basis of nonpolar interactions_gc columns pi
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GC SolutionsSeparation Science GC Solutions is the premier online resource or GC and GC/MS users working across the Asia Pacifc region.
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www.sepscience.com Issue 8: October 2010
Tech TipBasis o Interactions in Gas Chromatography,
Part 1 Non-Polar Interactions
To ully grasp the concepts o retention and selectivity o GC stationary phases,
one must frst understand the undamental intermolecular interactions that lead
to retention. This month we discuss the most dominant o those interactions
dispersive, non-polar interactions.
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There are ewer types o intermolecular interactions available orinteractions between solutes and stationary phases in gas chromatographycompared to those possible in liquid chromatography.In addition, themobile phase in GC (the carrier gas) plays no role in adjusting or modiyingthe nature o intermolecular interactions during the run.So, the range opossibilities in retention orces and selectivity is much more limited in GCthan in LC.
In gas chromatography, molecules can only interact with each other
through intermolecular orces that all under the umbrella o van derWaals orces.They are listed in Table 1.Van der Waals orces can be overcome or disrupted by thermal motion,
which increases as temperature increases. This is why liquids evaporateaster when heated. and also why solutes are less retained (elute aster) ingas chromatography as the temperature is raised.
In gas chromatography, the dominant intermolecular attractive orce isdispersive interactions.Dispersive interactions are known also as temporarydipole interactions, because they result rom transient random distortionsin the electronic clouds o molecules, London forces, ater the scientist whofrst described them.Dispersive interactions exist between all molecules.
Basis o Interactions in Gas Chromatography,Part 1 Non-Polar InteractionsTo ully grasp the concepts o retention and selectivity o GC stationary phases, one must frst understand the
undamental intermolecular interactions that lead to retention.This month we discuss the most dominant o thoseinteractions dispersive, non-polar interactions.
Matthew Klee
Name Description Molecular traits Characteristic
Non-Polar
London2 (1930)
dispersion
induced dipole in-
duced dipole
All compounds,
non-polar interaction
Transient polarization,
scales with molecular size
Hydrogen bonding Extreme dipoledipole
interaction: H acceptor
interacts with H donor
Signifcant with
compounds containing
OH or -NH groups
Polar
Keesom3 (1912) dipoledipole Interaction between
strong dipoles
Electronegative groups
(e.g., halogens, -OR, -NOx,
-SOx)
Debye4 (1923) Dipoleinduced dipol e I ntera ct io n between
a strong dipole and a
weak dipole
More polarizable = easier
induction
Table 1
Table 1:Van der Waals1 forces of interactions between molecules.
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Figure 1
Figure 1: London dispersive (non-polar) forces dominate the intermolecular interactions in gas
chromatography.They arise from spontaneous transient distortions, polarization, then coordinatedoscillations in electronic molecular orbitals. The larger the molecule, the larger the dispersive forces, thehigher the strength of interaction, and therefore the higher the retention.
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Dispersive interactions are non-polar (also called apolar).Dispersiveinteractions arise rom random distortion o the electronic cloud o amolecule, causing a slight electrostatic polarization one side o themolecule becomes more negative, the opposite more positive (Figure 1).
This spontaneous polarization then induces an opposite polarizationin neighbouring molecules.The opposite charges attract and draw themolecules closer, urther distorting the clouds..A stabilizing oscillationo the charge distortion results within the bulk liquid (in wall coatedopen tubular columns, the stationary phase is considered a liquid). The strength o dispersive interactions track with the size o themolecule; the larger the molecule, the higher its mass, the moreelectrons, the higher the strength o its dispersive orces.Largermolecules have larger electron clouds which are more able to handleelectrostatic distortions.So, the distortions can be o higher magnitudeand o longer duration.For this reason, both boiling points and elutiontemperatures o molecules track with the size o the molecule (Figure 2).
Although retention in GC is based on the sum o all possibleinteractions, polar + non-polar, some types o molecules such assaturated hydrocarbons (alkanes) can only interact through dispersiveinteractions.Even i a stationary phase were to have polar unctionalgroups and thereore a signifcant possibility or polar interactions,saturated hydrocarbons would only interact with the non-polar,
dispersive aspect. The basic premise o retention in GC is illustrated in Figure 3.Onecan see in Figure 3 that the solute represented by the blue triangleshas a higher proportion o molecules in the stationary phase than inthe gas phase. As such, it migrates slowly through the column (has ahigh retention time).In contrast, the majority o the solute representedby the green squares is in the gas phase, so it will migrate much asterthrough the column (have a much lower retention time).
Retention in gas chromatography is an exponential unction otemperature.As temperature is raised, there will be a temperature atwhich the more retained solute (blue triangles) will travel at the same
The strength of dispersive interactionstrack with the size of the molecule; the largerthe molecule, the higher its mass, the moreelectrons, the higher the strength of its
dispersive forces.
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Figure 3
Figure 3: Inner view o a capillary column. A flm thickness 0.5 m on a 250 m i.d. column represents a
phase ratio o 125, which is typical o capillary columns. As it is almost impossible to see the
stationary phase when drawn to scale, the inset shows a representation o the surace magnifed 100
times to better illustrated solute migration in/out o the phase.The solute with the weaker interactions
with the stationary phase (green squares) spends more time in the mobile phase and moves aster
through the column, eluting frst.The compound with stronger interaction with the stationar y phase
(blue triangles) spends less time in the gas phase and moves slower through the column.
250mi.d.
0.50mi.d.
magnified 100X
Column wall
stationary phase
carrier gas
column wall
Figure 2: (A) Retention time trend on-alkanes in a linear temperature programmed capillary GC run. (B)
Boiling point trend on-alkanes.Retention o in gas chromatography tracks boiling point because o
dominance o non-polar (dispersive) orces.
Figure 2
0
100
200
300
400
500
600
0 10 20 30 40 50
Carbon Numbern-Alkane
Boiling
Point(oC)
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50
Carbon number
RetentionT
ime
A
B
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Figure 4
0
100
200
300
400
500
600
700
800
900
1000
0 25 50 75 100 125 150 175 200 225
Temperature (oC)
Reten
tionFactor(k)
Figure 5
1
10
100
1000
0 50 100 150 200 250 300
Temperature (oC)
RetentionFactor(k)
Solute A
Solute B
Solute C
solute D
speed as the less retained one in Figure 3 didat the lower temperature.As illustrated inFigure 4, retention decreases approximatelyby or each 23 oC change in temperature.In Figure 5, example trends are plotted on alog scale. One can see that all solutes ollowa similar pattern to a rst approximationbecause the dominant intermolecular orceo interaction is dispersive.There are slightdiferences in slopes or homologs withdiferent unctionalities.These arise rompolar interactions and will be discussed nextmonth.
References
1. J. D. van der Waals, The Thermodynamic Theory o Capillarity
Under the Hypothesis o a Continuous Variation o Density,
originally published in Dutch in Verhandel. Konink. Akad. Weten.
Amsterdam, 1, 8, (1893)
2. Von R. Eisenschitz, F. London, ber das Verhltnis der van der
Waalsschen Krte zu den Homopolaren Bindungskrten Z.
Physik, 60, 491-527, (1930)3. W. H. Keesom, On the Deduction o the Equation o State
From Boltzmanns Entropy Principle, Communications
Physical Laboratory University o Leiden Supplement, Ed. By H.
Kamerlingh Onnes, Eduard Ijdo Printer, Leiden, Supplement 24a
to No. 121-132, 3-2. 0, (1912)
4. P. J. W. Debye, Die van der Waalsschen Kohsionskrte,
Physik. Zeitschr., 21, 178-87, (1920)
Dr Matthew S. Klee is internationally recognizedfor contributions to the theory and practice of gaschromatography. His experience in chemical, pharmaceutical
and instrument companies spans over 30 years. During thistime, Dr Klees work has focused on elucidation and practicaldemonstration of the many processes involved with GCanalysis, with the ultimate goal of improving the ease of useof GC systems, ruggedness of methods and overall quality ofresults.
Figure 4: Intermolecular interactions between a solute and a stationary phase lead to retention.
Retention o solutes is an exponential unction o temperature.Retention decreases by orapproximately every 23 oC change in temperature (solute and stationary phase dependent).
Figure 5: The exponential retention unction o several solutes plotted on log scale. More volatile solutes(e.g., solute B) will elute earlier and at lower temperatures in a temperature programmed run.To a rstapproximation, or a polar stationary phases the dependence o retention on temperature ollows the
same trend or all solutes because o dominance o dispersive (non-polar) orces o interaction,
diferences in slopes arise rom diferences in solute polarities (e.g., solutes A and C). Homologs usuallyhave the the same retention vs temperature slope on a given stationary phase (e.g., solutes B and D).
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Introduction
Butylated hydroxytoluene(BHT, 2,6-di-tert-butyl-4-methylphenol)is a common food additive. BHT is foundin many types of food including butter,meats, cereals, chewing gum, bakedgoods, snack foods, dehydrated potatoes
and beverages. It is used to preserve foododor, color and flavor. BHT is oxidizedpreferentially in fats or oils, protectingthe foods from spoilage.
Concern exists that long-term human consumption of BHT may have potentialhealth risks. It has undergone the additive application and review processrequired bythe U.S.Food and Drug Admini stration (FDA);the committee concludedthat no evidence in the available information on BHT demonstrates a hazard tothe public when it is used at levels that are now current and in the manner nowpracticed. However, uncertainties exist requiring that additional studies shouldbe conducted.1 The chemical properties which make BHT an excellentpreservativemay also be implicated in health effects. The oxidative characteristics andmetabolites of BHT may contribute to carcinogenicity. Some people may havedifficulty metabolizing BHT, resulting in health and behavioral changes.
Gas Chromatography/
Mass Spectrometry
a p p li ca t i on n ot e
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Perki nElmer,Inc.
Shelton,CT 06484 USA
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ApplicationNote
Determination of 2,4,6Trichloroanisolein corkand winewith HS-SPME/GCMS
-standard
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Corkstoppersusedforwinebottlescaneffectthetasteofthe wine.Themaincontaminantisthewellknown2,4,6-Trichloroanisole.Thisisanoff-flavorwhichisbelievedtobe producedbymethylationofphenolsoft hecorktreeandfinalbleachingofthecork.Human noseandtastecantracebackdowntoabout 5-10ng/L(5-10ppt).Forthequalitycontrol ofcorkstoppersthereforeanenrichmenttechniquelikesolidphasemicroextraction(SPME)isneeded.ThentheanalysiswithGC-ECDorGCMSisperformed.Heretheresultobtained
withGCMSarereported.Before the headspace SPME is done a l iquidextraction was performed with the corks. Forthis the corkstoppers were put into a 2 Lethanol water solution (12 %) for 24 hours atroomtemperature. Then an aliquot of 10 mlwere put into a 20 ml headspace vialsaturated with 3 g NaCl. The latter increasesthe effectivity of the adsorption of the TCAonto the fiber. As an internal standard adeuterized TCA is added ( 2H5-2,4,6-TCA).With these vials the automatized headspace-SPME experiments were performed using apolydimethylsiloxane fiber (PDMS, Supelco).The instrument used was a GCMS-QP2010with an AOC-5000 autosampler. In figure 1the incubatorof the AOC-5000 is shownwhen the headspace vial is placed into it.
Fig.1:20mlHeadspacevial placedintoincubatorof theAOC-5000
For the method optimization in a first step aliquid standard was injected. In the secondstep the liquid extract prepared a statedabove was spiked with TCA. To havemaximumintensity theMS was runinselected ionmonitoring (SIM).
6 . 0 7 . 0 8 . 0 9 . 0 1 0 . 0 1 1 . 0 1 2 . 0 1 3 .0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0( x10,000)
Fig.2:Top:SIM 1) dataofmasstrace195 relativetoanextractspikedwith17ppt TCAcomparedwithanunspikedextract.
In figure 2 the SIM data of an extract spikedwith 17 ppt compared to the blank extract isshown clearly indicating the 195 amu tracerelative to the 2,4,6, TCA which was used asthe quantifier ion. Qualifier ions usedwere210 and 212.The SPME parameters were:Extractiontemperature and time 50 C 30 min,desorption at 220 C for 2 min.The analysis conditions used for the GCMSwere: Injection splitless 2 min with highpressure pulse at 200 Kpa, Column: TRB 525 m, 0.25 mm, 0.25 m,5 0 C 2 mi n ,12C/min138C3 min20C/min260 2min linear velocity of the carrier gas 48.2cm/s, interface temperature: 270C.As a quantifier ion for the D-TCA the masstrace of 215 was used. Figure 3 shows thezoomed mass trace of 195 and 215 relativ toa standard of 0.7 ppt. The peak of the 2,4,6TCA is still clearly visible. The calibrationcurve is alsoshown in that figure.
SIM
trace1952,4,6 TCA
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Whenscreeningurine for drugsof abuse(DOA), reliabilityiscrucial. Detectionofcompoundssuchasmarijuana, cocaine,heroinand amphetamines, aswellastheir metabolites, iscommonlycarriedout byGC/MS, however the complexityof asample suchasurine canpresent ananalyticalchallenge. Highmatrixeffectsand frequent co-elutioncansignicantlycompromise reliabilityof identication,particularlyfor DOA at trace level,therefore these factorsneed tobeaddressed.
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Experimental
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8/7/2019 Basis of NonPolar Interactions_GC Columns PI
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GC Pressure and Flow Calculations iPhone/iPod ApplicationCompany: Agilent
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8/7/2019 Basis of NonPolar Interactions_GC Columns PI
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Advanced Topics in Capillary Gas Chromatography
Getting More from your GC3-4 November, 2010
Venue: Park Inn Manchester, Victoria, UK
Matthew Klees
GC Masterclass
CLICK HERE TO
REGISTER ONLINE
www.sepscience.com
2 day
695
Day 1
1. Capillary GC tune-up
2. Large volume injection
3. Auxiliary sampling and ocusing4. Fast GC how to painlessly and quickly migrate your current methods
Day 2
1. Multidimensional separations (instrumentation, practice)
2. Capillary column backfushing BF with benets
3. Automated sample preparation
4. Optimal mass spec.
5. Method development and troubleshooting - interactive discussion