2015 recent achievements in solidified floating organic drop microextraction

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Recent achievements in solidified floating organic dropmicroextractionPilar Vias a,*, Natalia Campillo a, Vasil Andruch b

a Department of Analytical Chemistry, Faculty of Chemistry, Regional Campus of International Excellence Campus Mare Nostrum, University of Murcia,E-30100 Murcia, Spainb Department of Analytical C hemistry, Pavol Jozef afrik University, SK-04154 Koice, Slovak Republic

A R T I C L E I N F O

Keywords:ComplexationDerivatizationDisperser solventGreen analytical chemistryLiquid-phase microextractionReal sample analysisSample pre-treatmentSFODMESolidified floating organic dropmicroextractionSolvent microextraction

A B S T R A C T

We give an overview of achievements in solidified floating organic drop microextraction (SFODME). Wefocus on the types of analyte investigated, the types of sample analyzed, the sample-pre-treatment pro-cedures used, including derivatization and complexation, and we cover the articles available on-line upto 30 November 2014. SFODME has been applied to the determination of organic compounds and inor-ganic analytes. Although most extraction methods based on SFODME have been applied to water samples,the technique has also been used for analysis of other types of samples. We also briefly discuss the effectsof different variables. We discuss the various modalities of SFDOME. Tables summarize the applica-tions, selected experimental conditions and the most important parameters of SFODME.

2015 Elsevier B.V. All rights reserved.

Contents

1. Introduction .................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ................ 492. A brief history of solvent microextraction .................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... . 493. Principle of solidified floating organic drop microextraction ..................... ..................... ...................... ..................... ..................... ...................... ..................... ......... 504. Applications to the determination of organic compounds ................... ..................... ...................... ..................... ..................... ...................... ..................... ................ 50

4.1. Types of analyte .................... ...................... ..................... ...................... ..................... ..................... ..................... ..................... ...................... ..................... ................. 504.2. Types of sample ..................... ..................... ...................... ..................... ...................... .................... ...................... ..................... ...................... ..................... ................ 514.3. Sample (aqueous phase) volume .................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ..... 51

Abbreviations (techniques and parameters): AA, Alcoholic-assisted; AAS, Atomic absorption spectrometry; AFS, Atomic fluorescence spectrometry; BE, Back-extraction;CAD, Charged aerosol detection; CE, Capillary electrophoresis; CLC, Capillary liquid chromatography; CV, Cold vapor; D, Displacement; DAD, Diode-array detection; DLLME,Dispersive liquid-liquid microextraction; DMAE, Dynamic microwave-assisted extraction; DS, Directly suspended; DSDME, Directly suspended droplet microextraction; ECD,Electron-capture detection; EF, Enrichment factor; ETAAS, Electrothermal atomic absorption spectrometry; ETV, Electrothermal vaporization; FAAS, Flame atomic absorp-tion spectrometry; FI, Flow injection; FID, Flame ionization detection; FLD, Fluorescence detection; FPD, Flame photometric detection; GC, Gas chromatography; HF, Hollowfiber; HG, Hydride generation; HPCE, High-performance capillary electrophoresis; HPLC, High-performance liquid chromatography; ICP, Inductively coupled plasma; IP, Ionpair; LC, Liquid chromatography; LL, Ligandless; LLE, Liquid-liquid extraction; LOD, Limit of detection; LOQ, Limit of quantification; LPME, Liquid-phase microextraction;

MAE, Microwave-assisted extraction; MS, Mass spectrometry; MS/MS, Tandem mass spectrometry; MSA, Magnetic stirring-assisted; MWCNT, Multiwalled carbon nanotube;OES, Optical emission spectrometry; SA, Surfactant-assisted; SD, Solvent demulsification; SDME, Single-drop microextraction; SFODME, Solidified floating organic dropmicroextraction; SFVCDME, Solidified floatingvesicular coacervative drop microextraction; SM, Supramolecular; SPE, Solid-phase extraction; SPME, Solid-phase microextraction;UA, Ultrasound-assisted; UAE, Ultrasound-assisted extraction; UASEME, Ultrasound-assisted surfactant-enhanced emulsification microextraction; UHPLC, Ultra-high per-formance liquid chromatography; USAE, Ultrasound-assisted emulsification;USAEME, Ultrasound-assisted emulsification-microextraction; UV-Vis,Ultraviolet-visible spectrometricdetection; VA, Vortex-assisted; VALLME, Vortex-assisted liquid-liquid microextraction; VASEME, Vortex-assisted surfactant-enhanced-emulsification microextraction.

Abbrevi ations (chemic als): APDC, Ammonium pyrrolidinedithiocarbamate; BDTA, Benzyldimethyltetradecylammonium chloride-dihydrate; BPHA, N-benzoyl-N-phenylhydroxylamine; 5-Br-PADAP, 2-(5-Bromo-2-pyridylazo)-5 diethylaminophenol; BTEX, Benzene, toluene, ethyl benzene and xylene; CTAB, Cetyltrimethylammoniumbromide; DAB, 3,3-Diaminobenzidine; DDTC, Diethyldithiocarbamate; DDTP, Diethyldithiphosphate; DMF, Dimethylformamide; DMSO, Dimethylsufoxide; DPC,1,5-Diphenylcarbazide; EDC, Endocrine-disrupting compound; Fmoc-Cl, 9-Fluorenylmethyl chloroformate; HDEHP, Di-2-ethylhexylphosphoric acid; HOC, Halogenated organiccompound; HQ, Hydroxy quinoline; IBCF, Isobutyl chloroformate; iBuOH, 2-Methyl-1-propanol; IL, Ionic liquid; OCP, Organochlorine pesticide; OPE, Organophosphate ester;PAH, Polycyclic aromatic hydrocarbon; PAN, 1-(2-Pyridylazo)-2-naphthol; PBDE, Polybrominated diphenyl ether; PCB, Polychlorinated biphenyl; PE, Phthalate ester; SDBS,Sodium dodecylbenzenesulfonate; SDS, Sodium dodecylsulfate; THF, Tetrahydrofuran; THM, Trihalomethane; TTA, 2-Thenoyltrifluoroacetone.

* Corresponding author. Tel.: +34 868 887 415; Fax: +34 868 887 682.E-mail address:[email protected](P. Vias).

http://dx.doi.org/10.1016/j.trac.2015.02.005

0165-9936/ 2015 Elsevier B.V. All rights reserved.

Trends in Analytical Chemistry 68 (2015) 4877

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4.4. Sample pre-treatment ................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ..................... ...... 524.5. Application of derivatization in solidified floating organic drop microextraction (SFODME) ..................................... ..................... ...................... .... 524.6. Salt addition and pH ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...... 524.7. Extraction temperature .................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ..................... ...................... .. 524.8. Figures of merit .................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ..................... ................. 53

5. Applications to the determination of inorganic compounds ...................... ..................... ...................... ..................... ..................... ...................... ..................... ......... 535.1. Types of analyte .................... ..................... ...................... ..................... ...................... .................... ...................... ..................... ...................... ..................... ................. 535.2. Sample (aqueous phase) volume .................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..... 535.3. Sample pre-treatment ................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ..................... ...... 53

5.4. Application of complexation in SFODME ................... ...................... ..................... ..................... ...................... ...................... ..................... ...................... ............ 625.5. Salt addition and pH ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...... 625.6. Extraction temperature .................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ..................... ...................... .. 625.7. Figures of merit .................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ..................... ................. 63

6. Different modalities of solidified floating organic drop microextraction .................... ...................... ..................... ..................... ...................... ..................... ......... 636.1. Nature and volume of the extraction solvent ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... .. 70

6.1.1. Organic compounds ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ............. 706.1.2. Inorganic compounds ..................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ......... 70

6.2. Selection of the disperser solvent for DLLME-SFO ................... ...................... ..................... ....................... ..................... ...................... ..................... ................ 706.3. Stirring rate ..................... ...................... ..................... ...................... ..................... ..................... ..................... ...................... ..................... ...................... ..................... ... 706.4. Extraction time ..................... ..................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ................. 706.5. Disruption of the cloudy solution ................... ..................... ...................... ..................... ...................... ..................... ...................... ..................... ...................... ..... 71

7. Concluding remarks and future trends ................... ...................... ..................... ...................... ..................... ..................... ...................... ..................... ...................... ......... 71Acknowledgments ............................................................................................................................................................................................................................................... 71References .............................................................................................................................................................................................................................................................. 71

1. Introduction

An inevitable requirement in modern chemistry is that chemi-cal procedures have the least possible impact on the environment[1]. Analytical chemistry is no exception to this trend[24]. An an-alytical procedure comprises severalsteps, and sample pre-treatmentis probably the most laborious and tedious of them. The develop-ment of novel, simple, green and low-cost sample pre-treatmentprocedures has therefore been an important topic in this area overthe past two decades.

In the mid-to-late 1990s, Dasgupta and co-workers published aremarkable series of papers on the potential application of amicrodrop. They demonstrated the usefulness of the unique fea-

tures of liquid drops through a series of novel liquid-drop-basedsystems[510]. These works can be considered as the beginning ofminiaturization in analytical chemistry. Miller and Synovec also dis-cussed various aspects of drop-based analytical measurements [11].Another interesting, challenging task is automation, the direct cou-pling of the sample preparation step with the detection system.Automated systems offer a number of advantages, such as minimiz-ingtheerrorsassociated with manual handling,reducing consumptionof sample and reagent, and improving sensitivity and precision.

Liquid-liquidextraction(LLE)is among theoldestpreconcentrationand separation techniques. However, in a conventional design, LLEhas many well-known drawbacks and limitations, the most impor-tant of which are the large volumes of sampleand extraction solventrequired, and consequently the large amount of waste generated,

as well as the low preconcentration factors provided, making nec-essarythe evaporation of theextractto dryness andthe subsequentre-dissolution in a smaller volume.

Some earlier attempts to miniaturize LLE can be found at the endof the twentieth century. Liu and Dasgupta reported a drop-in-drop system, in which the aqueous phase was continuously deliveredto the surface of a drop and was aspirated away from the bottompart of it; thus, the organic drop (1.3 L) was enclosed in the aqueousdrop. The feasibility of the suggested arrangement was demon-strated by the determination of an anionic surfactant through theformation of an ion associated with Methylene Blue reagent and itsextraction into chloroform[12].

Jeannot and Cantwell reported another approach to miniatur-ization of solvent microextraction: a small drop (8 L) of a water-

immiscible organic solvent was located at the end of a Teflon rod

immersed in a stirred aqueous sample solution. After the extrac-tion, the probe was withdrawn from the aqueous solution, and theorganicphase injectedinto an analytical instrumentfor quantification[13].

There are several valuable, well-founded review articles devotedto solvent microextraction;Table 1lists a selection. However, untilnow, only two review articles have directlyaddressed solidifiedfloat-ing organic drop microextraction (SFODME)[29,30]. In them, theauthors focused on the experimental factors that affect the extrac-tion efficiency and the coupling of SFODME with various detectiontechniques, such as gas chromatography (GC), liquid chromatogra-phy (HPLC, LC), atomic absorption spectrometry (AAS) andinductively-coupled plasma optical emission spectrometry (ICP-

OES), so we do not include these aspects of SFODME here.Our review focuses on the types of analyte and sample ana-

lyzed using SFODME and includes articles available up to 30November 2014. Tables summarize the application of SFODME.

2. A brief history of solvent microextraction

Numerous variations of solventmicroextraction have alreadybeendeveloped, including single-drop microextraction (SDME)[31,32],hollow-fiber liquid-phase microextraction (HF-LPME)[33], disper-sive liquid-liquid microextraction (DLLME) [34,35], solidified floatingorganic drop microextraction (SFODME)[36], ultrasound-assistedemulsification-microextraction (USAEME)[37], ultrasound-assistedsurfactant-enhanced emulsification microextraction (UASEME) [38],

directly suspended droplet microextraction (DSDME)[39,40]andvortex-assisted liquid-liquid microextraction (VALLME) [41].Electrodriven LME techniques have high efficiency in the extrac-tion of charged analytes in short times[26].

Until now, the majority of papers have reported manually per-formed solvent-microextraction procedures. However,severalarticlescan be found describing procedures with various levels of automa-tion (from semi-automated to fully automated); we discussed recentefforts at automation of solvent microextraction in a previous paper[42]. We therefore list certain procedures here, but we do notdiscussthem in detail.

Anthemidis et al. suggested automatedDLLME, in which the drop-lets of organic phase containing a complex of the analyte are retainedon a microcolumn[43,44], while a different option is based on a

flow-batch approach [45,46]. Another field of study has been

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automation of SDME[47,48]. Mitani et al. described on-line LPMEbased on a drop-in-plug sequential injection lab-at-valve plat-form for metal determination[49], while Burakham et al. appliedthe lab-at-valve system for the development of on-line LPME pro-cedures[50,51].

SFODME, like other solvent microextraction techniques, quickly

attracted the attention of researchers and remains very popularamong analytical chemists, as evidenced by its appearance in a con-tinually growing number of original papers (Fig. 1).

3. Principle of solidified floating organic drop microextraction

An aliquot of sample solution is put into a suitable glass vessel.A 1-Lvolumeof an appropriate extractionsolventis added, usuallyby rapid injection into the aqueous sample using a syringe, or, al-ternatively, it can be placed on the sample surface at the center ofthevortexin DS-SFODME [39,40,52,53]. Theextractions solvent shouldsatisfy not only the general criteria for an extraction solvent in LLE,but also the one specific criteria of SFODME, namely, it should havea melting point near room temperature, in the range 1030C. Thesolution is then agitated (usually by magnetic stirring) for a pre-scribedperiodof time; it is in this step that theanalytes areextractedinto the organic phase. After centrifugation, the glass vessel is im-mersedin an ice waterbath,andthe floatedorganic solvent solidifiesafter a short periodof time. Afterwards, thesolidified solvent is trans-ferredto a conical vial using a small spatula, quickly meltedat roomtemperature and injected (as a whole, as an aliquot or after properdilution) into the analytical instrument for quantification. Besidesmagnetic stirring, sample agitation can be performed using ultra-soundenergy(USAE-SFODME) [54] or vortex mixing (VA-SFODME)[55], or agitationcan be replaced byapplicationof a dispersersolvent(DLLME-SFO)[56]and enhanced by manual shaking[57]or usingsurfactants (SA-SFODME)[58].

The application of SFODME to the determination of organic andinorganic compounds is summarized inTables 2 and 3, respectively.

4. Applications to the determination of organic compounds

4.1. Types of analyte

The majority of SFODME procedures reported until now (61%)have been devoted to the determination of organic analytes. Thetechnique has been applied to the determination of various classesof organic compounds that are widely used in different areas ofhuman activity, but at the same time they (or their metabolites) maypose risks (Fig. 2).

Persistent organic compounds are of worldwide concern, becausesome of them are recognized as potential carcinogens and show tu-morigenic and endocrine-disrupting activities in mammals. They

Table 1

Selected review articles devoted to solvent microextraction(arrangedchronologically)

First Author Year Topic/Title Ref.

Miller 2000 Review of analytical measurementsfacilitated by drop formation technology

[11]

Anthemidis 2009 Homogeneous and dispersive liquid-liquid extraction for inorganics

[14]

Dadfarnia 2010 LPME, determination of metals [15]Rezaee 2010 Evolution of dispersive liquid-liquid

microextraction method[16]

Sarafraz-Yazdi 2010 Liquid-phase microextraction [17]Asensio-Ramos 2011 LPME, applications in food analysis [18]Mahugo-Santana 2011 LPME, determination of emerging

pollutants[19]

Tankiewicz 2011 Solventless and solvent-minimizedsample preparation techniques

[20]

Cruz-Vera 2011 Sample treatments based on dispersive(micro)extraction

[21]

Abadi 2012 LPME combined with UV-Visspectrophotometry

[22]

Pic 2013 Ultrasound-assisted extraction for foodand environmental samples

[23]

Delgado-Povedano 2013 Ultrasound-assisted emulsification-extraction

[24]

Kokosa 2013 Advances in solvent-microextractiontechniques

[25]

Hu 2013 LPME, analysis of trace elements and

their speciation

[26]

Spietelun 2014 Green aspects, developments andperspectives of LPME techniques

[27]

Bosch Ojeda 2014 Vortex-assisted liquidliquidmicroextraction

[28]

Fig. 1. The distribution of articles devoted to solidified floating organic drop microextraction (SFODME) from 2007 to 2014 (30 November) (Based on the data in Tables 2

and 3).

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exist ubiquitously in the atmosphere, soil, water and food {e.g., poly-cyclic aromatic hydrocarbons (PAHs) [36,5961]; halogenatedorganic

compounds (HOCs)[56]; trihalomethanes (THMs)[62,63]; poly-chlorinated biphenyls (PCBs), considered to be target compoundsof environmental regulations[6466]; volatile aromatic hydrocar-bons, such as benzene, toluene, ethyl benzene and xylene (BTEX)[6770]; nitrobenzene isomers[71,72]; mono nitrotoluenes[73];aliphatic amines[74]; aromatic amines, including aniline and othersubstituted derivatives[75]; and, chlorinated anilines[76]}.

Another group of analytes comprises flame-retardants {e.g., or-ganophosphate esters (OPEs)[77], decabrominated diphenyl ether[78]and polybrominated diphenyl ethers (PBDEs)}, which are wellknown for their anti-flaming properties[79,80].

Phthalate esters (PEs) are polymer additives and plasticizers thatcan migrate from the material to the environment and, consequent-ly, pollute water, soil, air and food products[8184]; esters of

p-hydroxybenzoic acid, commonly known as parabens, are widelyused as preservatives [85]; and, methyl methacrylate [86],benzotriazole ultraviolet stabilizers [87] and volatile aldehydes, rec-ognized as biomarkers of cancers[88], have also been determinedafter pre-concentration by SFODME.

Phenolic compounds are released in aquatic environments as pol-lutants during the production of plastics, drugs, pesticides andpetrochemicals. An EC directive[57]specifies a legal tolerance levelof 0.1 g L1 for each phenolic compound and 0.5 g L1 for the sumof all compounds in water intended for human consumption, anddifferent methods have been proposed for determining phenols[8994], nitrophenolic compounds[57]and alkylphenols[95].

Pesticides provide many benefits for increasing agricultural pro-duction, but their presence in water is strictly regulatedby legislation

to concentrations in the range


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decreased, because, when stirring the solution at a fixed rate witha completely filled vial, convection is insufficient in the aqueousphase, resulting in less extraction. Nevertheless, one can also findarticles in which higher sample volumes, such as 25 mL[96], 30 mL[67,97,107],40mL[79] oreven 200 mL[132], and210 mL[106], werechosen as optimal.

4.4. Sample pre-treatment

The treatment for water samples normally includes only filtra-tion and, in some cases, adjusting the pH and adding a salt.

A combination of solid-phase extraction (SPE) and DLLME-SFOwas designed and employed for sample preparation of organic com-pounds from water samples. The use of this combination enabledlower limits of detection (LODs) and higher enrichment factors (EFs)to be achieved. The SPE column is a key factor in the isolation andpurification efficiency of the target analytes. Multiwalled carbonnanotubes (MWCNTs) were chosen as the sorbent to isolate and toconcentrate OCPs because of their large surface area [106]. Also,otherSPE conditions, including the flow rate of the sample solution, thebreakthrough volume, the types and the volume of eluting solventand the salt concentration, were also tested in order to achieve ac-ceptable recoveries upon extraction of large volumes (210 mL) of

samples for environmental estrogens[132].The treatment of soil samples includes extraction with water and

sonication[112,140]. For sediments, extraction with water andshaking[78], or extraction with methanol and sonication[80,120]were applied.

For urine samples, most proceduresincluded using a superna-tant after centrifugation, adjusting the pH value, sometimes dilutionwith water or an organic solvent, such as acetonitrile, and addinga salt. After shaking and centrifugation, the extract can be used asthe sample for the SFODME step[58,79,90,93,98,136138,141143,146,147,150,158]. Most treatments for serum samples included ad-

justing the pH value, sometimes dilution with water or an organicsolvent to remove proteins, and the addition of a salt. After shakingandcentrifugation, the extractcan then be used as the aqueous phase

for the SFODME step[88,93,142144,146,148,150,159].For food samples, procedures required:

only filtration and centrifugation, or dilution with water for fruitjuices, wine and honey [52,66,110,115,116,121,122,124,126,128,151153,158,163];

extraction with an organic solvent and centrifugation[162]; extraction with an organic solvent and microwave-oven diges-

tion for fruits[66,117]and cereals[113]; extraction with an organic solvent and sonication for tomato

[130], fruits and vegetables[155], summer crops[101]or tea[109]; and,

using water and sonication for tobacco[161].

Due to its advantages over traditional techniques, the QuEChERSmethod was the most common method employed for sample prep-aration; it was used for analysis of fungicides in grapes [123].Traditional Chinese medicines were simply submitted to SFODMEor diluted with water[154,156,157]. For cosmetics, the treatmentincluded extraction with an organic solvent and dilution with water[82], or the use of an organic solvent and an acid, assisted by soni-cation[85].

4.5. Application of derivatization in solidified floating organic dropmicroextraction (SFODME)

Sometimes a derivatization step is needed prior to detection.Whenusing HPLCwith fluorescencedetection(FLD), derivatization

wasincluded forestrogenichormones usingp-nitrobenzoyl chloride

at 35C [131,133] and 9-fluorenylmethyl chloroformate at pH 8.5 fordetection of kanamycin in wastewater and soil samples[140].

When using UV detection, volatile aldehyde biomarkers in humanblood samples used 2,4-dinitrophenylhydrazine as a derivatizationreagent and formic acid as a catalyzer, and biogenic amines [153]were derivatized usingp-benzoyl chloride at pH 10.

Amino acids were derivatized with isobutyl chloroformate (IBCF)in aqueous solution, extracted and then determined in tobaccosamples using GC and mass spectrometry (MS) detection[161]. Theuse of GC also required a previous derivatization step to convert polarand non-volatile analytes into volatile derivatives, so this step wascarried out for phenolic compounds[91]and for haloanisoles andhalophenols using acetic anhydride and potassium carbonate[151].

4.6. Salt addition and pH

The salting-out effect has been used universally in LLE and solid-phase microextraction (SPME). Sodium chloride was added into thesample solution to increase the ionic strength, to decrease the sol-ubility of the analytes in the sample solution and to enhance theextraction efficiency. However, the decrease in extraction efficien-cy observed at higher salt concentrations can be explained by theaddition of a salt being able to restrict the transport of the analytes

to the extracting drop due to the increase in sample viscosity. Byincreasing the salt concentration, the diffusion of analytes towardsthe organic solvent becomes more and more difficult. In addition,NaCl dissolved in water might change the physical properties of theNernst diffusion film and thus reduce the rate of diffusion of thetarget analytes into the drop.

Frequently, the effect of salt addition on extraction efficiency wasevaluated by increasing the NaCl concentration up to 30% (w/v). Insome studies, the extraction efficiency of the analytes was notchanged by increasing the concentration of NaCl, and so it ceasedbeing used (32% of the total published studies). Other experi-ments showedan increase in the extraction efficiency in the presenceof salt, so values in the range 130% (w/v) NaCl were selected.

The value of pH plays a significant role in the extraction of ion-

izable compounds. Thus, very different pH values were selected indifferent applications {e.g., pH 3 for quercetin[155]; pH 4 for py-rethroids[109]; pH 2[91], 4 [90], 5 [93]or 6[92,94]for phenoliccompounds; pH 5[127]or 6 for triazole fungicides[114]; pH 5 fordecabrominated diphenyl ether[78]; pH 6 forpyrazoline derivatives[150]; pH 6.4[58]and pH 10.2[147]for am-phetamines; pH 7 for methyl methacrylate [86], fungicides [120,124],insecticides[128]and OCPs, PCBs and pyrethroid pesticides [65];pH 7.6 for haloanisoles and halophenols[151]; pH 8.5 for kanamy-cin [140] and antifungal drugs [143]; pH 9 for nicotine, cotinine [138]and opium alkaloids[144]; pH 10 for lovastatin[137]and halo-peridol [142]; pH 11 for herbicides [112], anilines [75] and

chloroanilines[76]; and, pH 12 for carvedilol[148], volatile alde-hydes with formic acid[88], chlorpyrifos with 0.2% (v/v) acetic acid[97], nitrophenols in 0.8 M HCl[57], neonicotinoids in HCl[116],aliphatic amines in 3% (w/v) NaHCO3 [74], macrolide antibiotics withsodium carbonate[141]and antidepressant drugs with KOH[146]}.

4.7. Extraction temperature

Generally, in LPME experiments, higher EFs can be obtained byincreasing the temperature, because heating facilitates the masstransfer of analytes from the sample to the organic solvent andthus increases the efficiency of the extraction. Normally, a simplewater bath was employed to heat the sample solution. The effectof sample-solution temperature on extraction efficiency was typ-

ically studied in the range 1080C. Results showed that, by



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increasing the temperature, the extraction efficiency was in-creased up to a maximum value near 5070C. However, in othercases (70% of the studies), values near room temperature up to40C were selected.

4.8. Figures of merit

Table 2sets out the linearity and the LODs obtained in the

SFODME methods developed for organic analytes.

5. Applications to the determination of inorganic compounds

5.1. Types of analyte

Inorganic analytes have alsobeen preconcentrated using SFODME,although there have been fewer applications than those for organiccompounds. A total of 30 elements have been preconcentrated byapplying classic SFODME or one of its variants (Fig. 3).

Aluminum[165,166], thallium[167169], lead[170179]andbismuth[179]are the metals from columns 1315 in the PeriodicTable of Elements thathave been determined. Antimony [180182],

arsenic [183186], selenium [187190] and tellurium [191] are themetalloids thathavebeen microextracted using SFODME.Withrespectto the transition metals, cadmium[177,179,192196], chromium[178,197199], cobalt[178,179,199204]], copper[199,204214],gold [169,215],iron [166,214,216,217], manganese [199,218],mercury[179,219,220], molybdenum [55,221], nickel [178,196,203,204,222],palladium[179,223225], rhodium[226], silver[227,228], vana-dium[229,230], yttrium[231], zinc[54,213]and zirconium[232]have all been quantified using SFODME. Several lanthanides eu-ropium[233], dysprosium[231], lanthanum[233]and ytterbium[233] and the actinide uranium[234]have all been determinedin different types of sample. Copper has been the element moststudied using the SFODME technique, followed by lead, cobalt andcadmium.

Most of the proposed methods have been developed for the de-termination of the total metal content; nevertheless, several of theminclude speciation studies. Thus, the speciation of antimony [181,182],arsenic[183186], chromium[198], iron[217], selenium[190]andthallium[168]has been carried out by submitting the sample tochemical reactions in order to change the oxidation state of theanalyte. Readers can also find some details in the last paragraph ofsub-section 5.6(below). The quantification of iron in its two oxi-dation states has been achieved by Moghadam et al.[216]usingchemometric methods.

5.2. Sample (aqueous phase) volume

For the volume of the aqueous phase submitted to the SFODMEprocedure, the most frequently selected values (in ~87% of cases)were 525mL, though some authors used volumes as high as 100 mL[194,205,207]and 160 mL[192], while intermediate values were4060 mL[168,182,216,229]. However, such volumes can lead todifficulties with phase separation due to the solubility of the organicsolvent in an aqueous sample.

5.3. Sample pre-treatment

The simplicity of water samples means that, in many cases, thereis no need for sample pre-treatment prior to preconcentration. Fil-tration to remove particulate matter from a suspension, especiallyfor environmental water, and, in some cases, acidification in orderto prevent adsorption of the metal ions on the inner flask walls, arethe unique treatments carried out. Water samples are generally fil-tered through0.45-m pore-size membrane filters shortly after beingcollected; they are then kept at 4C until needed for analysis. Severalauthors recommend that the plastic and glass materials used in theanalysis be maintained in acid over 24 h and then rinsed with high-

purity water; such a practice is very common in all metal-analysisprocedures[54,179,189].Even though, for most SFODME procedures, the matrix of water

samples does not influence the analysis, Dadfarnia et al. [188] founda relatively high degree of interference in the speciation of sele-nium in waters. They proposed using cation-exchange resins toremove the interfering metals. The combination of the SPE andSFODME steps has been proposed as an ultra-preconcentration tech-nique [185] for the determination of arsenic in water, increasing thesensitivity with respect to similar determinations[186]since thepreconcentrated sample volume is increased considerably. In thiscase, the SPE eluate is used as a disperser solvent.

Minimal sample treatment has been proposed for beverage andurine samples, as their matrices, slightly more complex than waters,

did not prevent microextraction being accomplished. Filtration isalso the only treatment required for beverages[196,209], as is cen-trifugation prior to filtration for urine samples[198].

When dealing with solid samples, the analytes need to be ex-tracted from the matrices in order to obtain solutions suitable forSFODME. Wet digestion by mixtures of mineral acids and, occa-sionally, hydrogen peroxide is the most widely used technique[168,169,175,191,195,196,210,211,214,220,221,225,226,228] . Some-times, digestion is assisted by microwaves[55,190,233], thusdecreasing the risk of contamination or analyte loss as well as

Fig. 3. Applications of solidified floating organic drop microextraction (SFODME) to the determination of inorganic compounds.



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Table 2 (continued)

Analyte Sample Mode Detection Sample pretreatment Extraction conditions

Triazole fungicides Water (lake, stream, well) VA-DLLME-SFO HPLC-DAD 10 mL water; 1.5 g NaCl; pH 6; filtration 12 L 1-dodecanol; 200 L metvortex mixing, 1 min; centrifug3500 rpm, 3 min; cooling, ice bamixed with 12 L methanol; 15

Triazole fungicides Juice USAE-SFODME HPLC-UV 0.5 mL filtrate juice; 9.5 mL water; 250 g L 1

NaCl50 L 1-undecanol; sonication, 30C; centrifugation, 4200 rpmmixed with 20 L methanol

Trihalomethanes Drinking water SFODME GC-MS 10 mL water; 3 M NaCl 7 L 1-undecanol; stirring, 15 m750 rpm; 60C; cooling, ice bat1 L injected

Trihalomethanes Water (tap, swimmingpool)

DLLME-SFO GC-ECD 18 mL water; 0.9 g NaCl 50 L 1-undecanol; 0.7 mL acetmanual shaking, 1 min; centrifu3000 rpm, 10 min; cooling, frid1 L injected

Steroid hormones Water (tap, r iver) DLLME-SFO HPLC-UV 5 mL water; 0.3 g NaCl; filtration 10 L 1-undecanol; 200 L metcentrifugation, 4500 rpm, 3 minice bath; mixed with 35 L DMinjected

Strobilurin fungicides Fruit juice UASEME-SFO HPLC-DAD 5 mL juice (diluted at 1:1 ratio with water);1% (w/v) NaCl

30 L 1-undecanol; 15 L 5 mgTween-80; sonication, 1 min, 6centrifugation, 3000 rpm, 5 minice bath, 10 min; 10 L diluted phase (1:1) injected

Sudan dyes Foodstuffs and water DLLME-SFO HPLC-UV 10 mL sample; 1 g NaClWater: filtrationFood: 2 g; 5 mL ethanol; shaking, 20 min,30C; centrifugation, 4000 rpm, 10 min;dried under nitrogen; diluted to 10 mL with10% (w/v) NaCl (pH 7)

100 L 1-dodecanol; 400 L of ewater bath, 20 min, 70C; centr4500 rpm, 5 min; cooling, ice bamixed with 50 L methanol; 20

Synthetic antioxidants Beverages DLLME-SFO HPLC-UV 5 mL beverage; filtration; pH 6; 0.3 g NaCl 90 L 1-octanol; 1 mL methanocentrifugation, 4000 rpm, 5 mininjected

Volatile aldehyde

biomarkers

Human blood DLLME-SFO HPLC-DAD Serum; 750 L methanol; 500 L of

supernatant diluted with water to 5 mL;0.75 g NaCl; 30 L 20 mM 2,4-dinitryophenyl hydrazine; 40 L formic acid;40C; 10 min

50 L 1-dodecanol; 50 L meth

centrifugation, 4000 rpm, 2 minice bath, 5 min; mixed with 50 methanol; 5 L injected


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reducing the time required in comparison with classic wet-acid di-gestion. Classic dry calcination in an oven, in order to obtain ashesbefore digestion treatment, has also been recommended in someinstances[176,181,212,230].

As previously stated, several SFODME applications tackle the de-termination of inorganic species in different oxidation states. Whena complexing agent reacts with only one form of an inorganic analyte,this can be exploited for the purpose of speciation, and themicroextraction has to be applied in two sample aliquots in whichthe metal ion is in different oxidation states. Thus, oxidation or re-duction processes have to be included in the sample pre-treatment.In such cases, a mixture of sodium thiosulfate and potassium iodidewas used to reduce As(V)[183186], concentrated nitric acid tooxidize Fe(II)[217], hydroxylamine hydrochloride to reduce Cr(VI)[198], both ascorbic acid/potassium iodide[181]and L-cysteine/hydrochloric acid [182] to reduce Sb(V), and hydrobromic acid [188]and hydrochloric acid[190]were used for the reduction of Se(VI).The total concentration of the analyte was then obtained from thetreated sample aliquot, and speciation was achieved by the differ-ence with respect to the non-treated aliquot.

5.4. Application of complexation in SFODME

A neutral form is required to extract inorganic ions intoan extractant organic phase. For this reason, practically allSFODME applications for metals are based on the formation ofhydrophobic chelates, sothiocarbamates [e.g., ammoniumpyrrolidinedithiocarbamate (APDC) and diethyldithiocarbamate(DDTC)] have been used in the determination of arsenic[183186],selenium [189,190], lead [171,173,175,176,179], antimony [181,182],cobalt [179,201,204], copper [204,208], palladium [179,224,225], tel-lurium[191], mercury[179,220], silver[228], nickel[204,222]andcadmium[179]. In addition, 8-hydroxyquinoline (8-HQ) has beenproposed for chelating copper[209,211,212], cadmium[195], mo-lybdenum[55,221]and cobalt [200], and the latter was alsocomplexed with 2-nitroso-1-naphthol [202]. Another naphthol usedto a greater extent is 1-(2-pyridylazo)-2-naphthol (PAN). The che-

lation of cadmium [193], copper [206,210,213], zinc [54,213], cobaltand nickel[203], thallium[167,168]and several rare-earth ele-ments[231,233]has been carried out with PAN, while the SFODMEdetermination of lead[170,172,174], cadmium and nickel[196]hasbeen accomplished in water and tea using dithizone. For the spe-ciation of arsenic[185]and selenium[188], diethyldithiophosphate(DDTP) and 3,3-diaminobenzidine (DAB) have been used,respectively.

The use of surfactants has also been considered for SFODME.Anionic surfactant sodium dodecylbenzenesulfonate (SDBS) has beenadded to the organic phase to facilitate the extraction of the hy-drophilic complex formed between Cu(II) and Neutral red; in thisway, the chelate acquires a more hydrophobic character [207]. Tritonhas been used with a similar objective for the determination of

cadmium and nickel[196]as well as cobalt[202]in water samples.Some authors have formed ion pairs (IPs) to achieve the extrac-tion of metals into an organic drop [166,169,187,192,197]. Moghadamet al.[166]proposed the simultaneous determination of iron andaluminum by extracting the IP formed between the cationic com-plexes of Fe(III)-morin and Al(III)-morin, with ClO4 as the bulkycounter anion. Surfactants are in some cases also involved in IP for-mation [169,187,192,197]. Dadfarnia et al. [192] determined cadmiumusing CdI42 complexes extracted in an organic solvent containingmethyltrioctylammonium chloride. Gold and thallium were ex-tracted into 1-undecanol through the formation of IPs between thesurfactant benzyldimethyltetradecyl ammonium and metal chloro-complexes (AuCl4 and TlCl4)[169]. A hydrophobic IP was formedbetween the cationic complex of Cr(VI) with 1,5-diphenylcarbazide

(DPC) and anionic surfactant SDS[197]. An indirect IP-DLLME-SFO

procedure has been developed for the determination of selenium:in this case, the triiodide anion, formed in the reduction of Se(VI),reacts with the surfactant cetyltrimethylammonium bromide (CTAB)contained in the extractant phase[187].

The chelating agent in about 75% of cases was added to theaqueous phase in order to extract the chelate thus formed[165,180,185,224,227]; however, some authors prefer to dissolve thisagent in the extractant phase and to add the mixture to the samplesolution[190,203,229].

The extraction of ionic analytes using SFODME has also been pro-posed in the absence of a chelating agent. The preconcentration ofrhodium can be achieved through a ligandless (LL) SFODME pro-cedure, in the form of its hydroxide in 1-dodecanol from a basicaqueous solution[226]. As previously indicated, the presence ofanionic surfactants in organic phase has been shown to extractanalytes efficiently from acid aqueous solutions. The micelles formedwhen the surfactant concentration is higher than its critical micel-lar concentration (CMC) cause adsorption or binding of the cations[194]. The addition of SDBS at a concentration higher than its CMCproduces an aggregation of surfactant molecules forming spheri-cal micelles with negative charge that extract metal ions from theaqueous phase. In this way, an LL-SFODME procedure has been pro-posed for the determination of cadmium in water samples[194].

Coacervates composed of reverse micelles formed with decanoic acidand tetrahydrofuran (THF) were used to extract Cr-DPC-SDS IPs [197].Lopez-Garca et al.proposed a saving in reactives by using undecanoicacid for the preconcentration of lead, cadmium[177]and mercury[219]. This weak acid acts as a complexing and extracting agent.

However, the effect of coexisting ions in real samples alwaysneeds to be considered for the recovery of the analytes when in-organic compounds are determined. The interference may be theresult of competition from other ions for the chelating agent andtheir subsequent coextraction with the analyte. The interferencecaused by Cd(II), Zn(II) and Cu(II) in the determination of Co andNi was removed by adding EDTA[203]. Rezaee et al.[165]foundthat Fe(III) and Cu(II) can interfere with the extraction recovery ofAl(III) ions and proposed their elimination using SCN and ascor-

bic acid/potassium iodide solutions, respectively. The interferenceby Fe(III) in the determination of Se(IV) was also eliminated usingEDTA[189]. An important degree of interference was observed inthe extraction of Se(IV) as a piazoselenol complex in 1-undecanol,with a cation-exchange resin used to eliminate the interferingcations[188]. The introduction of displacement reactions in amicroextraction procedure can improve the selectivity of the method[228].

5.5. Salt addition and pH

A decrease in analyte solubility in the aqueous phase leads toan increase in extraction efficiency; nevertheless, high salt concen-trations reduce the analyte diffusion rate to the organic phase and

thus decrease the extraction efficiency. In those procedures for whichthe presence of salt benefits extraction, concentrations between0.01 M [204,223] and ~0.2 M [226] were generally selected, with theuse of sodium chloride and sodium nitrate. Salt concentrations inthe 1.74 M range have also been recommended[184,209,212].

The pH selected for the aqueous phase is that which favors for-mation of the chelate and maintains it in a suitable form to beextracted {e.g., when APDC is the chelating agent, an acidic mediumis always used, in most cases with the pH adjusted to between 1[183,186]and 3[176,189]}.

5.6. Extraction temperature

Room temperature is generally selected during the extraction step

because the solubility of the organic phase can increase at high tem-



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peratures, and the complex of the inorganic analyte can be degraded.However, temperatures below 20Cprevent the extractant from beingefficiently dispersed. Nevertheless, 35C has been selected as theextraction temperature for the determination of uranium[234]andmercury [220], and, for SFODME using undecanoic acid, an extractiontemperature of 50C has been applied in order to keep the organicsolvent in liquid form[177,219].

5.7. Figures of merit

Table 3summarizes linearity, LODs and EFs obtained in theSFODME methodsdeveloped for inorganic analytes. For these species,EFs between 12[206]and 1520[185]were found in the literature.Nevertheless, comparison of the different values is difficult, becausethe authors used different criteria to obtain the EFs.

6. Different modalities of solidified floating organic drop

microextraction

Tables 2 and 3show the main features of the published proce-dures that focus on the determination of organic and inorganicspecies using SFODME for preconcentration purposes. Most of thesemethods applied the microextraction step in the conventional mode;

the rest employed some kind of modification (Fig. 4).In DLLME-SFO, the use of a disperser solvent to increase the

contact surface between the organic extractant and the aqueousphase produces a cloudy solution with no need for agitation, al-though some authors recommend agitation of the ternary mixture.In any case, the time required to achieve the extraction equilibri-um is reduced considerably compared with conventional SFODME.

In USAE-SFODME, dispersion of the extraction solvent is achievedwith the assistance of ultrasound energy instead of a dispersersolvent, and the method has been used with numerous applica-tions. Fazelirad and Taher[169]compared the extraction efficiencyachieved when dispersing the extraction solvent using ethanol orby applying ultrasound energy. Similar results were found in bothcases, and, in this case, USAE-SFODME was selected in order to de-

crease organic solvent consumption.A UASEME-SFO method for the determination of cadmium and

nickel using Triton to improve the emulsification has also beendeveloped[196].

VA-SFODME has been used for the determination of molybde-num[55,221], cobalt[202]and duloxetine[139]. VA has also beenused to accelerate reaching extraction equilibrium in DLLME-SFOfor the determination of benzotriazole ultraviolet stabilizers[87],phenolic compounds[92], organophosphorus pesticides[100],triazole pesticides[114], bisphenol A[90], 2-agonists [136],duloxetine[139], PBDEs[80], PEs[83], and triclosan and 2,4-dichlorophenol[94].

When the analyte is microextracted with coacervates com-posed of reverse micelles using extraction and disperser solventsand solidification of the enriched organic drop, the technique isknown as supramolecular solvent (SM)-DLLME-SFO. Coacervates areimmediately produced in the solution and the extraction equilib-rium rapidly reached. Lead [175], chromium [197] and parabens [85]have been determined in water and cosmetic samples using SM-DLLME-SFO. The reverse micelles were formed using decanoic acidand were dispersed in a THF-water mixture. THF acts as a disper-sant and in assembling the micelles.

BE into aqueous acid medium from an enriched organic phaseobtained by SFODME was required for the determination of sele-nium by HG-AFS, because the high viscosity and low volatility ofthe extraction solvent made it incompatible with hydride-generation analysis[189]. In this case, the BE step was assisted by

ultrasound. BE has also been recommended for the determinationof mercury in waters with a high calcium content in order toovercome the drawback caused by a high background appearingduring electrothermal atomization[219]. The same methodologywas applied for determination of several organic compounds usingCE[90,93,136].

Afzali et al. used a modified DLLME-SFO for the determinationof lead[171]and palladium[225]in water samples. A displace-ment reaction takes place, in which the targeted metal substitutesanother metal, whose complex has lower stability. The technique,called displacement (D)-DLLME-SFO, has proved effective in de-creasing interferences from coexisting metal ions and improvingthe selectivity of the procedure without using masking agents.For the determination of lead, zinc is displaced from its complex

with APDC[171], whereas, for the determination of palladium,copper is displaced from its DDTC complex[225]. The displace-ment procedure was used for the preconcentration of silver bySFODME[228].

Fig. 4. The different modalities of solidified floating organic drop microextraction (SFODME).



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6.1. Nature and volume of the extraction solvent

The selection of a suitable extraction solvent is crucial for op-timizing the SFODME process. The extraction solvent must meetcertain requirements:

low volatility and low water solubility in order to remain stableduring the extraction period;

a high affinity for analytes;

a melting point near room temperature (in the range 1030C);and,

in the case of being chromatographically analyzed, to be sepa-rated from the analyte peaks in the chromatogram and show goodchromatographic behavior.

6.1.1. Organic compoundsFor the extraction of organic compounds, 1-undecanol (melting

point: 1315C) was found to give the best extraction efficiency,while its chromatographic peak was easily distinguished from theanalyte peaks. In addition, because of its stability, low vapor pres-sure and low water solubility under extraction conditions,1-undecanol is one of the more frequently selected extraction sol-vents (accounting for 39% of all published studies). Another

frequently used solvent is 1-dodecanol (appearing in 43% of studies).Other solvents used include 2-dodecanol [56,86,93,98,161], 2-decanol[159], n-hexadecane [103105,145], cyclohexanol [151,152], 1-octanol[116,163], cyclohexane [75], a mixture of acetophenone/1-undecanol(1:8) [84], a mixture of 1-undecanol/chloroform [138] and1-hexadecanethiol[95].

Moradi and Yamini reported an efficient, environment-friendlymethod for the analysis of parabens using solidified floating vesic-ular coacervative drop microextraction (SFVCDME)[85]. 1 L ofsupramolecular solvent, produced from the coacervation of deca-noic acid aqueous vesicles in the presence of tetrabutylammonium(Bu4N+), was directly injected into an LC, with no need for dilutionor solvent evaporation[85].

Ionic liquids (ILs) consist of organic cations and organic or in-

organic anions with some special characteristics, such as negligiblevapor pressure, good chemical and thermal stability, a good abilityto dissolve both organic and inorganic compounds, and good mis-cibility with both water andorganic solvents. Nevertheless, we foundonly one article devoted to application of ILs in SFODME mode: Yanget al. used trihexyl(tetradecyl)phosphonium hexafluorophosphate([P14,6,6,6]PF6) for the determination of benzoylurea insecticide in fruit

juice by HPLC[128].

6.1.2. Inorganic compoundsAmong the organic solvents used for extracting inorganic analytes,

1-undecanol is by far the most widely used, specifically in 62% ofthe published procedures, and 1-dodecanol is the second most usedat 32%. The saturated fatty acids undecanoic acid[177,219]and de-

canoic acid have been proposed as extractants[175,197].Guo et al. used a mixture of solvents as extractant phase for thedetermination of heavy metals in environmental waters byETV-ICP-MS[179]. The relatively high boiling point of 1-dodecanolprevented the application of high temperatures in the ETV dryingstep without analyte losses. However,p-xylene, which also effec-tively extracts heavy metals and has a lower boiling point than1-dodecanol, could not be used because of its low viscosity, whichprevented the solidified droplet from being collected. Consequent-ly, a mixture 1-dodecanol/p-xylene was used as extraction solvent,and it was removed from the graphite tubes at 200C, withoutanalyte losses[179].

The effect of the organic solvent volume on the analytical signalwas studied in the range 4400 L. Normally, the analytical signal

increases slowly with increasing solvent volume in a narrow range.

It then decreases when the solvent volume is increased. Based on LLEequations, the rate of the analyte transport into the microdrop is di-rectly related to the interfacial area between the two liquid phasesand inverselyrelated to the organic-phase volume. Thus, by increasingthe drop volume, the effect of the interfacial area predominates andthe analytical signal is increased. By further increasing the microdropvolume, the dilution effect predominates and the analytical signal de-creases. Most of the proposed procedures use extraction solventvolumes of 520 L (organics) and 2090 L (inorganics), whereasvolumes out of these ranges have been selected less frequently.

6.2. Selection of the disperser solvent for DLLME-SFO

When the extraction solvent and disperser solvent are added tothe aqueous sample solution, the extraction solvent will form veryfine droplets after shaking the solution for a few seconds. The dis-perser solvent should be miscible with both water and the extractionsolvent, and thus produce very fine droplets of extraction solventwhen the mixture of extraction and disperser solvent is rapidly in-

jected into the aqueous sample. Methanol, ethanol, acetonitrile andacetone were chosen for most applications. The volume of the dis-perser solvent varied in the range of 0.052.5 mL.

6.3. Stirring rate

Sample agitation is another important parameter thatplaysa largerole in enhancing extraction efficiencyand reducing extraction time,as the thickness of the diffusion film in the aqueous phase de-creases. In most studies on organic compound preconcentration,samples were continuously agitated at different rates of up to1250 rpm using a magnetic stirrer. Normally, the relative peak areaincreases by increasing the stirring rate up to a maximum value,achieved at rates as high as 4001250 rpm[36,52,62,67,71,73,79,81,84,91,98,99,102,107,111,112,128,140,143,150,158]. Neverthe-less, very high stirring speeds can decrease the extractionefficiency due to spattering and damaging the organic droplet[176,190,194,205,233].

For analysis of inorganic compounds, stirring also enhances thekinetics of chelate formation; magnetic stirring speeds of 800 rpm[176,233] to1250 rpm [198] have beenapplied in SFODMEmethods.Those procedureswith stirring speedsaround 300 rpmgenerally cor-respondedtohighsample-volumeanalysis{i.e., 100 mL[194,205,207]}.Lpez-Garcaet al.[177,219]recommendedtwosteps of different stir-ring powers, the second at a lower speed in order to group all theenricheddroplets of undecanoic acid. In thiscase, the centrifugationstep usually applied to disrupt the cloudy solution is omitted.

6.4. Extraction time

SFODME depends on equilibrium rather than exhaustive extrac-tion. To increasethe repeatability and the sensitivityof the extraction,

it is necessary to choose an extraction time during which equilib-rium is reached between the aqueous and the organic phase. Therelative peak area for each analyte increases by increasing the ex-posure time and then remains constant, and it is this time that mustbe selected. Times of 1030 min [36,62,67,68,71,73,79,8486,91,99,102,105,107,111,112,128,140,150,162]or higher values of3090 min [52,67,68,70,73,81,84,86,98,143,158] have beenselected for the extraction of organic compounds. Most of the op-timized SFODME procedures for inorganic compounds have appliedextraction times in the 1030 min range. Lower and higher timeshave been proposed, such as 5 min[170]and 60 min[205]for thedetermination of lead and copper, respectively, in water samples.

In USAE-SFODME procedures, energy is applied by using ultra-sound baths for times of 220 min. In VA-SFODME, extraction

equilibrium is achieved in shorter times. Several SFODME



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applications assisted by vortex agitation are found in the literaturefororganic compounds[58,80,83,90,92,100,109,114,116,136,153,156]and inorganics[55,202,221]with extraction times of 0.52 min.

By contrast, extraction time is not an important variable forDLLME-SFO, because the surface areas between extraction solventand sample solution are infinitely large after formation of the cloudysolution. The mass transfer of analytes from sample solution to ex-traction solvent therefore happens so fast that the extractionequilibrium is achieved in a short time, and the analysis time forthe extraction procedure is shortened considerably. The time-consuming step in DLLME-SFO is the solidification of the floatingextractant in the ice bath for 510 min.

6.5. Disruption of the cloudy solution

In most of the proposed procedures, centrifugation is the way se-lected to disrupt the cloudy solution. Centrifuging speeds of4006500 rpmwere applied fortimes of 210 mingenerally.However,several authors omit the centrifugation step and aggregate the finedroplets of the extractant directly while coolingthe mixture. A coolingstep is included in all the published procedures, most employing anice bath within the mixture, and is maintained in the 310 min rangegenerally, although Sahin et al. proposed cooling in a refrigerator for

10 min [194,205,207]. The cooling step allows the enriched phase tobe easily collected, generally by being transferred into a conical vial,where it melts immediately, taking into account the low melting pointof the organic solvents used in SFODME. When undecanoic acid isused[177,219], heating of the enriched phase to 50C, once sepa-rated from the sample, has been recommended, taking into accountits slightly higher melting point and density than commonly usedsolvents, such as 1-undecanol and 1-dodecanol. Disruption of thecloudy state can also be achieved by adding a new portion of dis-perser solvent, which acts as demulsifier[104].

Several authors recommend dilution of the enriched phase priorto being analyzed in order to decrease its viscosity and thus facilitateits collection. Ethanol, methanol and acetonitrile are the solventsmost commonly used for this purpose, although 1-propanol and

dimethylformamide have been used to a lesser extent. Chen et al.[233] indicated the convenience of diluting the extract of1-dodecanol with THF to facilitate not only sample introduction intothe ETV, but also removal of the organic solvent through the ETVtemperature program. In this way, in the presence of THF, theremoval of the extraction solvent from the furnace has been carriedout at 220C without analyte loss. Moreover, enhancement of sen-sitivity in experiments with metal complexes in the presence of adiluent, THF, was attained. The volume of the diluted extract is gen-erally found in the 150500-L range, this value being conditionedmainly by the minimum volume necessary to carry out the ana-lytical measurement.

7. Concluding remarks and future trends

In the past decade, we have witnessed the attention of analyt-ical chemists constantly growing towards the development of newmicroextraction techniques that could better satisfy the require-ments of green analytical chemistry.

A considerable proportion of the publications describing SFODMEprocedures has been devoted to the analysis of various types of watersamples due to the simplicity of this matrix; however, applicationof the technique in other fields, such as analysis of biological fluids,foods,or soil and sediments, increased in recent years. In many cases,the simplicity of water samples enables avoidance of sample pre-treatment before the preconcentration step, or limits it to simplefiltration in order to remove particulate matter, especially inenvironmental waters. More complicated aqueous samples, such as

fruit juices, wine and honey, frequently require only filtration and

centrifugation or dilution with water. Solid samples, such as foods,fruits and vegetables, soils and sediments, necessitate the inclu-sion of a pre-treatment step for extraction of the analyte from thesolid matrices into an aqueous phase suitable for SFODME analy-sis. The conjunction of SPE and SFODME allows sensitivity to beincreased, since the preconcentrated sample volume is consider-ably increased. However, there arerelatively few publications devotedto coupling SFODME with SPE or QuEChERS.

Most of the reported SFODME procedures performed themicroextraction step in the conventional mode, while the restemployed certain modifications aimed at improving the stirringor agitation, and thus reducing the extraction time. This can beachieved by using a disperser solvent in DLLME-SFO, applyingultrasound energy in UA-DLLME-SFO and USAE-SFODME, or vortexmixing in VA-DLLME-SFO and VA-SFODME. The addition of a sur-factant canalso enhance the SFODME process. In addition to reducingthe use of organic solvents, the current trend in analytical chem-istry is towards automating procedures. However, automation ofSFODME is complicated and difficult, and has not yet beenaccomplished.

Most of the SFODME methods described for inorganic analysiswere developed for the determination of a total metal content; nev-ertheless, several of them include speciation studies. We can expect

an increase in publications in this area. However, we did not findany articles devoted to SFODME procedures for inorganic anions.Application of ILs is currently a hot topic in analytical chemistry,but we found only one article devoted to application of ILs inSFODME. We also anticipate growth in this field.

The SFODME method has advantages, such as simplicity, goodaccuracy and precision, short extraction times, low costs, andminimum organic solvent consumption. Since a fresh portion oforganic solvent is used for each extraction, there is no memory effect,and since the volume of the organic phase is very low, largepreconcentration factors are achievable. Since a specific holder isnot required to support the organic microdrop, stirring the samplesolution at very high speeds is possible. Also, by using a multi-stirrer, parallel extraction of many samples is possible.

Compared with other microextraction methods, the main draw-back of the SFODME method is the limitation in the selection of anextraction solvent. However, many solvents with suitable meltingpoints are available.

Acknowledgments

The authors acknowledge the financial support of the ComunidadAutnoma de la Regin de Murcia (CARM, Fundacin Sneca, Project15217/PI/10) and the Scientific Grant Agency of the Ministry of Ed-ucation of the Slovak Republic and the Slovak Academy of Sciences(Grant No. VEGA 1/0010/15). This work was supported in partthrough the developmentof bilateral cooperation between the Slovakand Spanish universities within the Operational Programme Edu-

cation financed by the European Social Fund (ESF) (Project ITMS26110230084).

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