamethyldisilazane, and formic acid (FA, 95%) were
purchased from Aldrich (Milwaukee, WI). Hydroxy-terminated polydimethylsiloxane
(PDMS) was purchased from United Chemical Technologies Inc. (Bristol, PA).
Tetraethoxygermane (TEOG), 3-cyanopropyltriethoxysilane (CPTES) were obtained
from Gelest, Inc (Morrisville, PA).
4.2.3 Preparation of sol-gel germanis-CN/PDMS-coated capillaries for CME
The coating sol solution was prepared in a clean polypropylene centrifuge tube
using 100 μL CPTES, 5 mg of hydroxy-terminated PDMS, 15 μL of TEOG, and 65 μL of
TFA (with 38 % of water) and 50 μL of methanol. The mixture was then vortexed for 5
min followed by centrifugation for 4 min to remove possible precipitates. The clean
supernatant was transferred gently to another clean vial. A hydrothermally pretreated
fused silica capillary (1 m) was filled with the clear sol solution using pressurized helium
(40 psi) in a filling/purging device. The sol solution was allowed to stay inside the
capillary for 40 min to facilitate the formation of a sol-gel coating. During this
in-capillary residence period, a 3-dimensional sol-gel network was formed in the sol
134
solution due to hydrolytic polycondensation reactions. During this process, patches of the
sol-gel network evolving in the vicinity of the capillary inner surface got chemically
bonded to the capillary walls due to condensation reactions between the sol-gel active
groups on the sol-gel network fragments and the silanol groups on the surface of fused
silica capillary. Following this, the capillary was purged with helium (50 psi) for an hour
to expel the unbonded portion of the sol solution leaving behind a dry surface-bonded
sol-gel coating on the capillary inner walls. The capillary was further conditioned under
helium purge by programming the temperature from 40 ºC to 250 ºC at 1 ºC/min and held
at the final temperature for 4 hours. The sol-gel coated microextraction capillary was then
rinsed with a 1:1 (v/v) mixture of methylene chloride and methanol (2 mL), and then the
capillary was conditioned again from 40 ºC - 250 ºC at 5 ºC/min, from 250 ºC to 300 ºC
at 1 ºC/min and held at final temperature for 1 hour. The conditioned capillary was then
cut into 10-cm long pieces that were ready for use in capillary microextraction.
4.2.4 Preparation of aqueous samples for CME-GC
Solute standards were first dissolved in methanol or THF to obtain the stock
solutions (10 mg/mL), which was further diluted to 0.1 mg/mL in the same solvent and
stored in the freezer in amber vials. The fresh aqueous sample was prepared daily by
further diluting this solution with DI water. All the glassware used was deactivated by
treating with HMDS solution to avoid analyte loss due to adsorption on the glassware
surface.
135
4.2.5 Sol-gel capillary microextraction by an in-lab gravity-fed sample dispenser
An in-lab designed gravity-fed sample dispenser was used for capillary
microextraction. It was constructed by modifying a Chromaflex AQ column (Kontes
Glass Co., Vineland, NJ) consisting of a thick-walled Pyrex glass cylinder concentrically
placed in an acrylic jacket. The inner surface of the glass cylinder was deactivated as
described previously [57]. In-lab dispenser provided a consistent means to pass the liquid
through the microextraction capillary under gravity. Therefore, it provides a convenient
means for (1) the flow-through microextraction to take place and (2) cleaning or rinsing
the inside of the capillary.
A 10 cm long segment of previously conditioned cyanopropyl-based sol-gel
germania coated fused silica capillary (250 μm i.d.) was vertically connected to the
bottom end of an empty sample dispenser. A 50 mL volume of the aqueous sample
solution was then filled into the dispenser, and the sample solution was allowed to flow
through the microextraction capillary under gravity. During this flow-through process, the
analyte molecules were extracted by the sol-gel germania hybrid coating residing on the
inner walls of the capillary. With the progression of this microextraction process, an
equilibrium was established between the sol-gel sorbent coating and the sample solution
(typically within 30-40 min). The capillary was then removed from the sample dispenser,
briefly purged with helium, and connected to the inlet end of the GC column using a
two-way press-fit connector. The free end of the microextraction capillary was introduced
into the GC injection port (from the bottom end of the port previously cooled down to 30
ºC) so that ~8 cm of the capillary resided inside the quartz liner in the injection port. A
136
graphite ferrule was used to secure a leak-free connection between the capillary and the
lower end of injection port. The extracted analytes were then thermally desorbed from the
capillary in the splitless injection mode by rapidly raising the temperature of the injector
(from 30ºC to 300ºC in five min) and the desorbed analytes were transported to the GC
column by the helium flow through the injector. The desorbed analytes were focused at
the inlet of the GC column held at 30 ºC. Analyte desorption and focusing was performed
over a 5 min period. The GC column temperature was then raised @ 20 ºC/min to
facilitate transportation of the focused solutes through the column providing their GC
separation. A flame ionization detector (FID), maintained at 350 ºC, was used for analyte
detection. The PDMS-based sol-gel GC silica-based sol-gel capillary column was also
prepared in-house following the procedure described by us elsewhere [32].
4.3 Results and discussion
4.3.1 Chemical reactions involved in the preparation of sol-gel Germania-CN/PDMS
coatings
Table 4.1 lists the chemical ingredients used in the sol solution to in situ create
germania-based sol-gel hybrid organic-inorganic coatings within fused silica capillaries
for use in solvent-free microextraction. All the chemicals presented in table 4.1 except
catalyst were dispersed and thoroughly mixed in solvent(s) first, and the catalyst was
introduced at the end to initiate the sol-gel process. Sol-gel process consists of two main
reactions: (1) hydrolysis of the precursor(s) and (2) polycondensation of hydrolyzed the
sol-gel-active species present in the sol solution. Silica-base sol gel system is the most
widely used, however, analogous non-silica material such as those based on zirconia-,
137
titania- and alumina have already been prepared [22, 26, 29, 58]. Since GeO2 is well
established as an isostructural analog of SiO2 it is compatible with the silica network [39].
Our previous work [32], germanium alkoxide precursor has been successfully used in
conjunction with hydroxyl-terminated polysiloxanes to prepare Germania-based sol-gel
extraction for CME as well as Ge stationary phase.
TEOG served as the sol-gel precursor and generated the inorganic component of
the resulting hybrid sol-gel organic-inorganic coating. The hydroxy-terminated PDMS,
CPTES served as the source of the organic component of the hybrid coating. CPTES
played a dual role: it served as a sol-gel co-precursor and also served as the source of the
organic (cyanopropyl) ligands providing the cyano functional group to the system.
Because of the high thermal stability and good film forming performance of PDMS, a
small amount hydroxy-terminated PDMS was incorporated in the sol solution to help for
the formation of stable the sol-gel coating.
138
Table 4.1 Chemical ingredients of the coating solution used in preparing sol-gel germania
organic-inorganic hybrid coatings chemically anchored to the inner surface of a fused
silica capillary
Ingredient Function Chemical structure
Tetraethoxygermane (TEOG)
Hydroxy-terminated
polydimethylsiloxane
(PDMS)
3-Cyanopropyltriethoxysilane
(CPTES)
Methanol
Formic acid (96%)
Water
Sol-gel
precursor
Sol-gel
active
polymer
Sol-gel
active
polymer
Solvent
Catalyst
Hydrolysis
reagent
Ge
OCH2CH3
OCH2CH3
OCH2CH3
CH3CH2O
Si
CH3
H
CH3
HO
n
O
Si
OC2H5
OC2H5
CNCH2CH2CH2 OC2H5
CH3OH
HCOOH
H2O
139
The creation of sol-gel hybrid germania coatings involved the following chemical
processes: (1) hydrolysis of the alkoxide-based precursors (Scheme 4.1), (2)
polycondensation of the partially and/or fully hydrolyzed precursors among themselves
and with other sol-gel active components of the solution (Scheme 4.2), (3) chemical
bonding of hydrolyzed CPTES and hydroxyl-terminated PDMS to the evolving sol-gel
germania network, and (4) chemical anchoring of the evolving hybrid organic-inorganic
polymer to the inner walls of the capillary (Scheme 4.3). The hydrolysis (partial or
complete) of TEOG produces a variety of reactive hydroxyl-containing species which are
capable of undergoing condensation reactions with neighboring sol-gel active chemical
species in the sol solution such as CPTES and hydroxyl-terminated PDMS. Condensation
of these reactive species leads to a hybrid organic-inorganic network via chemical
integration of an inorganic component (germania, originating from TEOG) and a
cyanopropyl-containing organic residual (cyanopropyl, hydrolyzed forms of CPTES).
Addition of PDMS into the system promoted the network formation and incorporation
and bridging of the chemical species arising from hydrolysis of TEOG, CPTES. The
silanol groups residing on the inner walls of the fused silica capillary are also sol-gel
active and under the catalyst they can take part in the condensation reactions with the
sol-gel network fragments in their vicinity resulting in chemical anchoring of a portion of
the sol-gel material on the surface of the capillary walls. This portion of surface bonded
sol-gel material is left on the capillary surface in the form of an organic-inorganic hybrid
coating and served as the sorbent for solvent-free microextraction. After coating and
thermal conditioning, the capillary was rinsed with a mixture of methylene chloride and
140
methanol to get rid of unreacted chemical species physically adhering to the coating.
Sol-gel hybrid germania coated capillaries were then ready for microextraction.
4.3.2 Extraction profiles for sol-gel germania-CN/PDMS capillaries
In capillary microextraction, it is desirable that the extraction equilibrium is
reached in the shortest possible time. In this work, we investigated the relationship
between extraction time and the amount of analytes extracted in CME with sol-gel
germania-CN/PDMS coating (Figure 4.1).
As can be seen in Figure 4.1, for undecanol, the extraction equilibrium was
reached within 20 min. For pentachlorophenol, a highly polar solute, the time to reach
equilibrium was around 30 min. However, for heptanophenone, which is moderately
polar compound compared with phenols and alcohols, the time to reach equilibrium was
at 40 min. These results indicated that polar solutes present stronger affinity to the
developed polar germania-CN/PDMS coating, which makes sorption time for them
shorter. Based on this observation, 40 min extraction time was used in CME experiments
presented in this paper.
Ge
OC2H5
OC2H5
OC2H5
C2H5O + 4H2O GeC2H5O4-n
OHn
+ nC2H5OH
TEOG
where : n=0,1,2,3 or 4
Scheme 4.1 Hydrolysis of germania precursor (TEOG)
141
+ + Si
CH3
O
CH3
HO
p
Ge O
O
HO Ge
O
O
O q
Si
CH3
O
CH3p
O Ge
O
O
Si
CH3
O
CH3
p
GeCH3O4-n
OHn
GeCH3O4-m
OHm
O Ge
O
Si
CH3
O
CH3
pO
+ Si
OC2H5
OC2H5
CN(CH2)3 OC2H5
Si
O
(CH2)3CN
O
Si
O
(CH2)3CN
O
Si (CH2)3CN
O
O
Scheme 4.2 Polycondensation of the hydrolyzed precursor and chemical bonding of the
sol-gel-active organic ligand (Y) to the evolving sol-gel network
142
OH
OH
OH
+
O
OH
OH
Ge O
O
HO Ge
O
O
O q
Si
CH3
O
CH3p
O Ge
O
O
Si
CH3
O
CH3
p
O Ge
O
Si
CH3
O
CH3
pO
Si
O
(CH2)3CN
O
Si
O
(CH2)3CN
O
Si (CH2)3CN
O
O
Ge O
O
Ge
O
O
O q
Si
CH3
O
CH3p
O Ge
O
O
Si
CH3
O
CH3
p
O Ge
O
Si
CH3
O
CH3
pO
Si
O
(CH2)3CN
O
Si
O
(CH2)3CN
O
Si (CH2)3CN
O
O
Scheme 4.3 Chemical anchoring of the evolving sol-gel germania hybrid
organic-inorganic polymer to the inner walls of a fused silica capillary
143
Figure 4.1 Capillary microextraction profiles of undecanol, heptanophenone,
pentachlorophenol on a sol-gel germania-CN/PDMS coated capillary. Extraction
conditions: 10 cm x 0.25 mm i.d. microextraction capillary, gravity-fed sample dispenser
for extraction at room temperature. Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS
coated GC capillary column; splitless analyte desorption by rapidly increasing the GC
injector temperature from 30 ºC to 300 ºC, 5 min; GC oven temperature programmed
from 30 ºC to 300 ºC at a rate of 20 ºC/min; helium carrier gas; FID 350 ºC.
144
4.3.3 Extraction characteristics of sol-gel germania-CN/PDMS in CME
Aqueous samples of different classes of analytes (ketones, alcohols, acids, and
phenols) were used. GC peak areas obtained from the extracted analytes were used as a
measure of the extracted amounts in CME. As can be seen from Table 4.2, the peak
area run-to-run R.S.D values for phenol samples were under 3.18 %, and those for GC
retention times were less than 0.2 %. For ketones, the RSD values for run-to-run peak
area and GC retention time were under 6.89 % and 0.07 %, respectively. For acids and
alcohols, the run-to-run peak area R.S.D value were under 4.51 %, 3.52 % and that for
retention times were within 0.07, and 0.54 %, respectively.
Using the developed sol-gel germania hybrid coatings for capillary
microextraction, ng/L level detection limits were achieved in CME-GC-FID for most
analytes. The detection limit value for different classes of analytes is shown in Table 4.3.
Capillary-to-capillary reproducibility is another important criterion to characterize
the consistency in capillary preparation. The results are presented in Table 4.4. For each
analyte, extractions were made in triplicates on each capillary and the mean of the
measured peak areas were used in Table 4.4 to evaluate capillary-to-capillary
reproducibility. The presented data showed that the capillary-to-capillary reproducibility
is characterized by an RSD value of less than 5.5 % for all four classes of compounds
used for evaluation. The low RSD value is indicative of excellent reproducibility.
145
Table 4.2 GC peak area and retention time repeatability data (n=4) for chlorophenols,
alcohols, free fatty acids and ketones extracted from aqueous samples by CME using
sol-gel germania-CN/PDMS coated microextraction capillary
Analyte
Peak area repeatability
(n=4)
tR Repeatability
(n=4)
Class Name
Mean peak
area
(aribitary
unit)
R.S.D. (%) Mean tR
(min)
R.S.D.
(%)
Ketoens
Phenols
Acids
Alcohols
Valerophenone
Hexanophenone
Heptanophenone
2,3-dichlorophenol
2,4,6-trichlorophenol
Pentachlorophenol
Decanoic acid
Undecanoic acid
Nonanol
Decanol
Undecanol
3.5
9.6
24.5
7.9
10.1
20.1
3.0
8.0
4.4
15.3
29.7
6.89
3.75
3.50
3.18
1.84
2.21
4.26
4.51
3.52
1.81
1.61
8.40
9.10
9.77
7.52
8.76
8.89
8.65
9.29
6.70
7.49
8.23
0.07
0.06
0.06
0.20
0.06
0.19
0.07
0.05
0.54
0.28
0.19
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min extraction
time; Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS open tubular GC column;
splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5 min; GC
oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID
350 ºC
146
Table 4.3 GC-FID detection limit data (n=4) for chlorophenols, alcohols, free fatty acids
and ketones extracted from aqueous samples by CME using sol-gel germania-CN/PDMS
coated microextraction capillary
Analyte Class Name Detection Limit (ng/L)
Ketoens
Phenols
Acids
Alcohols
Valerophenone
Hexanophenone
Heptanophenone
2,3-dichlorophenol
2,4,6-trichlorophenol
Pentachlorophenol
Decanoic acid
Undecanoic acid
Nonanol
Decanol
Undecanol
185.7
30.6
25.4
226.6
187.5
105.0
227.9
250.9
387.9
115.8
20.0
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min
extraction time; Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS open tubular GC
column; splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5
min; GC oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas;
FID 350 ºC
147
Table 4.4 Capillary-to-capillary reproducibility (n=3) data for extracted amounts in
CME-GC experiments conducted on sol-gel germania-CN/PDMS coated capillaries using
a chlorophenol (pentachlorophenol), an alcohol (undecanol), a free fatty acid (undecanoic
acid) and a ketone (heptanophenone) as test solutes
Analytes Mean peak area (arbitrary
unit)
R.S.D. (%)
Hepanophenone
Undecanol
Undecanoic acid
Pentachlorophenol
23.9
29.1
5.8
11.9
1.4
2.4
1.5
5.8
Extraction conditions: 10 cm x 0.25 mm i.d. microextraction capillary, 40 min extraction
time; Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS open tubular GC column;
splitless desorption of the extracted analytes in the GC injector, 30 ºC -300 ºC, 5 min; GC
oven temperature rose from 30 to 300 ºC at rate of 20 ºC/min; helium carrier gas; FID
350 ºC
148
Being polar compounds, alcohols demonstrate higher affinity for water and are
usually difficult to extract from an aqueous matrix. In the present work, this class of
analytes were extracted from aqueous sample using sol-gel germania-CN/PDMS coated
capillary (Figure 4.2) that did not require any analyte derivatization, pH adjustment, or
salting out effect. From the Figure 4.2, germania-CN/PDMS showed excellent extraction
ability to alcohols. The introduction of PDMS helped incorporate CPTES into the
germania-based sol-gel material because of the excellent network forming properties of
PDMS.
On the other hand, CPTES provided cyano polar functional groups into the system,
which possess stronger affinity to polar compounds. Compared with our previous
germania-PDMS [32], the introduction of small polar precursor such as CPTES provides
sol-gel Germania coatings capability of extracting polar analytes from aqueous media.
Extraction of free fatty acids and their analysis by GC is always difficult due to
the presence of highly polar carboxyl group in the structure of these molecules and the
propensity of these molecules toward adsorption. Therefore these molecules are often
derivatized to methyl esters and the generated esters are subsequently analyzed, which is
time-consuming. Besides, quantitative derivatization of fatty acids in trace concentrations
may be problematic. All these drawbacks of traditional approaches for the analysis of
fatty acids can be avoided by using cyanopropyl-based sol-gel Germania coatings. In
this work, sol-gel germania-CPTES coatings were used to extract free fatty acids from an
aqueous medium (Figure 4.3).
149
Figure 4.2 CME-GC analysis of a mixture of alcohols using a sol-gel
germania-CN/PDMS coated microextraction capillary. Extraction conditions: 10 cm x
0.25 mm i.d. microextraction capillary, extraction time, 40 min (Gravity fed at room
temperature). Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary
column; splitless analyte desorption by rapidly increasing the GC injector temperature
from 30 ºC to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a
rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Nonanol (2) Decanol (3)
Undecanol
150
Chlorophenols (CPs) are one of the major classes of environmental pollutants
constitutes an important group of priority toxic pollutants listed by EPA, with possible
carcinogenic properties. Chlorophenols (CPs) have been widely used as preservatives,
pesticides, antiseptics, and disinfectants [59], and they are often found in waters [60],
soils, and sediments [61]. CPs are highly polar compounds with significant affinity
toward water. In our study, four CPs were extracted from water samples using germania
based CN-PDMS coated capillaries. Low detection limits of ng/L level were achieved.
Figure 4.4 shows an example of excellent the ability of a sol-gel germania-CN/PDMS
coated CME capillary to accomplish preconcentration of some chlorophenols.
Ketones are relatively less polar compared with above classes of compounds, and
present health and environmental concerns. They are formed as by-products in the
drinking water disinfection processes. Many of these by-products have been shown to be
carcinogens or carcinogen suspects. However, it often requires their derivatization prior
to their extraction or chromatographic analysis. In sol gel germania based CME-GC
experiments (Figure 4.5), no derivatization step was necessary either for extraction or GC
analysis of ketone samples.
151
Figure 4.3 CME-GC analysis of a mixture of acids using a sol-gel germania-CN/PDMS
coated microextraction capillary. Extraction conditions: 10 cm x 0.25 mm i.d.
microextraction capillary, extraction time, 40 min (Gravity fed at room temperature).
Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary column;
splitless analyte desorption by rapidly increasing the GC injector temperature from 30 ºC
to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a rate of 20
ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Decanoic acid (2) Undecanoic acid (3)
Dodecanoic acid
152
Figure 4.4 CME-GC analysis of a mixture of chlorophenols using a sol-gel
germania-CN/PDMS coated microextraction capillary. Extraction conditions: 10 cm x
0.25 mm i.d. microextraction capillary, extraction time, 40 min (Gravity fed at room
temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary
column; splitless analyte desorption by rapidly increasing the GC injector temperature
from 30 ºC to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a
rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) 2,4,6-trichlorophenol (2)
Pentachlorophenol
153
Figure 4.5 CME-GC analysis of a mixture of ketones using a sol-gel
germania-CN/PDMS coated microextraction capillary. Extraction conditions: 10 cm x
0.25 mm i.d. microextraction capillary, extraction time, 40 min (Gravity fed at room
temperature). Other conditions: 5 m x 0.25 mm i.d. sol-gel PDMS coated GC capillary
column; splitless analyte desorption by rapidly increasing the GC injector temperature
from 30 ºC to 300 ºC, 5 min; oven temperature programmed from 30 ºC to 300 ºC at a
rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks: (1) Valerophenone (2)
Hexanophenone (3) Heptanophenone
154
4.3.4 Thermal stability of sol-gel germania coatings in CME
Sol-gel germania-CN/PDMS coating showed excellent thermal stability. After
coating and conditioning, the conditioning temperature was stepwise increased from 250º
C to 350ºC with an interval of 20 ºC. At each temperature, the capillary was conditioned
for one hour. The experimental peak area data obtained after conditioning the capillary at
each temperature were compared. In Figure 4.6 present, the extraction performance of the
sol-gel germania-CN/PDMS coated capillary was studied for nonanol (an alcohol) in the
course of stepwise conditioning of the capillary. The results suggest that sol-gel
germania-CN/PDMS coating retain its excellent extraction capability after being
conditioned at 330ºC.
155
Figure 4.6 Effect of conditioning temperature on the performance of sol–gel
germania-CN/PDMS microextraction capillary in the extraction of nonanol. Extraction
conditions: 10 cm x 0.25 mm i.d. microextraction capillary, extraction time, 40 min
(Gravity fed at room temperature). Other conditions: 10 m x 0.25 mm i.d. sol-gel PDMS
coated GC capillary column; splitless analyte desorption by rapidly increasing the GC
injector temperature from 30 ºC to 300 ºC, 5 min; oven temperature programmed from 30
ºC to 300 ºC at a rate of 20 ºC/min; helium carrier gas; FID 350 ºC Peaks.
156
4.4 Conclusion
A cyanopropyl-based sol-gel germania organic-inorganic hybrid coating was
developed for the preconcentration of polar analytes from aqueous by capillary
microextraction. The developed coating was prepared by using two different precursors
TEOG (germania component) and CPTES (silica component) and a small amount of
PDMS. PDMS is a non polar polymer. The sol-gel geamania-PDMS capillary reported in
previous work only showed excellent affinities for nonpolar and moderately polar
compounds. However, the sol-gel germania-CN/PDMS capillary was effective in
extracting polar analytes such as alcohols, fatty acids, and phenols, because presence of
high polar cyanopropyl ligands. Good run-to-run and capillary-to-capillary
reproducibility (peak area RSD) was achieved for most polar analytes (below 7 %) and
ng/L level detection limits were achieved in CME-GC-FID for most analytes, using the
developed sol-gel germania hybrid coatings for capillary microextraction. The developed
sol-gel germania-CPTES/PDMS capillary also showed excellent thermal stability, due to
the chemical anchoring of such polar coatings onto the fused silica surface.
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Appendix B
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ABOUT THE AUTHOR
Li Fang was born in Bengbu, Anhui Province, China. She obtained her Bachelor’s
degree in Chemistry Education from Anhui Normal University in 2000. She continued
her studies at University of Science and Technology of China (Hefei, Anhui) and received
her Master of Science degree in Analytical Chemistry in 2003. Later, she came to United
States and joined the Department of Chemistry, University of South Florida to pursue a
Doctorate Degree. The work presented here was conducted under the guidance of Dr.
Abdul Malik. She has three publications in international journals.
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