Trends in solid-phase microextraction for, Artykuły naukowe, SPME i HS-SPME
[ Pobierz całość w formacie PDF ]//-->trends in analytical chemistry, vol. 18, no. 8, 1999557Trends in solid-phase microextraction fordetermining organic pollutants inenvironmental samplesèìA. Penalver, E. Pocurull, F. Borrull, R.M. Marce *éèèèDepartment de Qu|mica Anal|tica i Qu| mica Orga nica, Universitat Rovira i Virgili, Imperial Tarraco 1,43005 Tarragona, SpainSolid-phase microextraction (SPME ) is arecent technique for sample preparation. Ithas been used successfully to analyze envi-ronmental pollutants in a variety of matricessuch as soils, water, and air. SPME is a sol-vent-free technique which has a number ofadvantages over more conventional samplepreparation techniques such as liquid^liquidextraction ( LLE ) and solid-phase extraction(SPE ). We describe the most recent develop-ments in SPME and some which are beingdeveloped, including its coupling to HPLCand CE, the use of new ¢bers, and the automa-tion of the entire SPME process and its appli-cation to ¢eld analysis. A summary is given ofthe most important parameters for applyingthis extraction technique to the analysis ofenvironmental samples.z1999Elsevier Sci-ence B.V. All rights reserved.Keywords:Solid-phase microextraction; Environmentalanalysis; Organic compounds; Extraction techniques1. IntroductionSolid-phase microextraction (SPME ) is a relativelynew extraction technique. Devised by Pawliszyn andhis coworkers [ 1 ], it represents a valuable advance insample preparation and has a number of advantagesover conventional techniques for extracting organiccompounds from environmental samples. Theseinclude liquid^liquid extraction ( LLE ) [ 2 ] andsolid-phase extraction (SPE ) [ 3,4 ] for semivolatileand non-volatile compounds, or headspace extraction[ 5 ] and purge-and-trap (P and T ) [ 6 ] for volatiles.Solid-phase microextraction is based on the parti-tion equilibrium of target analytes between a poly-*Corresponding author.0165-9936/99/$ ^ see front matterPII: S 0 1 6 5 - 9 9 3 6 ( 9 9 ) 0 0 1 4 5 - 4meric stationary phase, which is a coated fused silica¢ber, and the sample matrix. In order to extract analy-tes SPME does not require organic solvents, which areexpensive and may be harmful to health and to theenvironment. The technique is very simple, fast, easilyautomated, portable, and inexpensive. Also, onlysmall volumes of sample are needed. SPME can becoupled easily to gas chromatography ( GC ) and,with some modi¢cations, to high-resolution liquidchromatography ( HPLC ) [ 7^10 ]. Recently, SPMEhas also been coupled to capillary electrophoresis( CE ) [ 11,12 ], and the automated systems SPME^GC [ 13,14 ] and SPME^HPLC [ 9 ] have been devel-oped, which use conventional GC and HPLC autosam-plers. New developments in SPME devices and ¢bersmake SPME a very promising technique for ¢eld anal-yses [ 15,16 ]. Furthermore, solid-phase microextrac-tion has proved to be very useful for achieving chem-ical measurements such as in the determination of thefree concentration of organic compounds in complexsample matrices [ 17 ], and the water solubility and theoctanol-water partitioning of hydrophobic chlorinatedsubstances [ 18 ].The technique was introduced to determine rela-tively volatile compounds in environmental samples,but its use has now extended to the analysis of a widevariety of matrices and analytes. To date, SPME hasbeen used successfully to analyze gaseous, liquid andsolid samples. Also, a wide range of analytes fromvolatile to non-volatile compounds has been deter-mined by SPME. They include environmental pollu-tants such as pesticides [ 7,19^27 ], phenols [ 28^31 ],polychlorinated biphenyls (PCBs ) [ 32,33 ], polycy-clic aromatic compounds (PAHs ) [ 8,17,34 ] and, toa lesser extent, inorganic compounds [ 35 ].This review covers the most recent developmentsand applications of SPME for determining organicpollutants in environmental samples. We also summa-rize the application of the most recently developedSPME ¢bers and the effect of the various parametersß 1999 Elsevier Science B.V. All rights reserved.558trends in analytical chemistry, vol. 18, no. 8, 1999that should be considered when developing method-ologies based on solid-phase microextraction.2. SPME procedureThe SPME process comprises two steps. First, thetarget analytes are extracted from a sample matrix byexposing a coated ¢ber to the sample for a predeter-mined time. Secondly, the ¢ber is removed from thesample and the retained analytes are then desorbed inan analytical instrument in order to be separated andquanti¢ed. The desorption step is usually carried outby placing the ¢ber in a hot injector of a gas chromato-graph ( thermal desorption ). It can also be performedin an HPLC system by introducing an SPME^HPLCinterface. The entire process is very simple and can beautomated and coupled to GC [ 13,14 ] or HPLC [ 9 ].Two basic types of sampling can be performedusing SPME: direct extraction, and headspace extrac-tion, which is also called headspace solid-phasemicroextraction ( HS-SPME ) [ 1 ]. In direct sampling,the ¢ber is directly immersed in the liquid or gaseoussample, while in HS-SPME the ¢ber is suspended inthe space above the sample. Direct extraction can beapplied to the analysis of gaseous and relatively cleanliquid samples. HS-SPME is better for analysing dirt-ier liquid samples and can also be applied to solidsamples. As an example, Popp and Paschke [ 36 ] com-pared the extraction of BTEX compounds from waterby direct immersion or by extracting them from theheadspace by using two different ¢bers, 80Wmcar-boxen-polydimethylsiloxane ( carboxen-PDMS ) and100Wmpolydimethylsiloxane (PDMS ). Table 1shows the limits of detection ( LOD ) of BTEX com-pounds from both the direct immersion and headspacesampling modes. For example, with the PDMS coatingthe results for the most volatile compounds were betterusing the extraction from the headspace.The theory of the thermodynamic and kineticaspects of the SPME process, both using direct andheadspace extraction, have been studied widely[ 1,37 ]. Thermodynamic studies have shown that theamount of analyte extracted by the coating at the equi-librium time is directly proportional to the concentra-tion of the analyte in the sample, and is independent ofthe location of the ¢ber in the system. The terms `par-tition coef¢cient' or `distribution constant' betweenthe ¢ber coating and the sample matrix (Kfs), or theheadspace (Kfh), were introduced. The partitioncoef¢cients are temperature dependent and character-istic of each coating-analyte pair. Mathematical mod-els which describe the kinetics of the absorption proc-ess in both the direct and headspace extraction modes,have also been developed [ 1 ]. The equilibrium timedepends on the analyte's diffusion rate from the sam-ple into the coating and can be quite different if the¢ber is directly immersed in the sample or in the head-space. Usually, equilibration times are greater in theheadspace than with direct immersion.2.1. Parameters which affect the absorptionprocessThe amount of analyte extracted by the ¢ber inSPME can be affected by several parameters, e.g. thecharacteristics of the coating, the temperature and timeof the extraction process, the addition of salt or anorganic solvent to the sample, pH modi¢cation, agi-tation of the sample, and the sample volume. Matrixeffects and the introduction of a derivatization step canalso affect the extraction of analytes in SPME.Table 1Detection limits ( LOD ) of BTEX compounds for direct immersion and headspace sampling modes, with two different coatings,polydimethylsiloxane and carboxen-polydimethylsiloxane ( reprinted with permission from [ 36 ])SubstanceLOD ( ng l31)Headspace extraction80Wmcarboxen-PDMSBenzeneTolueneEthylbenzenem-Xylene+p-xyleneo-Xylene5550606055100WmPDMS480430225200215Direct extraction80Wmcarboxen-PDMS4535354035100WmPDMS1200550225215220trends in analytical chemistry, vol. 18, no. 8, 19995592.1.1. CoatingsThe choice of the most suitable coating is veryimportant for achieving good selectivity for the targetanalytes. The principle of `like dissolves like' can beapplied to ¢ber selection. As shown in Table 2, a num-ber of polymers is available commercially as coatingsfor SPME ¢bers. In addition to these commerciallyavailable ¢bers, some authors have developed othermethods for preparing `custom-made' ¢bers whichpresent speci¢c properties for extracting selected ana-lytes [ 38,39 ]. For example, Mangani and Cenciarini[ 38 ] have developed a method for coating a fusedsilica ¢ber with graphitized carbon black, Carbograph1. Fibers coated by phenyl, C8, and monomeric andpolymeric C18stationary phases have also been devel-oped and applied to determine PAHs in water samples[ 39 ].Polydimethylsiloxane and polyacrylate were the¢rst coated ¢bers to be used for SPME. PDMS is apo-lar and presents a high af¢nity for non-polar com-pounds such as BTEX compounds ( benzene, toluene,ethylbenzene and xylene ) [ 40 ], volatile organic com-pounds (VOCs ) [ 41^43 ] and some pesticides[ 25,27 ]. Polyacrylate is a more polar coating andextracts more polar compounds, such as phenols andtheir derivatives [ 28^31 ] and some pesticides [ 7,19^24,26,27 ]. Coatings containing the more porous andadsorbent materials, divinylbenzene ( DVB ), and car-boxen blended in PDMS or Carbowax ( CW ), havebeen introduced more recently: PDMS-DVB,PDMS-carboxen and CW-DVB. These ¢bers aremore polar than PA and are suitable for extractingmore polar compounds such as alcohols and ethers[ 44 ]. Moreover, carboxen-PDMS ¢bers have a largersurface area and show great potential for extractingorganic compounds, such as VOCs with low molec-ular weight, from the air [ 43 ]. As Table 1 shows,PDMS-carboxen ¢bers offer much better results thanPDMS ¢ber for extracting BTEX compounds fromwater. The DVB-TPR ¢ber, owing to the pore dimen-sion in the coating, is designed to reduce molecularweight discrimination between analytes which vary inchain length [ 11 ].The ¢rst SPME ¢bers were developed for GC use.Nowadays, some coating ¢bers have been developedfor use in HPLC. The desorption step in HPLC canonly be performed when the ¢ber coating is stable tothe addition of organic solvents. Only bonded phasesare compatible with all organic solvents. Table 2shows the recommended use ( GC, HPLC, or both )for the commercially available ¢bers.Table 2Fiber coatings commercially available for SPME useFiber coatingFilmthickness100Wmc30Wmc7Wma85Wmb65Wmb60Wmb75Wmb65Wmb50WmbRecom-mended useGC^HPLCGC^HPLCGC^HPLCGC^HPLCGCHPLCGCGCHPLCMaximumtemperature( for GC use )280³C280³C340³C320³C270³C^320³C265³C^ApplicationPolydimethylsiloxane (PDMS )Non-polar organic compoundssuch as VOCs, PAHs and BTEXPolar organic compounds suchas triazines and phenolsAromatic hydrocarbons andsmall volatile analytes such assolvents; air analysisVOCs and hydrocarbonsPolar organic compounds suchas alcoholsAnionic surfactantsPolyacrylate (PA )Polydimethylsiloxane-divinylbenzene (PDMS-DVB )Carboxen-polydimethylsiloxane ( Carboxen-PDMS )Carbowax-divinylbenzene ( CW-DVB )Carbowax-templated resin ( CW-TPR )aBonded phase.Partially cross-linked phase.cNon-bonded phase.b560trends in analytical chemistry, vol. 18, no. 8, 19992.1.2. Time and temperature of the extractionprocessSince SPME is based on an equilibrium distributionprocess, the maximum amount of analyte will beextracted at the equilibrium time. Stirring the samplereduces the time needed to reach equilibrium becauseit enhances the diffusion of analytes towards the ¢ber.Compounds with low distribution constants have longequilibration times, so an extraction time shorter thanthe equilibrium time has to be selected. In thisinstance, the exposure time must be controlled verywell to ensure good reproducible data.The extraction temperature has two opposingeffects on the SPME process. An increase in temper-ature during extraction enhances the diffusion of ana-lytes towards the ¢ber. Moreover, in the HS-SPMEsampling mode, the temperature helps transfer analy-tes to the headspace. On the other hand, this increase intemperature reduces the distribution constant of theanalytes because the absorption step is an exothermicprocess. Pawliszyn et al. [ 1 ] introduced a modi¢ca-tion of SPME, called internally cooled ¢ber SPME, tosolve this problem. This device allows the sample to beheated and the ¢ber to be cooled simultaneously, thusmaking the extraction process more ef¢cient.2.1.3. pH modi¢cation and addition of saltOne way of increasing the amount of some analytesretained in the ¢ber coating is given by adjusting thepH. The pH of the sample can be adjusted to valueswhich enhance the presence of neutral form in theextraction of acid and basic analytes such as phenolsand amines.Most studies have shown that by the addition of asalt, usually sodium chloride, the retention of the ana-lytes in the ¢ber coating increases. For example, forpolar analytes such as triazines, the sensitivity can beincreased by a factor of up to ten [ 27 ]. This addition ofsalt usually increases the ionic strength of the sample.This reduces the solubility of analytes which are moreeasily retained. This effect is not general and dependson the polarity of the analyte, the concentration of salt,and the sample matrix.2.1.4. Addition of solventThe addition of an organic solvent to aqueous sam-ples has not yet been widely investigated. The pres-ence of organic solvents in water samples usuallyreduces the amount of analyte extracted. For example,Eisert and Levsen [ 26 ] showed that increasing themethanol content up to 20% reduced the peak responseof triazine compounds by a factor of two. On the otherhand, in soils and sludges, the addition of water ororganic solvents to the sample matrix provides avery useful approach. Water or solvent is added toremove analytes from the matrix and to enhance thediffusion of analytes from the sample towards the ¢bercoating [ 1 ].2.1.5. Agitation of the sampleStirring of the sample enhances the diffusion of theanalytes towards the ¢ber coating and reduces theextraction time for both direct immersion and head-space extraction [ 1 ]. In HS-SPME, stirring also facil-itates mass transfer between the headspace and theaqueous phase. Magnetic stirring is the most com-monly used agitation technique but this does not mixthe sample ef¢ciently. Alternative stirring techniques,such as sonication and intrusive mixing, improve theextraction times but still do not provide perfect agita-tion of the sample. More recent developments, such as¢ber vibration and £ow-through cell design, should beconsidered, especially for automated SPME systems[ 13 ].2.1.6. Volume of the sampleThe sample volume is an important parameter to beoptimized in SPME because it is directly related to thesensitivity of the method. The volume of the sample isusually much higher than the volume of the ¢ber, andthe amount of analyte extracted is only proportional tothe partition coef¢cient, the sample concentration, andthe ¢ber volume. The partition coef¢cients of the ana-lytes between the sample matrix and the ¢ber shouldbe considered because compounds with largeKfsdonot achieve this approximation and are more affectedby changes in sample volume than compounds withsmall af¢nities to the ¢ber. For this reason, a goodcriterion for choosing the best sample volume usesthe value ofKfsfor the analytes [ 1 ].In HS-SPME, the analytes are distributed amongthe sample matrix, the ¢ber coating, and the head-space, and the headspace volumes must generally besmall in order to concentrate the analytes before theydiffuse towards the ¢ber coating. If the headspace vol-ume is too large, the sensitivity reduces considerably[ 22 ].2.1.7. Matrix effectsSome authors have investigated the effects ofmatrix on the extraction ef¢ciency of analytes [ 19 ].Organic matter such as humic and fulvic acids whichare present in real water samples can reduce theamount of analyte extracted, owing to the interactiontrends in analytical chemistry, vol. 18, no. 8, 1999561Fig. 1. SPME^HPLC interface: ( a ) stainless steel 1 / 16 in. tee; ( b ) 1 / 16 in. stainless steel tubing; ( c ) 1 / 16 in. PEEK tubing( 0.02 in. i.d. ); ( d ) two-piece, ¢nger-tight PEEK union; ( e ) PEEK tubing ( 0.005 in. i.d. ) with a one-piece PEEK union. Reprinted,with permission, from [ 1 ].between dissolved organic matter ( DOM ) and the ana-ëlytes. For example, Porschmann et al. [ 17 ] usedSPME to determine the binding state of low molecularmass pollutants such as phenols and PAHs in conta-minated water rich in humic organic matter.2.1.8. DerivatizationDerivatization can enable polar compounds in envi-ronmental samples to be determined by SPME. Thisstep and the SPME can be combined in three differentways; direct derivatization in the sample matrix, deri-vatization in the ¢ber coating, and derivatization in theGC injection port [ 45 ].The ¢rst approach is direct derivatization in thesample matrix, followed by extraction of the deriva-tives by SPME. For example, this has been used todetermine phenols by transforming them into the cor-responding acetate derivatives before SPME [ 31 ].Derivatization in the ¢ber coating can be achieved intwo ways: simultaneous derivatization and extraction,and derivatization after extraction. In the ¢rst case, the¢ber containing the derivatizing reagent is exposed tothe sample which contains the analytes. This approachis very interesting because it can be applied in ¢eldanalysis [ 1 ]. In the second case, the analytes areextracted by the ¢ber and then exposed to the deriva- [ Pobierz całość w formacie PDF ]