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ModernMethodsofSamplePreparationforGCAnalysisSjaakdeKoning1,&,Hans-GerdJanssen2,3,UdoA.
Th.
Brinkman41LECOInstrumente,DepartmentofSeparationScience,Marie-Bernays-Ring31,41199Mo¨nchengladbach,Germany;E-Mail:Sjaak.
dekoning@leco.
de2UnileverResearchandDevelopment,AdvancedMeasurementandImaging,P.
O.
Box114,3130ACVlaardingen,TheNetherlands3Van'tHoffInstituteforMolecularSciences,UniversityofAmsterdam,NieuweAchtergracht166,1018WVAmsterdam,TheNetherlands4DepartmentofAnalyticalChemistryandAppliedSpectroscopy,FreeUniversity,deBoelelaan1083,1081HVAmsterdam,TheNetherlandsReceived:7July2008/Revised:3November2008/Accepted:24November2008Onlinepublication:4February2009AbstractToday,awidevarietyoftechniquesisavailableforthepreparationof(semi-)solid,liquidandgaseoussamples,priortotheirinstrumentalanalysisbymeansofcapillarygaschro-matography(GC)or,increasingly,comprehensivetwo-dimensionalGC(GC9GC).
Inthepasttwodecades,alargenumberof'modern'sample-preparationtechniqueshasbeenintroduced,whichhavepartlysupersededtheir'classical'counterparts.
Thesenoveltech-niquesincludeoff-lineandon-line(sometimessemi-orfullyautomated)procedures,andexhaustiveextractionaswellasequilibriumtechniques.
Inordertoimproveoverallperfor-mance,aspectssuchasessentiallyorganicsolvent-lessapproaches,large-volumeinjectionandminiaturizationreceiveincreasingattention.
Inmostrecentapplications,massspectro-metricorelement-selectivedetectionhavebeenused.
Thepresentreviewdiscussestheadvantagesanddisadvantages,andrelativeperformance,ofmostofthemodernsample-preparationtechniquesandcitesanumberofillustrativeapplicationsforeachofthem.
KeywordsGaschromatographySamplepreparationIntroductionInthepast30years,sampleprepara-tion/pre-treatmentpriortochromato-graphicanalysishasrisenfromnear-obscuritytotheprominentplaceitnowholdsinmoststudiesonthetrace-leveldeterminationoforganicmicro-contaminantsinreal-lifesamples.
Traditionally,samplepreparationisstatedtobenecessaryforseveralrea-sons:improvementofthechromatographicbehaviouroftheanalyte(s),improvementofdetectabilityoftheanalyte(s),orisolationoftheanalyte(s)fromthematrix.
Today,therstaimhasbecomerela-tivelyunimportantbecauseofboththequalityofcolumnpackingsingas(GC)aswellascolumn-liquid(LC)chromatog-raphyandtheessentialsuperuousnessofderivatizingorlabellingpolaranalytestoallowtheirdeterminationbymeansofGC.
Theothertwoaims,viz.
improveddetectabilityandecientseparationfrominterferingsampleconstituents,are,however;asimportantastheyweresev-eraldecadesago.
Overtheyears,ithasincreasinglybeenrealizedthat,inmanycases,samplepreparationisthemosttime-consuming,tediousanderror-pronestepofthetotalanalyticalprocedure.
Inaddition,samplepreparationoftencan-noteasilybecoupledon-line(orat-line)withthesubsequentinstrumentalsepa-ration-plus-detectionstep,therebymak-ingautomationofsamplepreparation(butwithoutsamplepre-treatment;seeFig.
1below)plusGCanalysisessentiallyimpossible.
Moreover,itfrequently2009,69,S33–S78DOI:10.
1365/s10337-008-0937-32009TheAuthor(s).
ThisarticleispublishedwithopenaccessatSpringerlink.
comReviewChromatographiaSupplementVol.
69,2009S33adverselyaectstheoverallperformanceofananalysisthrougheectssuchaslossand/ordecompositionoftargetanalytes,andintroductionofextraneouscontami-nants.
Sucheectsself-evidentlyhavebecomemoreseriousinrecentyears,with(inter)nationaldirectivesandguidelinescontinuallydemandingimprovedper-formance—thatis,reliabledetection,identicationandquanticationateverloweranalyteconcentrations.
Overtheyears,manygroupsofworkershaveattemptedtoimprovethesituationbydesigningnewsample-prep-arationtechniques(somewhatlooselycalledmodernsample-preparationmethodsbymostauthors)toreplacetraditionalmethodssuchasSoxhlet,liquid–liquid(LLE)andambient-pres-suresolid–liquidextraction—whereoneshouldimmediatelyaddthattheformertwomethodsarestillwidelyusedtoday,specicallyinroutineapplicationsand,inthecaseofSoxhletextraction,forrefer-encepurposes.
Themodernsample-preparationtechniquesrangefromhighlyselectivemethodstobeusedforone,orafew,targetanalyte(s)ofspecialinteresttowide-ranging,andusuallyrathernon-selectiveproceduresprimarilymeantforscreeningpurposes,i.
e.
,fortargetana-lytesaswellasunknowns.
Manymethodscanbemadepartofon-line(and,thus,automatable)systems,whileotherstypi-callyareo-lineprocedures.
Toenabletheirimplementation,suitablesorbents,chemicals,membranes,low-dead-volumeconnections,cartridges,mini-columns,disks,etc.
,havebeensynthesizedand/ordesignedand,wheneverrequired,instru-mentationandancillaryequipmentwasconstructedand,frequently,commer-cialized.
Overtheyears,avarietyofapplicationsforwidelydierentanalyte/matrixcombinationshavebeenpublishedtodemonstratethepracticalityofthevariousapproaches.
Attentionhasbeendevoted,e.
g.
,todesigningintegratedanalyticalsystems,tominiaturizationandtoadequatelymatchingthesample-preparationandinstrumental-analysistime.
Themainaimswere,andstillare,toincreasesamplethroughput,improvetheoverallqualityofthesample-preparationprocedures,anddecreasetherequiredsamplesizesand/ortheuseoforganicsolventsandsorbents,andtheamountofwaste.
Onemoreaspectofinterestshouldbementionedhere,thatofimprovingdetectionlimits.
Inthepasttentofteenyears,therehasbeenanincreas-ing,andfullyjustied,emphasisontheproperidenticationand/oridentityconrmationofallanalytesofinterestineachsample.
Asaconsequence,quadrupole-orion-trap-basedmass-spectrometric(MS)detectionisthestate-of-the-artapproachtodayforalargemajorityofallchallenginganalyticalprocedures.
TheoverridingimportanceofMSdetectionwillreadilybecomeapparentfromthemanytablesincludedintheApplicationssectionofthisreview.
Evenelement-selectivedetectiononlyplaysamodestroletoday.
Itsmostprominentapplicationareasarethetrace-leveldeterminationoforgano-chlorine(and-bromine)micro-contami-nantsbyGCwithelectron-capturedetection,andtheselectivescreeningoforgano-sulphurcompoundsbyGCwithS-basedchemiluminescencedetection.
Today,awidevarietyofanalyticalmethodsisavailablefortheGCdeter-minationoforganicmicro-contaminantsinsampletypessuchasair,waterandotherliquidsamples,soilsandsedi-ments,shandfood,andbiota.
Atypi-calschematicwhichdisplaysmostofthemoreimportantroutesisshowninFig.
1.
Inthepresentreview,wefocusonthesample-preparationstep—withexamplesprimarilyrelatingtoliquidandsolidsamples—and,morespecically,onGaseousHSLLEMembranesP&TPyrolysisSBSESDMESPESPDESPMETrappingDTDHSMAEMSPDPLEPyrolysisSFESHWESoxhletUSEPost-treatmentGPCConcentrationLCSPEDerivatizationSulphurremovalAnalysisGC–GC*GC–FIDECDqMSIT-MSnToFMSFIDECDToFMSGrindingDryingSievingHomogenizationDerivatizationpHSaltingFiltrationDerivatizationSolidLiquidSamplingPre-treatmentSamplepreparationDryingGaseousHSLLEMembranesP&TPyrolysisSBSESDMESPESPDESPMETrappingDTDHSMAEMSPDPLEPyrolysisSFESHWESoxhletUSEPost-treatmentGPCConcentrationLCSPEDerivatizationSulphurremovalAnalysisGC–GC*GC–FIDECDqMSIT-MSnToFMSFIDECDToFMSGrindingDryingSievingHomogenizationDerivatizationpHSaltingFiltrationDerivatizationSolidLiquidSamplingPre-treatmentSamplepreparationDryingFig.
1.
TypicalstrategiesfortheGCdeterminationoforganicmicro-contaminantsinliquid,gaseousandsolidsamples.
SeeGlossaryforacronymsS34ChromatographiaSupplementVol.
69,2009Reviewthecharacteristicsofthemoderntech-niques,i.
e.
,thoseintroducedinthepasttwentyorsoyears.
Thesearemarkedingrey(electronicversioninred)inthegure.
Allacronymsusedinthisgureandthroughoutthereviewaresumma-rizedintheglossaryattheendofthisreviewarticle.
Inthesub-sections,eachoftheseparatetechniqueswillbebrieydescribed,andanumberofselectedapplications,strategiesandon-goingdevelopmentswillbegiventoillustratethemeritsanddemeritsofeachofthese.
Foreachtechnique,anumberofrecentreviewsand/orothergeneralreferencesourceswillbegiven;inmanycases,thesehavebeenusedasthebackboneofthischapter.
Aspectssuchasspikingandrecoveryofanalytes,andquantication(inclusiveofvalidationandmatrixeects)willnotbediscussed.
SamplePreparationMethodsPressurizedLiquidandSubcriticalHot-WaterExtractionPressurizedliquidextraction(PLE)involvesextractionwithsolventsatele-vatedpressures(uptoca.
20MPa)andtemperatures(uptoca.
200°C)withouttheircriticalpointbeingreached,toachieverapidandecientextractionoftrace-levelanalytesfroma(semi-)solidmatrix.
Sinceitsintroductionin1995[1],PLE,alsoknownasacceleratedsolventextraction(ASE)andpressurizeduidextraction(PFE),severalreviewshavebeenpublished[2–5]andthetechniquehasbeenshowntohavesignicantadvantagesovercompetingtechniquessuchasSoxhlet,Soxtec,andmicrowave-assistedextraction(MAE)extraction:enhancedsolubilityandmass-transfereectsandthedisruptionofthesurfaceequilibriumarethemainbenecialcau-ses.
Asaconsequence,comparedwithSoxhletextraction,bothtimeandsol-ventconsumptionaredramaticallyre-duced.
Originally,theuseofPLEmainlyfocusedontheisolationoforganicmi-cro-contaminantsfromenvironmentalmatricessuchassoil,sedimentandsewagesludge[1,6].
Today,thetech-niqueisalsousedfortheanalysisof,e.
g.
,foodandbiologicalsamples.
Insteadofanorganicsolvent,purewatercanalsobeusedforextraction.
Inthatcase,thetechniqueisusuallycalledsubcriticalhot-water(SHWE)orpressurizedhot-water(PHWE)extraction(seebelow).
Thebasicset-upofaPLEinstrumentisshowninFig.
2.
Thesystemconsistsofastainless-steelextractioncellinwhichthesampleisplaced;theprogrammedparameters(temperatureandpressure)arekeptattheirspeciedvaluesbyelectronicallycontrolledheatersandpumps.
Theliquidextractiscollectedinavial.
TheinstrumentusedinmostpublishedstudiesistheASE200(Dio-nex,Sunnyvale,CA,USA),inwhichupto24samplescanbeplacedinacar-rousel;extractioncellsof11–33mLareavailable,and40-and60-mLvialsforextractcollection.
Recently,Dionexintroducedtwonewsystems,ASE150andASE350.
Theformerisasingle-cellsystem;thelatterenablesautomatedextractionofupto24samples.
Bothsystemsaccommodateseven,1–100-mL,extractioncells.
Inseveralstudies,SFEextractorshavesuccessfullybeenusedforPLEofavarietyofsamples[8,9].
Inmostcases,PLEiscarriedoutinthestaticmode:oncethesamplehasbeenplacedintheextractioncell,organicsolventisaddedandthecellpressurized.
Afterheatingtotherequiredtempera-ture,staticextractioniscarriedoutfor,typically,5–20min.
Next,thevalveisopenedandthesolventallowedtoowtothecollectionvial.
Freshsolvent(some60%ofthecellvolume)isaddedtorinsethesystem,withanalbriefnitrogenpurgetoguaranteecompleteremovalofthesolventfromthesystem.
Inthedynamicmode,thesolvent(inmostapplications,water)iscontinuouslypumpedthroughtheextractioncellataconstantow-rate.
DynamicPLEisusuallycarriedoutinSFEextractorsorin-houseconstructeddevices.
Ifsamplesaresemi-solid,auniformdistributionoveraninertsupportsuchassandpriortopackingandcompletelylingthecellwiththemixturearerec-ommended.
Recently,Dionexintro-ducedachemicallyinertmaterialforsamplespre-treatedwithacidsorbases,Dionium.
Forheterogeneoussamples,grinding—frequentlyto63–150lmdp—isrecommended.
Grindingisanywaybenecialbecauseitwillshortenthediusionpathwaysandincreasethesurfacearea.
Dryingthesampleisimportantsincemoisturemaydiminishtheextractioneciency,specicallywhennon-polarsolventsareusedforextraction.
Ifmorepolarsolventsareusedtoextractwetsamples,thedryingstepbecomeslesscrucial.
Finally,ltersorglasswoolplugsshouldbeinsertedatbothendsoftheextractioncelltopre-ventblockingoftheconnectivetubingbysmallparticles.
Nexttowhathasbeensaidabove,severalparametersinuencingthePLEprocessshouldbebrieydiscussed.
Often,thesamesolventasusedforconventional,e.
g.
,Soxhlet,extractionsisinitiallytested.
Itisalsoimportanttotakeintoaccountthecompatibilitywithsubsequentstepsoftheproceduresuchasextractclean-uportargetanalyteFig.
2.
SchematicrepresentationofaPLEsystem[7]ReviewChromatographiaSupplementVol.
69,2009S35enrichment(actually,duringenrichment,achangeofsolventcanoftenbeeected).
Generallyspeaking,thepolar-ityofthesolventorsolventmixtureshouldbeclosetothatofthetargetcompound(s).
Whenanalytescoveringawiderangeofpolaritieshavetobeex-tracted,mixturesoflow-andhigh-polarsolventsgenerallyprovidebetterresultsthansinglesolvents.
Alternatively,twoextractions—onewithanon-polar,andthesecondonewithamorepolarsol-vent—canbeapplied[10,11].
Ingeneral,highertemperatureswillcauseanincreaseofthePLEeciencyduetoenhancedsamplewetting,betterpenetrationoftheextractionsolvent,andhigherdiusionanddesorptionratesoftheanalytesfromthematrixtothesol-vent.
Theyarethereforerecommendedprovidedtherearenolimitationsassoci-atedwiththermolabileanalytesand/ormatrices.
Toquoteanexample,atem-peratureof100°Cisoftenselectedas'defaultvalue'andusedforthePLEofPOPs(persistentorganicpollutants)fromavarietyofmatriceswithdierentsol-vents[12],whilemixturescontainingtol-ueneoftenrequiretemperaturescloseto200°Ctoprovidemaximumrecoveries.
Pressureessentiallyplaysnoroleotherthantokeeptheextractionsolventliquidatthehightemperaturesused[1,12,13].
However,withwetsamples[12]orhighlyadsorptivematrices[14],ahighpressurecanhelptoenhancethePLEeciencybyforcingtheorganicsolventintothematrixpores.
Thismayexplainwhylittleeectofthepressurewasob-servedduringPLEofherbicidesfromdrysoils,whileinthecaseofmoistenedsoilsincreasingthepressurefrom4to10MPawasbenecial.
SubcriticalHot-WaterExtractionSHWEisaPLE-typetechniquebasedontheuseofwaterasextractionsolventattemperaturesbetween100and374°C(criticalpointofwater,374°Cand22MPa)andatpressuressucienttokeepitintheliquidstate.
Undertheseconditions,thedielectricconstantofwater,e,i.
e.
,itspolarity,canbeeasilyanddramaticallyloweredbyincreasingthetemperature.
Purewateratambienttemperatureandpressurehasaneof79,whileincreasingthetemperatureto250°Catapressureof5MPaeectsasignicantreductiontoabout27[14].
Thisvalueissimilartothatofethanolat25°Cand0.
1MPaand,consequently,lowenoughtodissolvemanymedium-polaritycompounds.
AswithPLE,increasingthetemperatureatmoderatepressurealsoreducesthesurfacetensionandviscosityofwater,whichresultsinanenhancedsolubilityoftheanalytes.
Sincepressurehasonlyalimitedinu-enceonthesolventcharacteristicsofwateraslongasitremainsintheliquidstate,onecanincreasethepressuretoavoidtheformationofsteam—whichishighlycorrosiveandcandegradetheanalytes—atthehightemperaturesusedinSHWEwithoutcomprisingtheachieveddecreaseofpolarity.
Oneshouldnotethat,sincewaterisnotaGC-compatiblesolvent,afterSHWEtheanalytesintheextractmustbetransferredtoaGC-compatiblemedium,e.
g.
,byliquid–liquidextraction(LLE)[15],orbysolid-phasemicroextraction(SPME)orstir-barsorptiveextraction(SBSE)[16].
ApplicationsSelectedPLEandSHWEapplicationsfortheisolationofawiderangeofcompoundsfromavarietyofmatricesaregiveninTable1.
AsanexampleofatypicalPLE-basedanalysis,Frenichetal.
[17]reportedthemul-tiresidueanalysisoforganochloro(OCPs)andorganophosphoruspesti-cides(OPPs)inmuscleofchicken,porkandlamb.
5goffreeze-diedsampleweremixedwithHydromatrixandextractedbyPLEusingethylacetateasextractionsolvent.
AfterGPCclean-upfollowedbyconcentration,10lLofthenalextractwereanalysedbyGC–QqQ-MS;LODswereintherangeof0.
02–2lgkg-1.
ComparedwithSoxhletextraction,PLEwasfoundtoyieldimprovedextractioneciencyandprecision.
Moreover,theextractiontimewasshorterandtheconsumptionofsolventsmuchlower.
Oneaspectthatmeritsattentionisthat,formostapplications,PLE/SHWEhastobecombinedwithaclean-upsteptoremoveco-extractedmatrixconstitu-entssuchas,e.
g.
,lipids,pigmentsorresins.
Clean-upprocedurestypicallyarethesameasusedinclassicalprocedures.
Recently,severalauthorsusedmatrixsolid-phasedispersion(MSPD)forinsituclean-upintheextractionoftracecompoundsfromavarietyofsamples:sometimesMSPDconditions(seesectiononMSPDbelow)canbeselectedtore-tainparticularcompoundsbychoosinganappropriatedispersionmaterial/elu-entcombination.
AnovelapproachforPAHsinsoilsandsedimentsistopurifythePLEextractbydirectlarge-volumeinjection(LVI)inaprogrammedtem-peraturevaporiser(PTV)equippedwithalinerpackedwithanappropriatesor-bent[18].
ThePLEecienciesandper-formancedatacomparedwellwiththoseobtainedby6-hSoxhletextractionandotherconventionalprocedures[19].
Asanexample,Fig.
3showsa50-lLLVI–GC–MStraceobtainedafterminiatur-izedPLEofonly50mgofanaturallycontaminatedorganicsoiland100lLoftoluene.
AsregardsSHWE,Richteretal.
[15]reportedthedeterminationofpesticidesinsoilusingcontinuousSHWE(270°C,8.
2MPa,2mLmin-1,90min).
ThepesticidesintheaqueousextractwerequantitativelytransferredbyLLEwithdichloromethaneandinjectedintoaGC–MSsystem.
Forthe17pesticidesstudied,LODswere3–140lgkg-1.
ComparisonwithSoxhletextractionshowedtheanalyticalperformancetobequitesimilar.
ThemainadvantageofSHWEoverSoxhletextractionwasthetimeinvolvedintheextractionprocess:SHWEwassome10timesfaster.
Fur-thermore,lessthan10mLofsolventwasusedcomparedwith300mLforSoxhletextraction.
Severalapplicationsinvolvingon-linecouplingofSHWEwithGChavebeenreported(e.
g.
,[20,21]).
On-linecouplingofSHWEwithGCissimplerthancou-plingofPLE,becausetheaqueoussolubilityoftheanalytesdecreasesdra-maticallywhenthewateriscooledtoambienttemperature.
Trappingoftheextracton,e.
g.
,asolid-phasetrapisthusrelativelyeasy.
Usingasomewhatdierentapproach,Lu¨thjeetal.
[20]analysedpesticidesingrapesbySHWE–microporousmembraneliquid–liquidextraction(MMLLE)–GC–MS.
GrapeS36ChromatographiaSupplementVol.
69,2009ReviewTable1.
SelectedapplicationsofPLEandSHWEcombinedwithGCAnalytesSample(gormL)Pre-treatmentConditionsPost-treatmentDetectorLOD(lgkg-1)Recovery(%)Ref.
P(MPa)T(°C)Extractiontime(min)SolventPLEPCBs,OCPsFish(10)70gNa2SO41090–120395Hex–DCM(1:1),Hex–Acet(4:1)GPC,conc.
,dissolveECD––[22]PCBs,pesticidesSediment(5)Sieve,2gNa2SO46.
91005DCMConc.
,dissolve,SPE,conc.
,dissolveMS0.
2–0.
680–105[23]PCBs,PCDD/FsFoodGrind,Na2SO413.
8100295HepConc.
HRMS–81–97[24]PCBsMeat(0.
5)BlendwithNa2SO4andSiO2–H2SO4121002910HexSilicaECD0.
009–0.
3–[25]PAHsMussel(5),salmon(1.
3),shfeed(1.
5)Homogenise10.
3100295DCMFilter,GPC,conc.
MS0.
1–20ngkg-177–118[26]PAHsSoil(0.
050)Dry,sieve152001910TolPTVMS0.
8–30[18]OCPsSoil(1)Dry,0.
25gdiatom.
earth10.
31005Hex–Acet(1:1)Carbon,conc.
,dissolve–83–141[27]OCPs,OPPsChicken,pork,lamb(5)Freeze-dry,7gHydromatrix10.
8120295EtOAcConc.
,dissolve,GPC,conc.
,dissolveQqQ-MS0.
02–270–90[17]OCPsVegetables(0.
3)Grind,0.
075gdiatom.
earth101105Hex–Acet(1:1)Conc.
,SPEECD2–680–120[28]PesticidesSludge(1)Freeze-dry,grind,sieve,1gFlorisil,1gHydromatrix13.
8120295DCM–Acet(1:1)Conc.
,SPE,deriv.
MS1–3036–98[29]Chloroacetanilides,triazines,phenylureasSoil(15)Dry,0.
25gdiatom.
earth1050393AcetConc.
,dissolveMS0.
2–2>85[30]Alkylparabens,triclosanIndoordust(0.
5)3gFlorisil13.
8103391EtOAcConc.
,deriv.
MS/MS0.
4–176–98[31]OilcontaminationSoil(7)3gCelite545141005Hex–Acet(1:1)Conc.
FIDa––[32]SHWEPAHsSoil(–)130050Water(0.
5mLmin-1)MMLLEFID0.
2–0.
6[21]AtrazineKidney(0.
5)2gHydromatrix,dispersion(2gXAD-7HP),0.
3gdiatom.
earth51003910Water–EtOH(7:3v/v)SPMEMS20104[33]PesticidesGrapes(0.
5)Dry–12040Water(1mLmin-1)MMLLEMS0.
1–0.
69–28[20]VolatilesZiziphorataurica(1)Dry,grind615030Water(2mLmin-1)SPEToFMSa––[34]LigustilidesLigusticumchuanxiong,Angelicasinensis(0.
20)Dry,grind415010Water(2mLmin-1)HS-SPMEMS––[35]EssentialoilsAchilleamonocephala(1),Origanumonites(1.
5)Dry,grind615030Water(2mLmin-1)SPEToFMSa–>97[36,37]EssentialoilsFructusamomi(0.
050)Dry,grind52305Water(1mLmin-1)HS-SPMEMS–90[38]EssentialoilsCoriandrumsativumL.
(4)Grind2125120Water(2mLmin-1)LLEFID,MS––[39]aGC9GCinsteadofGCanalysisReviewChromatographiaSupplementVol.
69,2009S37samplesmixedwithseasandweredynamicallyextractedbySHWE.
TheextractwasledtothedonorsideoftheMMLLEunit(seesectiononmem-branesforuseofMMLLE)andMMLLEextractiontookplaceduringSHWE.
Next,the(static)acceptorsol-ventwastransferredon-linetotheGC–MSsystem.
However,therecoverieswereonly9–26%duetothelowe-ciencyoftheMMLLEstep;LODswere0.
1–0.
6lgkg-1.
Microwave-AssistedExtractionToday,MAEiswidelyrecognizedasaversatileextractiontechnique,especiallyforsolidsamples.
MAEutilizeselectro-magneticradiationtodesorbanalytesfromtheirmatrices.
Themicrowavere-gionisconsideredtoexistatfrequenciesof300MHzto100GHz.
Althoughthewholeofthisregionispotentiallyavail-ableforuse,all(domesticandscientic)ovensoperateat2.
45GHzonly.
ThemainadvantagesofMAEaretheusuallyhighextractionratesduetotheveryrapidheatingandtheelevatedtemperatures,andtheeaseofinstrumentoperation.
Adrawbackisthattheheat-ingislimitedtothedielectricconstantofthesample/solvent.
Theprimarymech-anismsforenergyabsorptioninMAEareionicconductanceandrotationofdipoles.
Ionic-conductanceheatingisduetotheelectrophoreticmigrationofionswhenamicrowaveeldisapplied.
Theresistanceofthemattertothisowwillgenerateheatasaconsequenceoffriction.
Dipolarmoleculescoupleelec-trostaticallytothemicrowave-inducedelectriceldandtendtoalignthemselveswithit.
Sincethemicrowaveeldisalternatingintime,thedipoleswillat-tempttorealignastheeldreversesandsoareinaconstantstateofoscillationatthemicrowavefrequency.
Frictionalforcescauseheattobedevelopedduetothemotionofthedipoles[40].
InMAE,sampleandorganicsolventaresubjectedtoradiationfromamag-netron.
ThereisahighcostdierentialbetweenmicrowaveovensfordomesticuseandforMAE,whichsometimesprecludesthepurchaseofadedicatedMAEsystem.
However,forsafetyrea-sons(explosionsinthepresenceofanorganicsolvent),itisstronglyrecom-mendedtouseonlydedicatedsystems.
Althoughtheapplicationofseveralbrandsandmodelsisreportedintheliterature,thereisatendencyforthemodelsofCEM(Matthews,NC,USA)andMilestone(Shelton,CT,USA).
Therearetwotypesofheatingsystem[41]—eitherthesampleisheatedinanopenglassvesselttedwithanairorwatercondenser[focusedmicrowave-as-sistedextraction(FMAE)],oraclosedsamplevesselconstructedinmicrowave-transparentmaterialisused[pressurizedmicrowave-assistedextraction(PMAE)].
Inanopen-stylesystem,theindividualsamplevesselsareheatedsequentially.
Thesystemoperatesat0–100%powerincrementswhichcanbeoperatedinstagesandfordierenttimeintervals.
Sampleandappropriatesolventareintroducedintoaglassvesselwhichisconnectedtothecondensertopreventlossofvolatileanalytesand/orsolvent.
Inacommonclosedsystem,uptotwelveextractionvesselscanbeirradiatedsimultaneously.
Safetyandrelevantexperimentalfeatures(temperatureandpressurecontrol,inoneextractionves-sel)areincorporatedinsuchsystems,andextractionconditionscanbevariedaccordingtoeitherthepercentagepowerinputorbyinsitumeasuringofthetemperatureandpressureinthemoni-toringvessel[41–43].
Figure4showstheschematicofaclosed-vesselMAEsys-temandofastandardasextractionvessel.
TheuseofPMAEispreferredinthecaseofvolatilecompounds.
How-ever,afterextractiononehastowaitforthetemperaturetodecreasebeforeopeningthevessel,whichincreasestheoverallextractiontime.
PMAEisquitesimilartoPLE,asthesolventisheatedandpressurizedinbothsystems,theonlydierencebeingthemeansofheating.
Consequently,asforPLE,thenumberofparametersislimited,whichmakesapplicationofthetechniquequitesimple[42,43].
However,oneshouldbeawarethat,inMAE,re-adsorptionoftheex-tractedanalytesisstillpossibleduringthenalcoolingstep,whilere-adsorp-tionisnegligibleinPLEwheretheextractionsolventisremovedfromthecellwhilestillwarm.
Withregardtotheextractioneciencies,FMAEandPMAEsystemswereshowntohavesimilarperformances[44,45].
ThenatureofthesolventisofprimeimportanceinMAE.
Nexttothefactthatthesolventshouldecientlysolu-bilizetheanalytesandbeabletodesorbthemfromthematrix,itsmicrowave-Fig.
3.
50-lLLVI–GC–MS(SIM)ofendogenousPAHsextractedfrom50mgofanorganicsoilwithaminiaturizedPLEusing100lLoftolueneat200°Cand15MPa.
Peakidentication:1=Naphthalene,m/z128/102;2=Acenaphthylene,m/z153/152;3=Acenaphthene,m/z153/152;4=Fluorene,m/z165/166;5=Phenanthrene,m/z178/176;6=Anthracene,m/z178/176;7=Fluoranthene,m/z202/101;8=Pyrene,m/z202/101;9=Benzo[a]anthracene,m/z228/226;10=Chrysene,m/z228/226;11=Benzo[b]uoranthene,m/z252/250;12=Benzo[k]uoranthene,m/z252/250;13=Benzo[a]pyrene,m/z252/250;14=Indene[1,2,3-cd]pyrene,m/z278/276;15=Benzo[ghi]perylene,m/z278/276;16=Dibenzo[a,h]anthracene,m/z278/267.
Slashinthex-axisindicateschangeintheionsmonitored[18]S38ChromatographiaSupplementVol.
69,2009Reviewabsorbingpropertieshavetobeconsid-ered.
Mostofthetime,thesolventischosentoabsorbthemicrowaveswith-outcausingstrongheatingtoavoidanalytedegradation.
Forthermolabilecompounds,themicrowavesmaybeabsorbedonlybythematrix,whichwillresultinheatingthesampleandreleaseofsolutesintothecoldsolvent[47].
Thislastmechanismcanalsobeusedwhenanabsorbingmaterial(e.
g.
,Weon)isad-dedtothesample[48,49].
ApplicationsPAHs,PCBs,phthalateestersandpesticidesareprominentclas-sesoftargetanalytesandsampletypesincludesoils[50,51],sediments[52]andvarioustypesofbiologicalmatrices[53,54].
RelevantinformationonaselectednumberofrecentMAE-basedapplica-tionsispresentedinTable3.
Post-treat-mentis(almost)alwaysneeded.
Theoperatingconditionshavetobeopti-mizedforeachanalyte–matrixcombina-tion,butitispossibletogivesomegeneralrecommendations:temperature,60–150°C;pressure,<1.
4MPa;extrac-tiontime,5–30min;solvent,5–50mLper0.
1–25gsample,withhexane–ace-tonebeingoftenused.
MAE-relevantcharacteristicsofthismixtureandofothersolventsalsofrequentlyusedarepresentedinTable2.
Asistobeexpectedfromtheabovediscussion,almostallMAEapplicationsinvolveo-lineprocedures.
However,inrecentyears,severalstudieswerepub-lishedwhichuseanon-lineapproach,whichisusuallycombinedwithdy-namicMAE(DMAE)[40,52,56].
Interfacingwasbasedonsolid-phasetrappingonacopolymersorbentwithsubsequentdryingwithnitrogenandlargevolumeinjection(LVI)toenableintroductionofthewholesampleex-tractintotheGCsystem.
MethanolwasusedforMAE,witha1:4dilutionwithwaterpriortothesolid-phasetraptoensureecientanalyteretention.
Figure5showsaschematicoftheon-lineDMAE–SPE–GCsystem.
Inonestudy[40],organophosphateestersweredeterminedinairsamples.
Thetotalsampling-plus-analysistimewaslessthan1.
5h,analyterecoverieswereover97%andNPD-basedLODswere60–190pgm-3.
Asregardsminiaturization,EricssonandColmsjo¨[52]insertedapreheatingcolumninfrontoftheextractioncellinthemicrowavecavity.
Usingthiscon-gurationtheauthorsdemonstratedthefeasibilityofDMAEcoupledon-linewithSPEfortheaccuratedeterminationofPAHsinareferencesediment(recoveries,88–104%;RSDs,1–10%)althoughonly60mgofsamplewereused.
Samplepreparationwascompleteinca.
45minandthenalextracts,col-lectedbyback-extractionoftheanalytesconcentratedona10mm92mmPLRP-SSPEcartridgewith400lLofMTBE,weredirectlyanalysedby1-lLinjectioninaGC–PIDsystem.
MAEwascomparedwithSoxhlet,USE(ultrasound-assistedextraction)andSFEfortheextractionof94com-poundslistedinEPAMethod8250[57].
FreshlyspikedsoilsamplesandtworeferencematerialswereextractedusingMAE(conditions:sample,10g;solvent,300mLhexane–acetone,1:1;tempera-ture,115°C;extractiontime,10min),Soxhletextraction(conditions:sample,10g;solvent,300mLhexane–acetone,1:1;extractiontime,18h),andSFE(sample,5g;solvent,10%MeOH-modiedsupercriticalCO2;pressure,45MPa;temperature,100°C;extractiontime,60min).
TherecoveriesforMAEandSoxhletwerefoundtobesimilar—thoseforUSEwereslightlyhigher,andforSFEclearlylower.
PrecisionwasbestwithMAEandworstwithSoxhletextraction.
Ultrasound-AssistedExtractionInultrasound-assistedextraction(USE),acousticvibrationswithfrequenciesabove20kHzareappliedtoextractanalytesfrompermeable(semi-)solidmatrices.
Thetopendofthefrequencyrangeislimitedonlybytheabilitytogeneratethesignals;frequenciesintheGHzrangehavebeenusedinsomeapplications.
Soundwavesareintrinsi-callydierentfromelectromagneticwaves:whilethelattercanpassthroughFig.
4.
Schematicof(a)aclosed-vesselMAEsystem,and(b)astandardlinedextractionvessel[46]Table2.
MAEsolventcharacteristics[55]SolventDielectricconstantBoilingpoint(°C)Closed-vesseltemperature(°C)aHexane1.
8968.
7–Hexane–acetone–52.
0b156Dichloromethane8.
9339.
8140Acetone20.
756.
2164Methanol32.
664.
7151Acetonitril37.
581.
6194aAt1.
2MPabExperimentallydeterminedReviewChromatographiaSupplementVol.
69,2009S39Table3.
SelectedapplicationsofMAEcombinedwithGCAnalytesMatrix(gormL)Pre-treatmentSolvent(mL)Temperature(°C)Extractiontime(min)Pressure(MPa)Post-treatmentDetectorLOD(lgkg-1)Recovery(%)Ref.
FMAEPCBsAsh(1.
5)DMSO(30)12010Dilutionwithwater,SPMEECD,MS–83–111[58]PCBs,chlorinatedalkanesSediments(5)Hex–Acet(1:1)(30)11515Florisil,conc.
ECD,MS0.
008–0.
02;1.
590[59]PBDEsMarinebiologicaltissuesGrindwithNa2SO4Pen–DCM(1:1)(25)11515GPC,conc.
MS<0.
189–97[60]PBDEsDomesticdust(0.
8)Hex(8)+10%NaOH(4)8015Na2SO4,Florisil,conc.
MS/MS0.
3–0.
692–114[61]PBDEsSediment(5)Freeze-dry,pulverize,sieveHex–Acet(1:1)(48)15224Filter,GPC,conc.
MS/MS0.
004–0.
0275–95[62]Organo-PameretardantsIndoordust(0.
5)Acet(10)13030Centrifuge,SPE,silica,conc.
NPD–85–104[63]PAHsAirborneparticlesAcet–Tol(5:95)(20)(150W)20Silica,conc.
MS–77–116[64]PesticidesSoil(1)SieveWater–MeCN–Hex(1:1:1)(1)+Hex(5)130(250W+900W)2+10Conc.
ECD–72–101[65]OCPsVegetables(0.
3)GrindHex–Acet(1:1)(15)(800W)4Filter,conc.
,SPEECD0.
2–280–120[66]OCPsSesame(5)GrindWater–MeCN(5:95)(40)10010Centrifuge,Na2SO4,FlorisilMS184–102[67]PyrethroidsSoil(2)Tol(10)+water(1)(700W)9Florisil,conc.
,copperwiresECD,MS0.
3–20097–106[68]PyrethroidsStrawberries(25)MeCN–water(1:1)(30)655SPMEMS1–14–[69]POPsMarinesediments(1)Water(8)8020LPMEhollowbremembraneMS0.
1–0.
773–117[70]VolatileorganicacidsTobacco(0.
4)Dry,grind10mMHCl(20)120(950W)20pH2–3,lterFID––[71]SVOCsSebumDepositonsebutapeHex–Acet(2:1)(10)6010SPMEMS20–30094–100[72]Nonylphenol,octylphenolPaper(2)Water(15)655SPMEMS0.
1(OP)5(NP)–[73]ChlorophenolsSludge(0.
5),sediment(1)Freeze-dry,sieveAcet–MeOH(1:1)(30)13020Centrifuge,SPE,deriv.
MS/MS–78–106[74]PMAEPAHs,PCBs,phthalates,nonylphenolsSediments(1)Mixwith1gactivatedcopperAcet(15)(80%offullpower)150.
15Florisil,conc.
MS0.
5–11(PAH)0.
4–1.
0(PCB)0.
5–20(Ph)100(NP)–[75]OCPsRiversediments(5)Hex–Acet(1:1)(25)120202Centrifuge,conc.
,silica,conc.
MS––[76]EndocrinedisruptersRiversediments(5)Drying(100°C,4h)MeOH(25)110151.
4Conc.
,silica(EtOAc–Hex(4:6)),conc.
,deriv.
MS0.
2–161–133[77]Irgarol1051Marinesediments(3)Water(30)115101.
4SPE,conc.
MS1–285–114[78]DMAEPAHsSedimentandsoil(0.
060)MeOH(24)(800lLmin-1)110203On-lineSPEMS(SIM)–88–108[52]Organo-PestersAir(62500)Gas-solidextractionMeOH(5)(500lLmin-1)110103On-lineSPENPD60–190pgm-397–103[40]S40ChromatographiaSupplementVol.
69,2009Reviewvacuum,soundwavesmusttravelinmatter,astheyinvolveexpansionandcompressioncyclestravellingthroughamedium.
Inaliquid,theexpansioncycleproducesnegativepressureandbubblesorcavitiesareformed.
Whenabubblecannolongerecientlyabsorbtheen-ergyfromtheultrasound,itimplodes.
Thewholeprocess,knownas'cavita-tion',takesplacewithinabout400ls.
Rapidadiabaticcompressionofgasesinthecavitiesproducesextremelyhightemperaturesandpressures,estimatedtobeabout5,000°Candroughly100MPa,respectively.
Thehightem-peraturesandpressurescausethefor-mationoffreeradicalsandothercompounds;forexample,thesonicationofpurewatercausesthermaldissocia-tionintohydrogenatomsandOHradi-cals,thelatterforminghydrogenperoxidebyrecombination[79].
Whencavitationoccursinaliquidclosetoasolidsurface,cavitycollapseisasymmetricandproduceshigh-speedjetsofliquid.
Liquidjetsdrivingintothesurfacehavebeenobservedatspeedscloseto400kmh-1.
Suchastrongim-pactcanresultinseriousdamagetoimpactzonesandcanproducenewlyexposed,highlyreactivesurfaces.
Theveryhigheectivetemperatures(whichincreasesolubilityanddiusivity)andpressures(whichfavourpenetrationandtransport)atthesolvent/solidmatrixinterface,combinedwiththeoxidativeenergyofradicalscreatedduringsonol-ysis,resultinhighextractivepower.
Sonicationtimesforreal-lifeapplica-tionsvarywidely,i.
e.
,from1–10to30–120min(Table4).
ForexcellentreviewsonUSEanditsapplications,thereadershouldconsultreferences[80,81].
Therearetwocommondevicesforultrasoundapplication,bathandprobesystems.
Thebathsaremorewidelyused,buthavetwodisadvantages,whichad-verselyaectexperimentalprecision,viz.
alackofuniformityofthedistributionofultrasoundenergy(onlyasmallfrac-tionofthetotalliquidvolumeintheimmediatevicinityofthesourcewillexperiencecavitation)andadeclineofpowerovertime.
Theprobeshavetheadvantageoverbathsthattheyfocustheirenergyonalocalizedsamplezoneand,thus,providemoreecientcavita-tionintheliquid.
Inbathsystems,thetransducerisusuallyplacedbelowastainless-steeltank,thebaseofwhichisthesourceoftheultrasound.
Sometanksareprovidedwithathermostaticallycontrolledhea-ter.
Theultrasoundpowerlevelsdeliv-eredbymostcommercialultrasonicbathsaresucientforcleaning,solventdegassingandextractionofadsorbedmetalsandorganicpollutantsfromenvironmentalsamples,butarelesseectiveforextractionofanalytesboundtothematrix.
Thepowershouldbegreatenoughtocausecavitationwithintheextractionvesselplacedinsidethebath.
Forabathwithasingletransduceronthebase,theextractionvesselmustbelocatedjustabovethetransducer,sincepowerdeliverywillbeatmaximumatthisposition(cf.
above).
Inordertoobtainreproducibleresults,thebathmustbeeitherthermostatedorpre-heatedatthemaximumtemperaturemeasuredintheliquidundercontinuousrunningconditionssincemostcleaningbathswarmupslowlyduringoperation.
Animportantdrawbackofmostclean-ingbathsisthelackofpoweradjustmentcontrol.
Intheliteraturenotarealten-dencycanbefoundinmodelsandbrandsofsonicationbathsapplied.
Probe-typesystemscandeliverupto100-foldgreaterpowertotheextractionmediumthanabath.
Onemainfeatureforthesuccessfulapplicationofultra-sonicprobesisthattheultrasonicenergyisnottransferredthroughtheliquidmediumtotheextractionvesselbutintroduceddirectlyintothesystem.
Theprobeconsistsofthefollowingcompo-nents:(1)ageneratorwhichisthesourceofalternatingelectricalfrequency,andwhichallowstuningtobecarriedoutforoptimumperformance;(2)thepossibilityofpulsed-modeoperationoftheultra-sonicprocessortoallowthemediumtocoolbetweensoundpulses;(3)theupperhornelement,apieceoftitaniumtowhichtheremovablehornisattached,formingboththeemitterorbooster,andthedetachablehornitself,usuallymadeofatitaniumalloy,whichallowsthevibrationofthexedhorntobetrans-mittedtoachemicalsystem.
Tiperosioncanoccurasaresultofcavitation.
SinceultrasoundirradiationbymeansofFig.
5.
SchematicofDMAE–SPE–LVI–GCsystem:1,Microwaveoven;2,Pre-heater;3,Extractionvessel;4,Mixingtee;5,Thermocouple;6,Temperatureregulator;7,Restrictor;8,SPEcartridge;9,PTV–GC–NPD;10,Fused-silicaleak;V1–V4,Valves;P1–P3,Pumps.
Workingmodes:(a)extractionandtrapping;(b)systemclean-upanddryingwithnitrogen;(c)transferandGCanalysis[40]ReviewChromatographiaSupplementVol.
69,2009S41Table4.
Selectedapplicationsof(D)USEcombinedwithGCAnalytesSample(gormL)Pre-treatmentSolvent(mL)Sonicationtime(min)Post-treatmentDetectorLOD(lgkg-1)Recovery(%)Ref.
USEbathPCBsSediment(3)DryHex–DCM(4:1)(50)120Na2SO4,Florisil/alumina,conc.
MS––[92]PAHsSediment(15)DryHex–acet(1:1)(50),28°C2930Filter,conc.
MS–75–119[93]PesticidesSoil(5)–EtOAc(5)3915Na2SO4,conc.
,dissolveMS0.
05–769–118[87]FungicidesSoil(5)SieveEtOAc(4)2915Filter,conc.
,Na2SO4ECD,NPD,MS(SIM)2–1087–111[94]PyrethroidsAir(100L)TenaxTAEtOAc(1)10–lECD<197–106[95]FlumethrindegradationproductsHoney(5)Hex–DCM(1:1)(20),25°C2920Conc.
,SPE,conc.
ECD0.
9–1.
090–106[89]VolatilesCitrusower(5)–Pen–diethylether(1:2)(30),25°C10MgSO4,conc.
MS––[96]VolatilesHoney(40)Mixwith22mLwaterand1.
5gMgSO4Pen–diethylether(1:2)(15)10AddNaCl,centrifuge,conc.
MS––[96]VolatilesWine(25)–DCM(10)15–FID23–269–50[97]NicotineChewinggumGrindHep(15)60DiluteFID––[98]USEprobePhthalatesPlastics(1)GrindHex(10)2910Conc.
,C18-SPE,conc.
,dissolveMS(SIM)1082–106[99]TotalfatSunower,soybean,rapeseed(–)Mill,sieveHex(100),75°C90Soxhletcycles910secConc.
FID99–100[90]TriterpenesOliveleave(1)Dry,millEtOH(30)20Centrifuge,conc.
,deriv.
(USE;5min)MS83–103[91]DUSEbathOrganophosphatesAir(180L)CuttingHex–MTBE(7:3),70°C2920Conc.
NPD–100[82]OrganophosphatesAir(180L)CuttingHex–MTBE(7:3),200lLmin-1,70°C3On-line-PTV–NPD25–180pgm-386[83]DUSEprobeNitro-PAHsSoil(4)Sieve,dryDCM(8),2mLmin-1forward-backward,20°C10Conc.
,dissolveMS/MSLowpg–[100]EnvironmentalpollutantsSediment(1)Mixwith3gsandHex(6),1mLmin-115Conc.
,dissolveToFMSa––[84]aGC9GCinsteadofGCanalysisS42ChromatographiaSupplementVol.
69,2009Reviewprobesgeneratesalargeamountofheat,somecoolingofthesonicationvesselisrequired.
Oneshouldalsobeawarethatvolatilesampleconstituentscanbelostduetothe'degassing'eectoftheultrasoundpower.
TheprobesystemmostlyusedfortheapplicationsreportedintheliteratureistheSonier450(Branson,Danbury,CT,USA).
MostUSEapplicationshavebeendevelopedusingabathoraprobe.
Dynamicsystems(DUSE)havebeenusedinafewcasesonly,eventhoughthisapproachwillspeeduptheUSEprocessconsiderably.
TherearetwoDUSEapproaches,openandclosedsystems.
Inopensystems,freshextractantowscontinuouslythroughthesample,sothemasstransferequilibriumisdis-placedtothesolubilizationoftheana-lyte(s)intotheliquidphase.
Thismodehasthedisadvantageofseriousextractdilutionwhichimpliesthatsubsequenttime-consumingconcentrationbysol-ventevaporation[82]orcouplingtoSPEisrequired.
Somewhatsurprisingly,despiteitseaseofimplementation,thelatterapproachhasnotbeenreportedyet.
Sanchezetal.
[83]coupledDUSEtoLVI–GCutilizingaPTVinjectortoanalyseorganophosphateestersinair.
AirltersweredesorbedbyDUSEwitha200lLmin-1owofhexane–MTBE.
WiththePTVinthesolvent-ventmode,theentireextractvolumewasintroducedintotheGC–NPDsystemwithoutanyclean-up.
TheLODsoftheorgano-phosphateesterswereintherangeof25–180pgm-3(averagerecovery,86%,RSD,5–14%(n=5)at1ng/lter).
Inclosedsystems,apre-setvolumeofextractantiscontinuouslycirculatedthroughthesolidsample.
Consequently,dilutionislessseriousthanwithanopensystem.
Thedirectionoftheextractantcanbechangedatpre-setintervalstoavoidundesirablecompactionofthesampleandanyincreaseinpressureinthedynamicsystem.
Afterextraction,avalveeitherdirectstheextractforcol-lectioninavialordrivesittoacontin-uousmanifoldforon-lineperformanceofotherstepsintheanalyticalprocess,suchaspre-concentration[84].
ApplicationsAselectedlistof(D)USEapplicationsfortheisolationofarangeofcompoundsfromavarietyofmatricesisshowninTable4.
USEismainlyusedforenvironmental(soil,sediment,air)andfoodandbeverage(soybean,honey,wine)samples.
Inmostapplications,USEiscombinedo-linewithGC,buttherearealsoseveralexamplesofon-lineset-ups[83–86].
AsanexampleofatypicalUSE-basedanalysis,wequotetheprotocolforpesticideresidueanalysisinsoil,de-signedtoexpandtherangeofapplica-bilityofEPAMethod3550C[87,88].
5gofsoilwereplacedinasmallErlenmeyeraskand5mLethylacetateadded.
Fig.
6.
GC–MS(SIM)chromatogramofaUSEextractofasoilspikedat50-lgkg-1concentrationlevel[87].
1=Dichlorvos;2=Desethylatrazine;3=Hexachlorobenzene;4=Dimethoate;5=Simazine;6=Atrazine;7=Propazine;8=Lindane;9=Terbutyl-azine;10=Propyzamide;11=Fonofos;12=Diazinon;13=Metribuzin;14=Parathion-methyl;15=Simetryn;16=Alachlor;17=Heptachlor;18=Fenitrothion;19=Malathion;20=Metolachlor;21=Aldrin;22=Chlorpyrifos;23=Parathion-ethyl;24=Iso-drin;25=Chlorfenvinphos;26=Pendimethalin;27=Heptachlorepoxide;28=Chlorfenvinphos;29=Procymidone;30=c-Chlordane;31=Tetrachlorvinphos;32=Endosulfan,I;33=Fenamiphos;34=4,4'-DDE;35=Dieldrin;36=Endrin;37=Endosulfan,II;38=4,4'-DDD;39=Endosulfan,sulfate;40=4,4'-DDT;41=Azinphos-methyl;42=k-Cyhalothrin;43=a-Cypermethrin;44=Delta-methrinReviewChromatographiaSupplementVol.
69,2009S43AftermanualagitationthesamplewasexposedtoUSEfor3915min.
Aftereachperiod,extractswerecollectedbypouringtheextractantthroughafunnelpluggedwithcottonwoolandoverlaidwithanhydroussodiumsulphate.
Thenal15-mLextractisevaporatedtodrynessandredissolvedin200lLethylacetate,and1lLwasanalysedbyGC–MS.
LODswereinthe0.
05–7.
0lgkg-1range.
Figure6showsachromatogramofa50-lgkg-1spikedsoil.
Theproce-dureisstraightforwardandanalytedetectabilityisfullysatisfactory.
How-ever,thetotalanalysisissomewhattime-consumingandincludesriskysolventevaporation.
Asanotherexample,Zhouetal.
[89]usedUSEforthedeterminationof4-uoro-3-phenoxybenzaldehydecyano-hydrin(FPBC)and4-uoro-3-phenoxy-benzaldehyde(FPB),twodegradationproductsofumethrin,inhoney.
A5-ghoneysample,dissolvedinacetone–dichloromethanewasextractedinamixtureofhexane–dichloromethaneusingasonicationbath.
Afterclean-upbySPEandconcentration,theextractwasanalysedbyGC–ECD;theLODswere1–2ngg-1withrecoveriesof90–106%.
Luque-Garcaetal.
[90]com-binedUSEwithconventionalSoxhletextractionfortheanalysisoftotalfatinoleaginousseeds.
AwaterbathwasmodiedsuchthattheSoxhletchamberwaslocatedinit.
Thebathwassonicatedbyaprobetoacceleratetheextractionprocess(Fig.
7).
Theeciencywassim-ilarto,orevenbetterthan,thoseofconventionalSoxhletextractionandtheocialISOmethod,savingbothtimeandsamplemanipulation.
Recently,thetwofoldapplicationofUSEinasingleanalyticalprotocolwasreported[91].
Themaintriterpenes—eleanoicacid,ursolicacid,uvaolanderyuthodiol—werequantitativelyleachedfromoliveleavesby20-minUSEwithethanol.
Thiscomparesfavourablywiththe5hre-quiredbyconventionalproceduresinvolvingmaceration.
AnaliquotoftheleachatewassilylatedpriortoGC–MS.
Ultrasound-assistedsilylationtookonly5min,asagainst0.
5–3hforconven-tionalsilylation.
SupercriticalFluidExtractionOneareathatstimulatedaninterestinenhanceduidextractions,wassuper-criticaluidextraction(SFE).
Thisisalongestablishedmethod,whichhasbeenusedindustriallyformanyyears.
How-ever,itwasnotuntilaninterestwasshowninsupercriticaluidsaschro-matographicmediathatitstartedtobeseriouslystudiedasanextractiontech-niqueonananalyticalscale.
Ithassincebeenthesubjectofnumerousbooksandreviews(e.
g.
,[4,101–103]).
AlmostallSFEemployscarbondioxide(criticalpoint,30.
9°C,73.
8bar)asthesupercriticaluid:itisanalmostidealsolventsinceitcombineslowvis-cosityandhighanalytediusivitieswithahighvolatility(whichmakesanalyterecoveryverysimpleandprovidessol-vent-freeconcentrates),andisinexpen-siveandenvironmentallyfriendly.
AnimportantdrawbackofCO2isitsnon-polarcharacter.
Inordertowidentheapplicationrangeofthetechniquetoincludemorepolaranalytes,thepre-ferredrouteistoemploypolarmodierssuchasmethanol,ethanol,acetoneandacetonitrile(1–10%addition,preferablybymeansofaseparatemodierpump).
Inadditiontoamodierpump,thebasiccomponentsofanSFEsystem(Fig.
8)are:asupplyofhighpuritycarbondioxide;aCO2pump;anovenfortheFig.
8.
SchematicofabasicSFEsystem[41].
BPR,backpressureregulator(withattachedcontrollerunit)Fig.
7.
Schematicofultrasound-assistedSoxhletextraction[90]S44ChromatographiaSupplementVol.
69,2009Reviewextractionvessel;apressureoutletorrestrictor;andasuitablecollectionvesselforrecoveryoftheextractedanalytes.
Samplecollectioncanbeperformedbypurgingtheextractthroughasolventoroverasuitableadsorbent,suchas,Florisil.
SFEcomprisestwointegratedparts,extractionoftheanalytefromthesamplematrixandsubsequentcollection—ortrapping—oftheanalytes.
Therearethreemaincollectionmodes:(1)collec-tioninavesselcontainingsolvent;(2)trappingonacartridgepackedwithanadsorbingorinertsolid-phasematerialand(3)collectioninadevicethatisconnectedon-linewiththechromato-graphicsystem.
Comparedto'o-line'solventcollectionorsolid-phasetrap-ping,theon-linetechniqueoersbetteranalytedetectabilitybecausetheentireextractratherthananaliquot,canbetransferredtothechromatographicsys-tem.
However,samplesizeshouldbelimitedsinceco-extractedfatorwatermayeasilycontaminatetheinterfaceusedand/orruintheanalyticalcolumn.
Fortherest,itisgoodtoaddthatallthreetypesofcollectionrequirecarefuloptimization,withsolventcollectionprobablybeingthesimplestsystemtouseandtheeasiesttooptimize,andsolid-phasetrappingoeringselectivitybythetwo-steptrapping/elutionproce-dure.
On-linecollectionprovidesthebestsensitivitybecausetheentireextractisintroducedintotheGCsystem.
ApplicationsOvertheyears,SFEhasbeenusedfortheextractionofPAHs,PCBsanddioxins,aliphatichydrocar-bonsandpesticidesfromsoil,sedimentandair-borneparticulates,infoodandfragrancestudies,especiallyforessentialoilsandfats,fortheextractionofpoly-meradditives,naturalproducts,anddrugsandtheirresidues.
Specialatten-tionhasalwaysbeengiventotheextractionofthermolabilecompoundsbecausethemildconditionsofCO2-basedSFEwillminimizetheirdegradation.
IllustrativeexamplesaresummarizedinTable5.
Inordertogiveanimpressionofthewidevarietyofanalyte/matrixcombina-tionsforwhichSFEhasbeenusedassample-preparationmethod,threestud-iesincludedinTable5arebrieydis-cussed.
TheextractionofonionoilfromfreshonionsbymeansofSFEwasre-portedbySeangcharoenratandGuyer[104].
Onionswerepeeled,cutandjuiced.
ThejuicewaslteredtoseparateitfromthepulpandfedtoanAmberliteXAD-16polymericsorbentbed.
TheonionoilwasextractedwithsupercriticalCO2(20.
7–28.
7MPa,37–50°C)intheup-owdirectionand,afterdilutionindichloromethane,theextractwasana-lysedbyGC–MS.
Rissatoetal.
[105]usedSFEfortheanalysisofpesticidesinhoney.
A5ghoneysamplewasmixedwith3mLwaterandheatedat40°Ctoimprovehandling.
Afterlyophilization,thehoneysampleswerepouredintoastainless-steelextractioncellinthesandwichmode,usingsilanizedglasswoolatthebottomandtop.
ExtractionwasperformedwithCO2with10vol%acetoneasamodier,at200barand60°Cduring5min.
Thepesticideswerecollectedon-lineonFlorisilat10°C.
Afterrathertime-consumingelutionwithtwo5mLsolventmixtures,con-centrationandredissolutionin1mLacetone,only1lLwasanalysedbyGC–ECD(Fig.
9).
TheLODswerebetterthan0.
01mgkg-1(recoveries,75–94%).
ComparedwithconventionalLLE,samplecontaminationwasgreatlydiminishedassamplehandlingwasminimizedandtheuseoforganicsol-ventswasreduced(consequently,solventevaporationwasmuchfaster).
Garrigosetal.
[106]usedSFEfortheanalysisofstyreneinpolystyrene.
Styrenewasex-tractedwithsupercriticalCO2withcol-lectionindichloromethane.
Afterconcentration,theextractwasanalysedbyGC–MS.
SFEwasfoundtobemoreselectivethanMAE,SoxhletandHS(lessextractionofmatrixcomponents)andgaveananalyterecoveryofabout100%.
Thefactorsthatgoverntheextrac-tionofananalytefromamatrixarethesolubilityoftheanalytesinthesuper-criticaluid,themasstransferkineticsoftheanalytefromthematrixtothesolu-tionphase,andinteractionsbetweenthesupercriticalphaseandthematrix(Fig.
10)[107–110].
Toputitdierently,despitequiteanumberofpromisinginitialresultsobtainedwhenCO2hasbeenusedfortheextractionofnon-polarmicro-contaminantsfromsediments[111],naturalproductsfrombiologicalsamples[112]oressentialoilsfromplantmaterial[113],SFEhasnotbecomeaswidelyandaseasilyusefulasinitiallyexpected.
OnemainreasonisthatSFEhasbeenfoundtooanalyte-and,spe-cically,toomatrix-dependenttobereadilyandroutinelyapplicableformuchworkinvolvingcomplexenviron-mentalandfoodsamples.
Thisisespe-ciallytrueforenvironmentalsampleswhereanalyte/matrixinteractionsoftenbecomestrongerinageingsamples:optimizationonthebasisofspikerecoveriesmaythenleadtoquiteerro-neousresults.
Inaddition,methoddevelopmentisratherdicultsincequiteanumberofparametershavetobeoptimized,andthereareoftentechnicalproblems.
Inbothrespects,PLE—another'modern'compressed-uidtech-nique—issuperior.
Moreover,PLEcanbeusedwithmostconventionalsolventsandcanthereforehandlepolaraswellasnon-polarcompounds,whereasSFEispreferentiallyemployedfornon-polaranalytesonly.
Ontheotherhand,on-linecouplingtoGCismucheasierwithSFE[114],itisasolvent-freemethodandminiaturizationshouldnotmeetwithanyproblems[4].
Dedicatedattentionisobviouslyrequiredtounderscorethemeritsofwhatisnowsomewhatofa'niche'technique[115].
MatrixSolid-PhaseDispersionTheanalysisof(semi-)solidenviron-mental,foodorbiological—sometimesfat-containing—matricesisachallengingproblem,withrapidandecientanalyteisolation—andsubsequentpurica-tion—beingofkeyinterest.
In1989,Barkeretal.
[126]introducedmatrixsolid-phasedispersion(MSPD)andthetechniquehassincethenbeendiscussedinseveralreviews[127–130].
MSPDin-volvesthedirectmechanicalblending(forsolidsamples)ormixing(forsemi-solidandliquidsamples)with,usually,analkyl-bondedsilicaSPEsorbent—but,occasionally,alsoplainsilica,Florisilorsand.
TheaddedabrasivepromotesthedisruptionofthegrossReviewChromatographiaSupplementVol.
69,2009S45Table5.
SelectedapplicationsofSFEcombinedwithGCAnalytesMatrix(gormL)Pre-treatmentCO2modierTemperature(°C)Extractiontime(min)Pressure(MPa)CollectionmodePost-treatmentDetectorRecovery(%)Ref.
PesticidesGazpacho(20)DrywithMgSO4–50–902030–50EtOAcMS[116]PesticidesHoney(5)10%Acet602020FlorisilcartridgeECD75–94[105]PesticidesFood(2)MixwithHydromatrix–503012.
3Stainless-steelballsECD,NPD70–133[117]PesticidesFishmuscleFreeze-dry–36–64–10–24FlorisilECD[118]PesticidesBabyfood(2)Extreluttodehydrate15%MeCN705517.
2DCMC18-SPE,conc.
MS11–37[119]VolatilesBuniumpersicumBoiss.
seed(3)MespilusgermanicaL.
seed(3)Grind,mixwithsand–453520DCMConc.
MS[120]VolatilesWine(170)–5020EtOH–FID[121]EssentialoilsEquisetumgiganteumL.
(40)Dry,grind–30–4030012–30Flask–MS[122]EssentialoilsHypericumperforatumL.
(50)Grind,lter–14–401508–10––MS[123]EssentialoilsLaurusnobilisL.
(60)Grind,mixwithseasand4%EtOH607525––MS[113]OnionoilOnionGrind,lter–37–50–10.
3–28.
7––MS[104]CholesterolCowbrain(0.
1)Freeze-dry,grind–6030025–Deriv.
FID[112]FAMEsInfantpowder(2)–15%EtOH1002046.
5C18-trapDeriv.
MS[124]StyrenePolystyrene–1053048.
3DCMMS[106]SqualeneTerminaliacatappaleavesandseeds(1)Freeze-dry,grind–40–601513.
8–27.
6––MS[125]S46ChromatographiaSupplementVol.
69,2009Reviewarchitectureofthesamplewhile,withabondedsilica,sampleconstituentswilldissolveanddisperseintothebondedphase,causingacompletedisruptionofthesampleanditsdispersionoverthesurface.
Whenblendingormixingiscomplete,thehomogenizedmixtureispackedintoanemptycolumnorcar-tridge(with,usually,frits,ltersorplugsatbothtopandbottom).
Obviously,thereisonemaindierenceherebetweenMSPDandSPE:withtheformertech-nique,thesampleisdistributedthroughoutthecolumnandnotonlyretainedintherstfewmillimetres.
Elutionwith,preferably,alimitedvol-umeofsolventisthenalstepoftheremarkablysimpleprocedure.
Theuseofsmallparticlesforthedispersionsorbent,shouldbeavoidedtopreventundulylongelutiontimesorcolumnplugging,and40lmorlessexpensive40–100lmparticlesareusedmostfrequently.
Thesample/sorbentratiousuallyisabout1:4,butmayvaryupto1:1.
Thenatureofthesorbentusedforaspecicapplicationalsohastobeconsidered.
Forexample,foranalyteextractionfromanimaltissue,C18-bon-dedsilicaisthemostpopularsorbent,whileC8-andC18-bondedsilicasandFlorisilarepreferredforplantsamples.
Florisilhasbeenappliedsuccessfullyalsoforothertypesofsample,e.
g.
,fruitjuices,soilandhoney.
Amoreselectivesorbent,cyanopropyl-bondedsilica,hasbeenusedtoisolatepolaranalytessuchasveterinarydrugsfrombiologicaluidsandtissues.
Recentdevelopmentsin-cludetheuseofacidicsilica,whichwillstronglyretainbasiccompoundsandfacilitatebasic/acidgroupseparations.
Afterelutionofthebasicanalyteswithanon-polarsolvent,thelatterclassofcompoundscanbeelutedwitharela-tivelypolarsolvent.
Silicatreatedwithsulphuricacidhasalsobeenusedforecientfatremoval.
Sandissometimesselectedtoallowtheearlyelutionofinterferencesthatwouldnotberetainedbyanysorbentduringtheelutionofthetargetanalytes.
Theelutionsolventshouldeectanecientdesorptionofthetargetanalyteswhilethebulkoftheremainingmatrixcomponentsshouldberetained.
Intheliterature,awidevarietyofsolventshasbeentested,rangingfromhexaneandtoluene,viadichloromethaneandethylacetate,toalcoholsandwateratelevatedtemperatures.
Notsurprisingly,pesti-cidesareusuallyelutedwithlow-ormedium-polarsolvents,anddrugsandnaturallyoccurringcompoundswithmorepolarones.
Generallyspeaking,thenatureofthepreferredsorbent/solventcombinationismainlydeterminedbythepolarityofthetargetanalytesandthetypeofsamplematrix.
Keepingthiscommon-senseconsiderationinmindwillfacilitateMSPDoptimization.
Insomecases,eluatesfromanMSPDcolumnaresucientlycleantopermitdirectinjectionintotheGCsys-tem[131].
However,moreoftenaddi-tionalclean-upisrequired.
Forsomeapplications,e.
g.
,theanalysisoffruitsandvegetables,washingtheMSPDcol-umnwithwaterpriortoelutionoftheanalytesgenerallysuces[131,132].
Post-MSPDtreatmentmayrangefromsimpleltrationorcentrifugation,toevaporation-plus-redissolutionoraque-ous-to-organicextraction,andmoreversatileSPE.
Inthelast-namedcase,asuitablesorbentcanbepackedatthebottomoftheMSPDcolumnortheMSPDcolumncanbeelutedo-oron-lineontoaconventionalSPEcartridgeordisk.
AninterestingdevelopmentistocombineMSPDandPLE,i.
e.
,toin-creasethespeedoftheanalysisbyapplyingelevatedtemperaturesandpressures,althoughtheseshouldberelativelymildinordertomaintaintheselectivityoftheMSPDprocedure[133].
ApplicationsThreeapplicationareasinwhichMSPDisfrequentlyusedarethedeterminationofdrugs,organicmicro-contaminantsandnaturallyoccurringcompounds(however,withthelast-namedgroup,MSPDisusuallycombinedwithLC,notGC).
Table6summarizesaFig.
9.
GC–ECDchromatogramofahoneysampleobtainedbySFE[105].
1=Dichlorvos;3=Triuralin;4=Hexachlorobenzene;5=Dicloran;7=Dimethoate;8=Chlorothalo-nil;9=Vinclozolin;10=Aldrin;13=Chlorpyrifos;16=a-Endosulfan;17=Hexaconaz-ole;20=b-Endosulfan;27=Tetradifon;29=CyutrinI;30=CyutrinII;33=CypermetrynII;34=CypermytrinIIIAnalytesMatrixSolventPhysicalandchemicalinteractionsKineticsSwellingPhysicalandchemicalinteractionsKineticsAnalytephysicalparametersSolute-solventinteractionsSolubilityFig.
10.
FactorstobeconsideredwhenstudyinganSFEextractionprocess[108]ReviewChromatographiaSupplementVol.
69,2009S47Table6.
SelectedapplicationsofMSPDcombinedwithGCAnalyteSample(gormL)Sorbent(g)Elutionsolvent(mL)Pre-treatmentPost-treatmentDetectorLOD(lgkg-1orlgL-1)Recovery(%)Ref.
PCBs,PBDEsBiota(0.
5)Florisil(1.
5g)Hex(20)Dry0.
5gsamplewith2gNa2SO4Acidsilica,neutralsilicawith20mLhex+12mLDCM–hex(20:80)ECD0.
1[141]PCBsEggs,clams,mussel,oyster(2)Florisil(4)DCM–Pen(15:85),40°C,14MPa(55)–Florisil,conc.
,HexECD,MS–MS0.
001–0.
004/0.
002–0.
0790–105[133]OCPs,PCBsChickenegg(1)Florisil(2)DCM–Hex(1:1)(10)DiscardshellsConc.
,H2SO4ECD/MS0.
2–0.
780–110[142]OCPsHumanserum(1)Florisil(2)Hex,DCM–FlorisilorC18ECD0–110[143]FungicidesFruits,vegetables(0.
5)C18(0.
5g)EtOAc(10)–SilicaNPD,ECD,MS(SIM)3–3060–100[138]Pesticides(266)Applejuice(10)Diatom.
earth(20g)Hex–DCM(160)–Conc.
MS(SIM)3–18[135]PesticidesOlives(1)Aminopropyl(2g)MeCN–Florisil,conc.
,MeCN–water(1:1)orMeCNMS10–6080–10575–105[144]HalogenatedcompoundsAquaculturesample(1.
5)C18(1g)Hex(30)–Acidsilica,alumina,conc.
MS/MS0.
02–0.
91–8[145]OPPsFruitjuices(1)Florisil(2)EtOAc(295)Mix1:1withMeOHFilter,conc.
NPD,MS0.
1–0.
670–110[146]OPPs,permethrinFruit(0.
025)C8(0.
025)EtOAc(0.
1)8mLwateror––MS4–9085–120[131]OPPs,amidine,carbamateHoney(1.
5)Florisil+Na2SO4(2.
5+1)Hex–EtOAc(9:1)(295)–Filter,conc.
NPD,MS6–1580–100,60[147]InsecticidesHoneybees(0.
5)Florisil,silica(1.
5)Hex–diethylether(9:1,8:2,7:3)+Hex–EtOAc(7:3)(4915)15mLHexAluminaorsilica,conc,hex–acetNPD/ECD5–5070–110[148]InsecticidesFruitjuicesFlorisil(4)EtOAc(295)–Conc.
,Na2SO4ECD,MS1–575–110[149]PesticidesLiver(0.
5)C18(2)EtOAc(4+3+3)Florisil,conc.
,cyclohexaneMS/MS0.
01–970–115[150]ChloramphenicolMuscletissue(2)C18(3)MeCN–water(1:1)(10)10mLHex+12mLMeCN–water(1:1)C18,LLEEtOAc,conc.
,deriv.
ECD/MS1.
695–100[151]S48ChromatographiaSupplementVol.
69,2009Reviewnumberofrecentexamplesofeachofthese,andprovidesrelevantinformationontheexperimentalconditionsandana-lyticalperformance.
Inmoststudies,theamountofsampleisseentobeinthe0.
5–2grange.
Largeglasscolumnshavebeenusedforapplicationsinvolvinghighsampleamountsinordertodeterminetrace-levelconcentrationsofPCBsandPCDD/Fs[134]andpesticides[135].
Inonestudy,Chuetal.
[135]mixed10gapplejuicewith20gdiatomaceousearth,transferredthemixturetoaglasscolumnandleachedthepesticideresidueswith160mLhexane–dichloromethane(1:1).
Theeluentwasconcentratedto1mLand1lLwasinjectedtoGC–MSinSIMmode.
LODsfor266pesticideswere3–18lgkg-1,withanalyterecoveriescloseto100%.
Thereare,ontheotherhand,alsoseveralpaperswhichfeatureminiaturizedMSPDof,typically,some25–100mgofsample[131,136,137].
Toquoteanexample,Ramosetal.
[136]analysedPCBsinfreeze-driedmeat,whereonly0.
1gmeatwasdispersedwith0.
1gofacidsilica.
Therecoverieswere80–130%andtheLODsforECDdetectionwerebelow0.
3ngg-1.
Figure11showsaGC–lECDchro-matogram.
Theapproachmeritsatten-tionbecauseof(1)itspracticalityifsamplesizeislimitedas,e.
g.
,withsingle-insectstudies[137],and(2)thesignicantlyreducedvolumeofelutionsolvent,whichfacilitatesfurtherhan-dling.
Actually,withsolventvolumesaslowasafewmillilitres,onewouldex-pecton-linecouplingofMSPDandGC,orLC,tohavebeenimplementedbuttothebestofourknowledge,nopapersdealingwiththistopichavebeenpublishedsofar.
Navarroetal.
[138]comparedMSPDandLLEfortheanalysisoffungicidesinvegetableswithbothtechniquesusingethylacetateassol-vent.
Theresultsshowedsatisfactoryagreement,buttheLLEextractscontainedmuchmoreinterferingcom-pounds.
Picoetal.
devotedtwo(LC-based)studies[132,139]toacomparisonofMSPD,SBSEandSLE(solid–liquidextraction)forthedeter-minationofpesticidesinfruitwithMSdetection.
TheauthorsconcludedthatMSPDshouldbepreferredbecauseitiseasiertoperformandfaster,andshowsequalaccuracy.
InacomparisonofMSPDandMAEforthedetermination(admittedly,byLC)of16PAHsinsoil,theanalyticalperformancedataofthetwotechniqueswerefoundtobecloselysimilar.
AsforMSPD,extractionandclean-upofthelyophilizedsampleswerecarriedoutinasinglestep,usingaFlorisil/silicasorbentmixture[140].
DirectThermalDesorptionThermaldesorption(TD)isavaluablealternativetoheadspacetechniquesfortheisolationofvolatilecompoundsfromnon-volatilesolid,semi-solidand,occa-sionally,liquidmatrices,andawidevarietyofapplicationshasbeenreportedintheliterature.
AlthoughTDisnotreallyanewtechnique,fullyautomatedsystemsareonlyinuseforslightlyover10years.
Oneoftherstexampleswastheuseofautomatedthermaldesorption(ATD)forthedeterminationofvolatileconstituentsofplantsandfood[152].
Typically,a1–40mgsampleisplacedinadesorptioncartridgebetweentwoglass-woolplugs.
Byheatingthecar-tridgeforapre-settime,thevolatilesaredesorbedand,next,adsorbedonacoldTenaxtrap.
HeatingofthetrapeectsrapidtransferoftheanalytestotheGCforfurtheranalysis.
Similarly,TD–GC–MScanbeusedasascreeningmethod,e.
g.
,forchlorinatedhydrocarboncon-taminationinsoil[153].
Inthiscase,adual-tubesystemwasusedtoenablefocusingoftheanalytesonaTenax-plus-carbontrappriortotheirreleaseandtransfertotheGCsystem.
Totalanalysisincludingthesamplepreparation,re-quiredlessthan1h.
TDisalsousedtostudytherelativelylow-molecular-masscomponentspresentin(oil-containing)Fig.
11.
GC–lECDchromatogramsofanon-spikedmeatsample(uppertrace)andaprocedureblank(lowertrace)usingminiaturizedMSPD[136].
Samplesize,0.
1g.
Peakidentication:1=CB28;2=CB52;3=CB95;4=CB101;5=CB81;6=CB77+CB110;7=CB123+CB149;8=CB118;9=CB114;10=CB153;11=CB132;12=CB105;13=CB138;14=CB126+CB129+CB178;15=CB183;16=CB167;17=CB156;18=CB157;19=CB180;20=CB169;21=CB170;22=CB189;23=CB194;TCN(1,2,3,4-tetrachloronaphthalene),externalstandard;PCB209,externalstandardReviewChromatographiaSupplementVol.
69,2009S49rocks[154,155].
Pyrolysistechniquesareusedtostudythevery-high-molecular-massstructuresinsuchsampleswhicharenotdirectlyamenabletoGC[156,157].
Aninexpensiveanduser-friendlysystemformulti-stepTD–pyrolysis–GCforuseingeochemicalanalysiswasde-signedbyvanLieshoutetal.
[158].
ThermaltreatmentwasperformedinsideaPTVinjectorwhichservedasboththeTDunitandthepyrolysersystem.
Sampleamountsrangingfromsub-mgamountsupto2gwereweigheddirectlyintothelineroftheinjector.
Thesystemwasalsousedforpolymercharacteriza-tion[159].
TDisbeingincreasinglyusedfortheanalysisofaerosols(see,e.
g.
,[160,161])and,alsointhisareaofapplication,primarilyinordertoreplacetime-con-sumingproceduresinvolvingsolventextraction(andevaporation)bymoredirectapproachesinwhich(partsof)aerosol-loadedltermaterialispackedintoaGCinjectorlineranddirectlysubjectedtothermaldesorptionplusinstrumentalanalysis[162].
Themainadvantagesarereducedsamplehandlingandimprovedanalytedetectability(9–500timesbetterLODsthansolventextraction[162]),whilethereisnoneedtomodifytheGC–MSset-up.
Thepracticalusefulnessofthis,so-calleddirectthermaldesorption(DTD)isdis-cussedinsomemoredetailbelow.
Asindicatedaboveandinthesectiononapplicationsbelow,thebasicinstru-mentationneededfor(D)TDstudiesisrathersimple.
However,becauseofthe(semi-)solidnatureofmostsamples,automationofthesampleintroductionisdicult.
In2002,deKoningetal.
[163]designedasystemwhichfeaturesfullyautomatedlinerexchange.
Tothisend,aFocusXYZsamplepreparationrobotwasequippedwithanewlydevelopedinjectorheadtoopenandclosetheOptic2(ATASGL,Veldhoven,TheNether-lands)injectioninterface.
InFig.
12theinjectorheadisshownintheopen(left)andclosed(right)position.
Thespeciallydesignedliners,cappedwithastandardcrimpcap,areplacedinasampletrayandtransportedtothethermaldesorp-tiondevice.
Bothlinertransportandlinerexchange(whichcanbeperformedaftereachanalysis)areautomated.
Twosystemsarecommerciallyavailableto-day,theALEX(AutomaticLinerEX-change)fromGerstel(Mu¨lheim,Germany)[164]andtheLINEX(LINerExchanger)fromATASGL[165].
Asarstapplication,thewoodpreservativeN-cyclohexyl-diazeniumdioxide(HDO)wasquantiedin10mgofsapwoodpowderbymeansofDTD–GC–MS(m/z114).
Thereproducibilityoftheprocedure(5–10%)andtheLOD(4mgHDO/kgwood)werefullysatisfactory[166].
ApplicationsThenumberofapplica-tionsofDTD–GC–MS(andDTD–GC9GC–MS)isstillratherlimitedbut,ontheotherhand,thepublishedexamplesdoshowthattheapproachcanbeusedsuccessfullyforawiderangeofsamples,andyieldinterestingresults(Table7).
Recentworkbythreegroupsofauthorsisbrieydiscussedbelow.
O¨zelandco-workersusedDTDcombinedon-linewithGC9GC–ToFMStoanalysetheessentialoilofpista-chiohulls[34]andthevolatilecompo-nentsofCheddarcheese[167].
Inbothcases,10mgofsamplewereplacedinaGCinjectorliner,glasswoolbeingusedtoholdthesampleinplace.
Afterabriefpurgeatambienttemperaturetoremovewatervapour,theDTDprogrammewasstarted.
Theheadoftherst-dimensionGCcolumnwascryo-cooledtoensuretrappingoftheanalytes.
Withtheessentialoil,some100compoundswereidentied—withthecheese,some55.
Zimmermannandhisgroup[168–171]collectedparticulatematter(PM<2.
5lm)onquartzbreltersandplacedlterpunchesrepresenting1–2.
5m3ofsampledairintoaninjectorlinertogetherwithanISmixtureforquantication.
DTD–GC–ToFMSre-vealedthepresenceofsome1,500com-pounds,outofwhichsome200couldbe(semi-)quantied.
WhenGC9GCwasusedinsteadofGC,some10-foldmore,i.
e.
,over10,000compoundswerede-tected.
AnexampleofaDTD–GC9GC–ToFMScontourplotisgi-veninFig.
13.
AtechniquewhichisstronglyrelatedtoDTDisDMI(orDSI:dicultmatrix/sampleintroduction)whichwasrstdescribedbyAmiravetal.
[172,173].
Theauthorsusedanexchangeablemicro-orl-vialwhichholdsthesampleandismanuallyplacedintheGCinjec-torusingaChromatoProbe(Varian,PaloAlto,CA,USA)[160,174].
Afterpurgingtheinjectorisheatedtoevapo-ratetheanalytes.
Attheendoftherun,thel-vialwhichcontainsnon-volatilesampleconstituentsisremovedfromtheFig.
12.
DTDautomatedlinerexchanginghead[163].
Left,open;right,closedS50ChromatographiaSupplementVol.
69,2009ReviewTable7.
SelectedapplicationsofDTDcombinedwithGCAnalytesSample(mgorlL)Pre-treatmentDesorptionparametersCarriergasTrapDetectorLODRef.
DTDPAHsAerosollter25°C–12°/s–300°C20mLmin-1Glasswool,-100°CMS[183]PAHs,alkanesAerosollter100°C–275°C(7min)55kPaNoMS5–240ngm-3[162]VolatilesPlantmaterialGrind,dry180°C,15min20mLmin-1Tenax,-30°CMS[184]VolatilesOakwood(125),plantmaterial(2–15)Grind,dry180°C,15min20,50mLmin-1Tenax,-30°CMS[185,186]VolatilesOliveoil(10)20°C–30°/min-40°C(20min)100mLmin-1Cryo,-120°CMS[187]VolatilesPlants(5–25)Dry,grind180°C(15min)50mLmin-1Tenax,-30°CMS[152]VolatilesOliveoil(5)40°C–16°/s-70,175,250or600°C(10min)210kPaCryo,-150°CToFMSa[188]VolatilesApricot(2–5)Cut,dry150°C(5min)–Carbonblack+mol.
sieve,-30°CToFMS[189]SVOCsAerosollter50°C(2min)–1°/s-320°C(15min)2,5mLmin-1–ToFMSa[168–171]SVOCsAerosollter320°C(15min)3mLmin-1–MS[190]SVOCsAerosollter120°C–3°/s-350°C(3min)4.
5mLmin-1–MS[191]ExplosivesPTFEwipe45°C–40°/min-280°C(3min)-80°C285mLmin-1Tenax,40°CECD2–3ng[192]ExplosivesWipe50°C–30°/min-170°C(2min)6.
4mLmin-1–ECD,NCIMS30–350pg,0.
1–6ng[193]SolventadditivesWaterbornepaints(0.
3)60°C–20°/min-190°C(solvents)190°C–20°/min-280°C(additives)550°C(pyrolysis)50mLmin-1CryoMS[194]ResidualsolventsPrintedpaper27°C(1min)-3°C/min-40°C(10min)10mLmin-1–FID[195]PreservativesPinesapwood(10)Grind45°C(0.
5min)-10°/s-200°C(1min)128kPa–MS4mgkg-1[166]EssentialoilPlantmaterial(10)Dry,grind40°C(2min)-400°C/min-150°C(5min)255kPaCryoToFMSa[34,167,196,197]DMIPesticidesTomatoesBlendwithAcet.
90(1min,solventvent50mLmin-1)-300°/min-250°C(0.
5min)-900°C5mLmin-1–PFPD[173]FattyacidsGreenmicroalgae(1.
8lg)TMSHmethylation40°C(5s)-16°/s-350°C70kPa–MS[180]FattyacidsAquaticmicro-organisms(1.
8lg)TMSHmethylation40°C(5s)-16°/s-350°C70kPa–MS[181]PesticidesFood(5)ExtractinEtOAc50°C(2min,solventvent150mLmin-1)-6°/s-280°C1mLmin-1–ToFMS1–10ngg-1[175]PesticidesFood(5)ExtractinEtOAc70°C(3.
3min,solventvent)-4°/s-280°C1mLmin-1–ToFMS<10ngg-1[176]aGC9GCinsteadofGCanalysisReviewChromatographiaSupplementVol.
69,2009S51injector.
DeKoningetal.
[175]includedthisapproachintheliner-exchangeset-updiscussedabovetoanalysepesticidesinfoodbyGC–ToFMS.
TheXYZsampleprocessingrobotnowholdsatraywithanumberofsampleextracts,whileanadditionaltraycontainsanequalnumberoflinerscontainingal-vial.
Justbeforeanalysis,afreshlinerisplacedintheinjector.
Afterthesamplepreparation,therobotinjectsanamountofsampleextractinthel-vialinthelinerforGCanalysis.
TheLODswere1–10ngg-1,whichmeetstheEuropeandirectivesforbaby-foodanalysis.
Patelandco-workerspublishedarelatedstudy[176]ontheuseofDMIincontractlaboratories.
SilanizationoftheDTDlinerswasfoundtobeparticularlyimportanttomaskactivesitespresentinthefrit.
EliminationofacommonlyemployedGPCorSPEclean-upstepacceleratedsampleprocessingandpro-videdasignicantreductionofthesol-ventusage.
Otherauthors[177–179]combinedrapidanalyteisolationbymeansofliquidpartitioningplusdis-persiveSPE(toremovefatsandwaxes)withDMItodeterminepesticideresi-duesinvegetablesandfruits.
Blokkeretal.
[180]usedtheDMIapproachtorecordthefattyacidprolesofmicro-algaeandvegetableoils(whichincludedin-unittransestericationofthetargetcompoundsintoFAMEs)andthechemicalanalysisofsporesandpollen(whichcouldbecarriedoutwithlessthantenpollenperanalysis).
Akotoetal.
[181]usedthesameapproachforthefattyacidGC–MSprolingofrawbiologicalsamples.
Theauthorsstatedthatupto18algalandmicrobialcellsamplescouldbeanalysedperday.
O¨zeletal.
[34]comparedtheper-formanceofDTD,steamdistillation(SD)andSHWEforthedeterminationofvolatilecompoundsfromplantleaves.
TheauthorsconcludedthatthechemicalcompositionsofthevolatilefractionsobtainedbySDandSHWEweresimilar,butagreaternumberofcompoundswasisolatedwhenusingDTD.
Theconclusionpartlyagreeswithamuchearlierstudy[182]whereitwasshownthat,althoughthechromato-graphicprolesofplantvolatilefrac-tionsobtainedbySDandDTDweresimilar,therecoveryofbothlow-volatileandthermolabilecompoundswerebetterusingDTD.
Solid-PhaseExtractionInthelate1970s,SPEwasintroducedforthepre-treatmentofaqueoussam-ples.
Sincethattime,o-lineand,Fig.
13.
DTD–GC9GC–ToFMStotalioncurrentcontourplotofanaerosolsample:(a)showsthefullchromatogramoftheanalysedaerosoland(b)theenlargementofaselectedsection;(c)representsthissectionoverlaidwithabubbleplotgeneratedfromthepeakapicesofthesamesection[168]S52ChromatographiaSupplementVol.
69,2009Reviewspecically,on-linetraceenrichmentandclean-upbymeansofSPEusingpre-columnsor(disposable)cartridgeshasbecomeaverypopular—probablythemostpopular—column-switchingtech-niqueinLC.
Mosttechniquesandmuchofthehardwareusedtodayforo-lineSPE-GCandon-lineSPE–GCwereadaptedfromthecorrespondingLCtechniques.
Inthe1990s,semi-andfullyautomatedsystemsweredesignedforbothchromatographictechniques,andscoresofo-line,at-lineandon-lineapplicationswerereported.
Conse-quently,manyofthemoreinformativereviews[198–200]werepublishedinthatperiod—withenvironmentalapplica-tionsbeingthemaineldofinterestforGC-basedstudies.
SPEcartridgeshavedimensionsof,typically,10–20mmlengthx1–4.
6mmID.
Inmostinstancesthecartridgesarepackedwith10–30lmsorbentssuchasC18-orC8-bondedsilicaorastyrene–divinylbenzene(SDB)copolymer.
Theseareessentiallynon-selectivesorbentsbe-causeformanyapplicationstheSPEstepshouldprimarilyguaranteetheenrich-mentofanalytescoveringawiderangeofpolarities,withthesubsequentchro-matographicseparation(plusdetection)stepensuringtheproperrecognitionoftheindividualcompounds.
Sincesepara-tion-plus-detectionismuchmorepow-erfulinGCthaninLCanalysis,withtheformertechniquethebondedsilicasandthecopolymerarevirtuallytheonlysorbentsusedinreal-lifeapplications.
Atypicalset-upforSPE–GCisdepictedinFig.
14.
Aftercartridgeconditioning,asamplevolumeof,often,some10mLisloadedataspeedofseveralmLmin-1,thecartridgeiscleanedwithafewmilli-litresofwater,anddriedforsome20–30minwithnitrogenatambienttem-perature.
Next,theenrichedanalytesaredesorbedwithaslittleas100lLofanorganicsolvent—frequentlyethylormethylacetate—andtransferredon-linetotheGCpartofthesystemforfurtheranalysis.
Thereisabundantexperimentalevidence[201–203]thatwith,e.
g.
,GC–MS,GC–NPDorGC–AEDasinstru-mentalanalyticaltechniques,LODsof5–50ngmL-1canbeobtainedforawidevarietyofmicro-contaminantsin10-mLreal-lifesamples.
Theaboveconclusionisanimportantonebecause(semi-)automatedSPE–GCisindeedaverypowerfultechniquebutis,atthesametime,somewhatmorecomplexthanisappreciatedbymanyanalystswho,therefore,prefertouseano-lineprocedure.
Theprotocolis,then,essentiallythesameastheonegivenaboveand,ifdesired,theSPEpartoftheprocedurecanbecarriedoutfullyauto-matedlyonastand-aloneinstrumentsuchastheSymbiosis(Spark,Emmen,TheNetherlands)—thesuccessorofthehighlysuccessfulProspekt—theMPS2-SPE(Gerstel)ortheASPECXL(Gil-son,Middleton,WI,USA).
However,theSPEeluatecontainingtheanalytesisnowcollectedinavialand,typically,some25lLareinjectedbymeansofLVI–GC.
Inotherwords,thereisafourfoldlossinperformancecomparedwiththeon-lineoperation(25outof100lL),whichcaneitherbeaccepted(ifanalytedetectabilitydoesnotcreateproblems)orcanbecompensatedbyloadingafourfoldlargervolume(whichusuallywillnotcausebreakthroughproblems:experienceshowsthatthesedonottendtooccurforsamplevolumesoflessthan100mL).
Onemainadvantageofthevariouson-lineset-upsbrieyreferredtoabovewasthesubstantialsample-volumereductionfromtheconventional100–200mL(combinedwithclassical1-lLinjectionvolumes)to,typically,10andsometimeseven1–2mL,whichcouldbeeectedwithoutadverselyaectingtheanalyticalperformanceoftheprocedures.
Here,oneshouldaddthatSPEalsoisarewardingtechniquewhenultra-tracelevelsof,e.
g.
,0.
01–0.
5ngmL-1,ofmi-cro-contaminantshavetobedeterminedinmarinewaters.
Insuchcases,samplevolumestypicallyareaslargeas5–20Lando-lineproceduresinvolvingtheuseof47–90mmdiameterC18orSDBdisksorstackedcartridgespackedwithgraph-itizedblackcarbonarepreferablyused.
ApplicationsForthereasonsoutlinedabove,mostoftheselectedon-lineapplicationsincludedinTable8arefromthe1990sratherthanthepastfewyears.
Forreadersinterestedinsettingupasystemoftheirown—whereaspectssuchascompleteremovalofwaterfromtheloadedcartridgestopreventGCcolumnproblems,re-useofcartridgesandcompleteretentionofevenvolatileanalytesarerelevantissues—twootherpapersarerecommended[202,204].
Thetablealsofeaturesseveralvery-large-volumeapplications.
OnetypicalexampleisdescribedbyHankemeieretal.
[202]whousedSDB-SPE–GC–MStoanalyse10-mLriverwatersamples(withoutandwithspikingFig.
14.
Schemeofanon-lineSPE–GC–MSsystem[202]ReviewChromatographiaSupplementVol.
69,2009S53Table8.
SelectedapplicationsofSPEcombinedwithGCAnalyteSample(mLorg)Pre-treatmentSorbentDesorptionsolvent(mL)Post-treatmentDetectorLOD(ngL-1)Recovery(%)Ref.
On-lineMicrocontaminantsWater(20)SDBEtOAc(0.
1)IR[213]MicrocontaminantsWater(10)SDBEtOAc(0.
1)MS20–5070–115[202]MicrocontaminantsRiverwater(75)FilterSDBEtOAc(0.
75)MS,AED20–200[214]MicrocontaminantsWater(50)SDBEtOAc(0.
1)MS,AED20–1,000[215]MicropollutantsWater(7.
5)SDBEtOAc(0.
05)MS(SIM)0.
5–575–95[216]PesticidesWater(10)30%MeOHSDBEtOAc(0.
1)MS(SIM)2–2015–100[217]PesticidesWater(10–100)SDBEtOAc(0.
1)MS/MS0.
01–4[218]OPPsWater(100)SDBEtOAc(0.
1)AED0.
5–1.
560–105[201]EndocrinedisruptorsWater(15)Add50%MeOHSDBEtOAc(0.
3)MS1–3530–110[205]Alkyl-,chloro-,mononitrophenolsWater(10)Deriv.
SDBEtOAc(0.
1)MS(SIM)1–15[206]Large-volumeMicropollutantsWater(50)SDBEtOAc(0.
3)MS1–2[219]PesticidesSeawater(10,000)SDBEtOAc(3930)+Hex–EtOAc(4:1)(50)Conc.
,silicaclean-up,conc.
MS0.
1–0.
760–130[220]Triazines,OPPs,acetanilides,OCPsMarshwater(10,000)90mmC18diskDCM–Acet(1:1)(40)Filter,conc.
MS0.
05–27–130[221]SelectiveSemaridineHumanplasma(0.
6)LLEMIPHep(1.
7)WashingNPD[210]CholesterolYolk(10)SaponicationMIPTCM–EtOH–EtAc(3:1:1)(3)Deriv.
FID[209]Organo-ScompoundsWater(10)Pb(II)-loadedcation-exchange1%CS2intoluenePFPD500–3,000[222]OPPsRiverwater(1000)FilterMIPDCM–MeOH(9:1)NPD10–3082–99[223]TriazinesWater(10)IAGlycinebuerSPEFID,NPD15–25,1.
5–2.
5[212]S54ChromatographiaSupplementVol.
69,2009Reviewof86micro-contaminantsatthe0.
5lgL-1level).
Full-scanMStracesareshowninFig.
15.
LODswereinthe20–50ngL-1rangeorlowerforessen-tiallyallanalytes.
Theidenticationpo-tentialoftheprocedureisillustratedbym/ztracesofthefourcharacteristicionsofpeak11(benzaldehyde)intheraw,i.
e.
,non-spiked,water.
Itsconcentrationwascalculatedtobeapprox.
40ngL-1.
Asimilarapproachwasusedfortheanalysisofendocrinedisruptorssuchasatrazine,hexachlorobenzene,DDTandbenzo[a]pyrenebySPE–GC–MS[205].
Inthiscasethe15-mLwatersamplecontained50%methanoltopreventsorptionproblems.
100lLethylacetatewereusedforanalytedesorption.
TheLODsforthetargetanalyteswere1–40ngL-1.
Jahr[206]usedautomatedSPE–GC–MStodetermine26alkyl-,chloro-,andnitrophenols(aftertheirin-samplederivatization)indrinkingwaterandriverwater.
Time-scheduledSIM-MSenabledtargetanalysisdownto,typically,LODsof2–10ngL-1.
Sofar,nomentionhasbeenmadeofmorespecializedSPEphasessuchasre-stricted-accessmedia(RAM),molecularimprintedpolymers(MIP),immunoaf-nityextraction(IAE)phasesandotherclass-orcompound-selectivesorbents.
ThisisbecausealmostallapplicationswhichutilizeoneoftheseselectivetypesofsorbentuseLCforsubsequentanal-ysis(see,e.
g.
,[207,208]).
Althoughthisis,therefore,anarealargelybeyondthescopeofthepresentreview,afewperti-nentexamplesareincludedinTable8.
Shietal.
[209]analysedcholesterolinyolk.
Aftersaponicationandtheaddi-tionofwaterandhexane,1mLoftheorganicphasewasloadedontheMISPEcartridge.
Afterrepeatedwashing,elu-tionwasdonewith3mLchloroform–ethanol–aceticacid(3:1:1).
Theeluatewasevaporatedtodrynessandtheresi-duedissolvedinpyridinewithsub-sequentderivatizationwithBSTFA.
AnalysisbymeansofGC–FIDshowedthatMISPEcreatedmoreselectivitythanC18-SPEtreatment.
However,mostoftheextraclean-upwascreatedinpartsoftheGCchromatogramfarremovedfromtheanalyteposition.
ArathersimilarconclusionholdsfortheMISPE-baseddeterminationofsemaridineinplasma[210].
Fortheselectiveextractionoftributylphosphate(TBP)fromdiesel,Harvey[211]injected20lLofdieselonaMIPcolumn(3793.
0mm).
Afterelution,theTBP-containingfractionwasanalysedbyLVI–GC–FID.
Figure16,Fig.
15.
TICchromatogramforSPE–GC–MSof10mLofriverRhinewater(a)spikedatthe0.
5lgL-1levelwith86microcontaminantsand(b)non-spiked.
A50-lLvolumeofmethylacetatewasusedaspresolvent.
Forpeakassignment,seeref.
[202].
Theinsert(c)showsthemasschromatogramsoffourcharacteristicmassesofbenzaldehyde(m/z51,77,105and106).
ThetimescaleforthemasschromatogramistwiceaslargeasfortheTICchromatogramFig.
16.
GC–FIDofTBP-spikeddieselsample(top)andthefractionretainedbytheTBP-specicMIP(bottom)[211]ReviewChromatographiaSupplementVol.
69,2009S55showsthechromatogramofthatfrac-tionwhichis(notsurprisingly!
)muchcleanerthanthechromatogramoftheoriginaldieselsample.
On-lineIASPE–GC–FID/NPDwasusedtodeterminetriazinesin10-mLwatersamples[212].
Sincethematerialisnotcompatiblewithanorganicsolvent,afterenrichmenttheanalyteswereelutedwithanaqueousglycinebuerandtransferredon-linetoanSDBcartridge.
Afterclean-upanddryingofthecar-tridge,theentireextractwasdesorbedwithethylacetateandtransferredon-linetotheGCcolumn.
Theselectivitywassuchthatanon-selectiveFIDcouldbeusedfordetection(Fig.
17),withLODsof15–25ngL-1.
Withaselectivedetec-tor,i.
e.
,anNPD,theLODscouldbeimproved10-fold.
Solid-PhaseMicro-ExtractionIn1990,solid-phasemicro-extraction(SPME)wasintroducedbyArthurandPawliszynasanorganic-solvent-freeextractiontechnique[224].
Thetheoryandpracticeofthemethodhavebeenexaminedinconsiderabledetail[225–228]andnumerousapplicationshavebeenreportedandreviewed[229,230].
Basically,thetechniqueenablesthetraceenrichmentofanalytesbytheexposureofafused-silicabrecoatedwithanappropriatesorbentlayer,foraselectedtime,toagasorliquidsample,withthesubsequent(rapid)desorptionofthetargetanalytesbyheatingtheexposedbreintheinjectionportofaGC.
Anumberofbrecoatings,whichoerarangeofanalytesolubilitiesandporosi-ties,arecommerciallyavailable.
Theseincludethehighlypopularnon-polarpolydimethylsiloxane(PDMS)andmorepolarcoatingssuchasPDMS–divinylbenzenecopolymers,polyacry-latesandmixturesofcarboxen(aninorganicadsorbent)andPDMSordivinylbenzene.
Theirmutuallydierentphysicochemicalcharacteristicshelptowidentheapplicationrangeofthetech-nique.
Fibrecoatingsareavailableinincreasingthicknessesfrom7–150lm,whichincreasesthepartitioningratioofthetargetanalytes—and,hence,analytedetectability—butalsoincreasesequili-brationtimes.
TheschematicofanSPMEdeviceisshowninFig.
18.
Thebreismountedinasyringe-likedeviceforprotectionandeaseofhandling.
TheneedleservestoconvenientlypiercetheseptumofasamplevialortheGCinjector.
Thatis,duringanalyteextractionanddesorp-tion,thebreisexposedbutduringtransferofthesampletotheGCinjector,itisinsidetheprotectiveneedle.
Obvi-ously,thisisanelegantapproach,andthefactthatnosolventisrequiredisadistinctadvantage.
Ontheotherhand,itisadisadvantagethatthebresarera-therfragile,eventhoughtheyareshiel-dedwhenoutofthesampleorinjector;theycanalsobedamagedbythebuild-upofinvolatilematerialfromthesam-ples.
[Toimprovetherobustnessofthetechnique,Lipinski[231]introduced(automated)solid-phasedynamicextraction(SPDE)whichusesneedlespreparedfromstainless-steelcapillarycolumns,withPDMS-coatedinnerwalls.
]InatypicalSPMEexperiment,thecoatedbreisexposeddirectlyimmersedin,ortotheheadspaceof,asmallvol-umeofliquidorsampleextract,usuallysome2–5mL.
TheanalytespartitionintothestationaryphaseuntilplateauFig.
17.
GC–FIDchromatogramsofanextractobtainedby(a)on-lineSPEand(b)on-lineIASPEof10mLofmunicipalwastewater,spikedwith1mgL-1ofseventriazines.
(c)BlankrunofIASPE–GC–NPDof10mLofHPLC-gradewater.
1=Atrazine;2=Terbuthylazine;3=Sebuthylazine;4=Simetryn;5=Prometryn;6=Terbutryn;7=Dipropetryn[212]Fig.
18.
CommercialSPMEdevice[234].
(a)SPMEbreholder;(b)sectionviewofSPMEholderandbreassemblyS56ChromatographiaSupplementVol.
69,2009Reviewconditionsarereached,whichtypicallytakes2–60min.
Theprocesscanbeai-dedbysalting-out(additionof,e.
g.
,25%NaCl)and/orpHadjustment,sampleagitation(tospeedupanalytetransportfromthebulkofthesolutiontothevicinityofthebre)andheating[232,233].
Adversematrixeectscanbeavoidedbyapplyingastandardadditionprocedureforquanticationor,lessfre-quently,usingprotectivemembranestopreventadsorptionofmatrixcompo-nentsonthebre[226].
Ifselectivedetection,suchasMSintheSIMmodeorECD,isused,LODsforbothvolatileandsemi-volatileana-lytestypicallyareinthelow-ngmL-1,andsometimesinthengL-1,range.
However,oneshouldconsiderthatSPME(asisalsotruefore.
g.
,SBSE;seesectiononSBSE)isanequilibriumtechnique.
Thatis,althoughfavourableanalytescanbeextractedessentiallyquantitatively,therearealsomanyclas-sesofcompoundsforwhichthisiscer-tainlynottrue—actually,itisnotunusualtondrecoveriesoflessthan10%inthepublishedliterature(Table9).
Forsuchclassesofcom-pounds,conventionalSPE(cf.
sectiononSPE)canalwaysprovide(substantially)betteranalytedetectability.
Admittedly,non-equilibriummethodscanalsobeusedforSPME—andalsoforSBSEandHS—butthiswilldecreasemethodsen-sitivityandwillrequirehighlyprecisetimingprocedures.
Asalreadyindicatedabove,therearethreemodesofSPME,viz.
theoftenapplieddirect-immersionextraction(DI-SPME)andheadspaceextraction(HS-SPME)andtherarelyusedmembrane-protectedSPME(Fig.
19).
ItwillbeclearthatDI-SPMEisaverystraight-forwardtechniquewhichdoesnotre-quirefurtherdiscussion.
However,exposingthefragilebrestohighlycomplexsamples—which,inaddition,cancontainhighNaClconcentrationsand/orhaveatooextremepH—maywellcausedamageand,consequently,leadtoerroneousresults.
Theincreas-inglypopularHS-SPMEmodeprimarilyservestoprotectthebrecoatingfromsuchdamagebyhigh-molecular-massmaterialsuchashumicsubstancesorproteinsandothernon-volatilespresentinthesamplematrix.
Self-evidently,modifyingthesamplecompositionnowdoesnotcreateanyproblemseither.
OneshouldnotethattheamountsofanalyteextractedintothebrecoatingarethesameatequilibriumforDIandHSsamplingprovidedthesamplevial,andthevolumesoftheliquidsampleandthegaseousheadspacearethesame.
Thisisduetothefactthattheequilibriumconcentrationisindependentofthebrelocationinthesample/headspacesystem.
Iftheaboveconditionsarenotsatised,asignicantsensitivitydierencebe-tweenthetwoapproachesexistsonlyforveryvolatileanalytes.
Withmembrane-protectedSPMEthemainpurposeofthebarrieralsoistoprotectthebreagainstdamage,viz.
whenverydirtysampleshavetobeanalysed.
Inaddition,membranepro-tectioncanbeusedforthedeterminationofanalyteshavingvolatilitiestoolowfortheheadspaceapproach.
Inprinciple,asuitablemembranecanaddadegreeofselectivitytotheextractionprocess.
However,theanalystshouldconsiderthatthekineticsofmembraneextractionaresubstantiallyslowerthanfordirectextraction,becausetheanalytesmustdiusethroughthemembranebeforetheycanreachthecoating.
Intheliterature,rathermuchatten-tionisdevotedtoextendingtheappli-cationrangeofSPMEtomorepolarcompounds.
Generallyspeaking,thisisanapproachwhichisnottoberecom-mendedtoday,sincemostclassesofpo-larcompoundscanbeanalysedsuccessfullybymeansofLC–MStech-niques(alsoseesectiononStir-BarSorptiveExtraction).
WiththeLC-basedroute,theintactcompoundscanbesubjectedtoanalysisandtime-consum-ingderivatization(which,moreover,of-tengeneratesartefactsandisfrequentlynotsuccessfulattheultra-trace-level)isavoided.
Thereare,however,alsoin-stanceswhentheLCroutecannotbeusedandSPME-cum-derivatizationhastobeapplied[235].
Derivatizationcanbeperformedindierentways,withdirectderivatizationinthesamplematrix[236]andonthebre[237]beingmostpopu-lar.
DerivatizationintheGCinjectionportisalsoused[238].
Asregardson-brederivatization,therearetwomodesofoperation,viz.
(1)samplingofthetargetanalytesonthebrewithsub-sequentexposureofthebretotheHS-derivatizingreagentsolution,and(2)exposureofthebretotheHSanalytesolutionafterithasbeenexposedtothederivatizingreagentsolution.
Practicalexamplesofeachoftheseapproacheswillbegivenbelow,inthesectiononapplications.
TheSPMEtechniqueismarketedbySupelco(Bellefonte,PA,USA).
Mostreportedapplicationsareofthemanualtype.
However,automatedanalysiscanbeperformedbyusingsystemscommer-cializedbyVarian(PaloAlto,CA,USA)[174,236]andCTC(Zwingen,Switzer-land)[239–241].
Recently,PawliszynandhisgroupreportedtheautomationofSPMEona96-wellplateformat[242],whichtheyclaimtobeaviableapproachcompatiblewithbothGCandLCplat-forms.
ApplicationsIntheearlyyears,SPMEwasusedprimarilyforthedeterminationofrelativelyvolatilecompoundsofenvironmentalinterest[243,244].
Today,therearealsomanyapplicationsinthebiomedicaleld[245]andforfoodanalysis[246].
Moreover,aswasdis-cussedabove,thetechniqueisalsousedforlessvolatilecompounds[234].
Anumberofrelevantapplicationsarelis-tedinTable9.
Someofthesearebrieydiscussedbelow.
ApopularapplicationofSPMEistheanalysisofaromacompoundsinwine.
Togiveanexample,Penaetal.
[247]determinedmonoterpenesbyadd-ingNaClto7mLofwinetoobtainanalsaltconcentrationof25%.
SPMEwasperformedbyimmersinga100-lmPDMS-coatedbrefor15mininthesample,withstirringat1,100rpm.
Withanalyterecoveriesof71–91%,theLODs(TICMS)were11–25lgL-1.
Intheenvironmentaleld,HS-SPMEwasusedtodeterminevolatileorganochlorinesinlandllleachates[248].
10mLofsamplewereputina12-mLvial.
Nosaltwasaddedandthesamplewaskeptatroomtemperature.
TheHS-SPMEprocedure,whichuseda10-lmPDMS-coatedbreandstirringat900rpm,wascompletein2min.
WithLODs(SIMMS)of0.
05–0.
10ngmL-1andanalyterecoveriesofReviewChromatographiaSupplementVol.
69,2009S57Table9.
SelectedapplicationsofSPMEincombinationwithGCAnalytesSample(gormL)Pre-treatmentExtractionDesorptionDetectorLOD(ngmL-1orngg-1)Recovery(%)Ref.
DirectimmersionPAHsVegetableoil(0.
2)DilutewithHex30minThermalToFMSd0.
1–1.
5[250]PAHmetabolitesUrine(5)Enzymehydrolysis45min,35°CaThermalMS[237]PAHmetabolitesWater(1)40min,900rpmMeCNToFMS0.
06–0.
2[242]FenitrothionandistmetabolitesPoplarleavesLiquidextraction40minThermalMS(SIM)0.
5–2.
5[251]MonoterpenesWine(7)25%NaCl15minThermalMS11–2570–90[247]AmphetaminesUrine(1.
2)pH,deriv.
b16minThermalMS5–151.
5–10[236]HeadspacePCBsWater(20)MAE(30W)10min,100°CThermalECD0.
3–1.
5ngL-155–160[252]VOCsRiverwater(20)30min,70°CThermalECD0.
2–11ngL-195–110[253]VOXsLandllleachates(10)2minThermalMS(SIM)0.
05–0.
1090–100[248]DichlorvosFruitsandvegetables(2)MAE(10min,132W,pH5)10minThermalECD105[254]BenzenesWorkplaceair10minThermalMS[243]AromaproleWine(20)15min,750rpmThermalMS[255]AmphetaminesBlood(0.
5)1MNaOH15mincThermalMS5–102.
0–6.
5[238]SulphurvolatilesCheese(2)Cutincubesof0.
5cm30min,50°C,250rpmThermalPFPD[256]VolatilesIcewine(3)1gNaCl5min,45°CThermalToFMS[239]Phenols,haloanisolesWine(5)0–2gNaCl60min,70°CThermalMS/MS0.
01–0.
1595–100[241]a,b,cProcedureinvolveson-bre(a),direct(b)orinjection-port(c)derivatizationdGC9GCinsteadofGCanalysisS58ChromatographiaSupplementVol.
69,2009Review93–100%,theresultswerecloselysimilartothosefoundwithconventionalhead-space(HS)analysis.
However,HS-SPMEwasfasterthanHS(2minvs.
15min);ontheotherhand,HSgavemorepreciseresults.
Asforderivatization,thedirectap-proachhasbeenusedfortheautomatedSPMEdeterminationofamphetamines(ascarbamates)inbueredurinesam-ples,withpropylchloroformateasderiv-atizationagent[236].
Analyterecoverieswerelessthan10%andtheLODs(TICMS)weresomewhathigh(5–15ngmL-1).
ThesamecompoundswerealsoanalysedinwholebloodviaHS-SPMEandinjection-portderivatiza-tionwithheptauorobutyricanhydride[238].
Desorption-cum-derivatizationtookonly3min.
Finally,theon-brealternativewasappliedfor,e.
g.
,PAHmetabolitesinurine[237].
SPMEwithan85-lmpolyacrylatebrewasrathertime-consuming,i.
e.
,45minat35°Cunderstirring.
Afterextraction,thebrewasplacedintheheadspaceofavialcon-tainingBSTFA;derivatizationat60°Ctook45min.
Thebrewasthentrans-ferredtothehotinjectionportoftheGCanddesorbedfor3min.
AsanalternativetoSPME,andalsoSBSE,Burgeretal.
[249]introducedtheuseofasample-enrichmentprobe(SEP),whichwasdevelopedprimarilyforHSanalysis.
TheSEPconsistsofathinrodofaninertmaterial,providedatoneendwithashortsleeveofPDMSforthehigh-capacityanalyteenrichment.
Afterenrichment,theendoftherodcarryingthesiliconerubberisintroducedintotheinjectorandtheanalytesaresubjectedtoTD–GC.
SEPissimilartoSPME,butamaindierenceisthatamuchlargermassofthesorptivephaseisemployed.
Resultsofthetwotechniquesfor(semi-)volatileorganiccompoundsarestatedtobecomparable.
Stir-BarSorptiveExtractionIntheprevioussection,therelativelysmallvolumeofboundstationaryphaseusedforanalyteextraction,wasquotedasamainlimitationofSPME.
ThispromptedthedevelopmentofanotherminiaturizedextractiontechniquebyBaltussenetal.
[257],stir-barsorptiveextraction(SBSE),marketedastheTwisterbyGerstel.
Thetechniquehasbeenreviewedinseveralrecentpapers[258–261].
InSBSE,amagneticstirbarof,typically,10–30mmlength,andcoatedwith24–47lLofpolymethyldisiloxane(PDMS),isrotatedinanaqueoussam-pleatsome1,000–1,500rpmforapre-settimewhichisoftenverylong,i.
e.
,60minormore.
After(near-)equilib-riumhasbeenreached,thestirbarisremovedbyhandwithtweezers,dippedbrieyindistilledwatertoremove,e.
g.
,absorbedsugarsorproteins,placedontissuepapertoremoveresidualdroplets.
Rinsingdoesnotcausesoluteloss,be-causetheadsorbedsolutesarepresentinsidethePDMSphase.
Asanalterna-tive,liquidrinsingwithanon-polarsol-ventsuchashexanecanbeused.
Finally,thestirbarisplacedinthelinerofathermaldesorptionsystemtoenableGCanalysis[258].
Afterthermalorliquiddesorption,thestirbarscanbere-used.
SamplevolumesinSBSEtypicallyareontheorderof2–20mL.
Thereare,however,alsoseveralapplicationswhichfeaturesamplesizesof80–200mL.
Sincethedimensionsofthestirbarsselectedforanalyteextractionarethesameaswhenusingmoremodestvolumes,stir-ringtimesnowfrequentlyareexcessive,i.
e.
,3–15h[16,262–264].
AsinSPME,analyteextractionfromtheaqueousphasetotheextractionmediumiscontrolledbythepartitioncoecientbetweenthetwophasesand,consequently,theKo/w.
Sincetheamountofsorbentcoatedonastirbaris50–100-foldlargerthanonanSPME-bre,thereisahigherphaseratiothaninSPMEand,hence,ahigherextractioneciency,whichresultsinimprovedanalytedetectability.
Today,onlyPDMSisavailableasanextractionphaseoncommerciallyavailablestirbarsandthelargemajorityofapplica-tionsthereforeusethiscoating.
Hereoneshouldaddthatattemptstoapplyothercoatingshavefailedmainlybecauseofirreproduciblecoatingorexcessivebleedingduringthermaldesorption[258].
Inthiscontext,arecentinnovationshouldbementioned,viz.
theintroduc-tionofdual-phasetwisterswhichcom-binetheconcentratingcapabilitiesoftwosamplingmaterials,PDMSandcarbon,whichoperateindierentways,i.
e.
,bysorptionandadsorption,respec-tively[265].
ThesestirbarsconsistofanouterPDMScoatingholdinganacti-vatedcarbonmaterialinside.
Twomag-neticstopperswhichcloseotheendsofthePDMStube,enablestirring.
Increasingrecoverieswerefoundforveryvolatilecompoundsemittedfromplantmaterialandforpolarsolutesinwater.
MostapplicationsofSBSEdealwithaqueoussamplescontaininglowcon-centrationsoforganiccompounds.
Samplescontaininghighconcentrationsofsolvents,detergents,etc.
shouldbedilutedbeforeextraction.
Ifveryhydro-phobicsoluteshavetobedetermined,suchas,e.
g.
,PAHsandPCBs,some10vol%ofanorganicisaddedtominimizewalladsorption,asisalsodonein,e.
g.
,SPE.
Thenegativeeectonthepartitionofthetargetcompoundscanbene-glectedbecauseoftheirhighKo/wvalues;actually,theoverallselectivityoftheFig.
19.
ModesofSPMEoperation:(a)DI-SPME,(b)HS-SPME,(c)membrane-protectedSPME[226]ReviewChromatographiaSupplementVol.
69,2009S59procedurewillimprovebecausemanylesshydrophobiccompoundswillbe(partly)ushedtowaste.
Inquiteanumberofpapers,SBSEiscombinedwithinsituderivatization[260,266–269],especiallyinordertoimprovetherecoveriesofpolaranalyteswiththeirlowKo/wvalues.
Derivatizationreactionsthatcanbeperformedinaqueousmediaincludeacylationofphenolsusingaceticanhydride,estericationofacids,acyla-tionofaminesusingethylchloroformateandoximationofaldehydesandketonesusingPFBHA.
However,ineverysingleinstancetheanalystshoulddulyconsiderwhetherthetime-consumingSBSE-cum-derivatizationprocedureshouldbeusedortheintactanalytessubjectedtoanLC-basedanalysis.
SBSEisalsousedforheadspacesorptiveextraction(HSSE).
Astirbarishungintheheadspaceofasample,oftenbyattachingthemagneticstirbartoapaperclip,whichpiercestheseptumofaheadspacevial,bymagneticforce.
HSSEhasbeenappliedtoheadspacesamplingofawidevarietyofinterestingsampletypes.
Theseincludearomaticandmedicinalplants[270],chiralmonoter-penesinessentialoilsincombinationwithenantio-MDGC–MS[271],coee[272]andvolatilemetabolomicsfromtoxigenefungi[273,274].
ApplicationsSBSEismainlyusedfortheGCanalysisofbiologicalandfoodsamples(Table10).
Someselectedapplicationsarebrieydiscussedbelow.
Sandraetal.
[275]determinedfungi-cidesinwine.
Theauthorspoured10mLofundilutedwineina20-mLheadspacevialandusedastirbarcontaining24lLPDMStostirthesamplefor40minat1,400rpm.
TheabsorbedcompoundsweretransferredtotheGC–MSsystembythermaldesorptionofthestirbar.
Althoughtherecoverieswereratherlow(7–35%),LODswereintherangeof0.
2–2ngmL-1.
Inordertodeterminethesevenso-calledBallschmiterPCBsinhumansperm,Benijtsetal.
[276]soni-cated1mLofspermtobreakthemem-braneofthespermatozoaanddilutedthesamplewith9mLwater–MeOH(1:1).
ForSBSE,theresultingsolutionwasrotatedfor25minat1,000rpmbyaPDMS-coatedstirbar.
Afterthermaldesorption,GC–MSwasperformedinthetime-scheduledSIMmode.
Withanalyterecoveriesof30–40%,theLODswere0.
1–3pgmL-1.
Kawaguchietal.
[267]appliedSBSEforthedeterminationofchlorophenolsin2mLofhumanurine.
ThesamplepHwasadjustedto11.
5priortotheadditionofthederivatizationreagent,aceticacidanhydride.
SBSEwasperformedfor60minwithstirringat500rpm.
GC–MSintheSIMmoderesultedinLODsof10–20pgmL-1withquantitativeanalyterecoveries.
Theex-tracted-ion-chromtogramsforthestudiedchlorophenolsderivatesareshowninFig.
20.
HSSEwasthesamplingmethodusedbyDemyttenaereetal.
[273]tomonitorthemycotoxinproductionoffungi.
Thefungiwerecultivatedin22-mLvials,andaPDMSstirbarwasheldintheheadspacefor1h.
Themycotoxinswereanalysedbythermaldesorption–GC–MS.
TheauthorsconcludedthatSPMEisfaster(30minextraction)andsimpler,becauseitdoesnotrequireaspecialthermaldesorptiondeviceand,also,becausetheconcentrationsofthetargetanalyteswererelativelyhigh.
Moreover,SPMEcaneasilybeautomatedandusedforfastdetection.
MembraneExtractionSeparationbymeansofamembranecanbeachievedinmanywaysandverygenerally,amembranecanbedenedasaselectivebarrierbetweentwophases.
Whenadrivingforceisappliedacrossamembrane,transportofmatteroccursfromthedonortotheacceptorphase,givingtheso-calledux.
Separationisachievedwhensomespeciesaretrans-portedtoalargerextentthanothersand,intheidealcase,componentsofinterestaretransferredquantitatively,whileallothersamplecomponentsremaininthedonorphase.
Membraneseparationprocessescanbeclassiedbymeansofthedrivingforcesinvolved.
Themostimportantonesaredierencesof(1)concentration,whichcauseamolecularux(transportofmolecules),(2)electricpotential,whichcauseanelectricalux(transportofcharge)and(3)pressure,whichcauseavolumeux(transportofbulkliquidorgas).
Veryoften,morethanonedrivingforceispresentinamembraneseparationprocess.
Awidevarietyofmembranemateri-alscanbeused.
Inmanycases,amem-braneisaporousnetworkofasyntheticpolymer,suchaspolypropylene,poly-sulphoneoracellulosederivative.
Sep-arationisbasedonlyonsize-exclusion:sucientlysmallmoleculescanpermeatethroughtheporesbutlargeronescan-not.
Moreselectivitycanbeobtainedwith,e.
g.
,ion-exchangemembranes,whichhavepositivelyornegativelychargedgroupscovalentlyattachedtothepolymericmembranematerial.
Sep-arationisnowbasedonbothsizeandchargedierencesofthevarioussolutes.
Non-porousmembranesarearatherdierentclass:theyconsistofaliquidorpolymerlm,intowhichamoleculemustactuallydissolveinordertobeabletopassthrough.
Foraparticularcom-pound,theeciencyofmembranetransportnowlargelydependsonitspartitioncoecientsbetweenthedier-entpartsofthemembraneseparationsystem.
Onlycompoundswhichareeas-ilyextractedfromthedonorphaseintothemembraneand,inaddition,easilyextractedfromthemembraneintotheacceptorphasewillbetransported.
Sep-arationisthereforebasedonthesameprincipleasinLLEwithasubsequentback-extractionandanalyteswithdif-ferentphysicochemicalpropertieswillbeextractedtoadierentextenteveniftheyareofequalsize.
Fourmembraneseparationtech-niquesarefrequentlyusedforsamplepreparation.
Threeofthese—dialysis(concentration-driven),electrodialysis(electricallydriven)andltration(pres-sure-driven)—utilizeporousmembranesandarecombined(mainly)withLC[289,290].
Theyarethereforebeyondthescopeofthisreview.
Onetechnique,so-calledmembraneextraction,usesnon-porousmembranesandiscombinedwithLCaswellasGC.
Themostfrequentlyusedmembrane-extractionsystem,referredtoassup-portedliquidmembrane(SLM),consistsofaporousmembranesupportimpreg-S60ChromatographiaSupplementVol.
69,2009ReviewTable10.
SelectedapplicationsofSBSEcombinedwithGCaAnalytesSample(mLorg)Pre-treatmentStirringDetectorLODRecovery(%)Ref.
SBSEPCBsHumansperm(3)Dilute1:9(water/MeOH1:1)45min,1,000rpmMS(SIM)3pgmL-130–40[276]25PCBsWater(8)2mLMeOH120min,1,000rpmMS(SIM)0.
05–0.
2ngL-175–95[277]PBDEsWater(100)20%MeOH25h,900rpmMS0.
3–10ngL-195–105[278]PesticidesBrewedgreentea(20)Teabrewing(1.
25g/200mL),centrifuge,30%NaCl60min,1,500rpmMS0.
6–110ngL-110–70[279]PesticidesFood(15)Homogenize,USEwithMeOH,centrifuge,dilutewithwater60min,1,000rpmMSLowngg-1–[280]OCPs,chlorobenzenesSoil(10)SHWE(water:MeCN75:25,130°C,100bar,3910min)180min,1,000rpmMS0.
002–5ngg-160–130[16]OPPsCucumber,potato(10)SLEwithAcet,centrifuge,dilute1:10,30%NaCl30min,600rpmTSD0.
003–0.
2ngg-195–105[281]FungicidesWine(10)40min,1,400rpmMS0.
2–2lgL-110–35[275]VolatilesWine(25)–90min,700rpm,60°CMS0.
001–6lgL-1–[282]35SVOCsWater(100)Add20%NaCl240min,1,400rpmMS0.
04–10ngL-1–[263]PhenolsWine(25)Dilutewithwater60min,900rpmMS(SIM)6–375lgL-190–100[283]ChlorophenolsWater(10)pH11.
5,deriv.
240min,500rpmMS(SIM)1–2pgmL-195–105[267]ChlorophenolsHumanurine(2)pH11.
5,deriv.
240min,500rpmMS10–20pgmL-195–110[267]Nonylphenols,octylphenolsWater(2)–60min,500rpmMS0.
002–0.
02lgL-195–105[268]EstrogensRiverwater(10,50)pH11.
5,deriv.
240min,500rpmMS0.
2–5pgmL-195–105[266]PhenolicxenoestrogensRiverwater(10)pH10.
5,deriv.
90min,1,000rpmMS(SIM)0.
5–2pgmL-195–115[284]AromaproleWine(20)Dilutewithwater60min,800rpmMS––[255]ExplosivesWater(10)–30min,4,000rpmIMSb0.
1–2ngmL-1–[285]AromatichydrocarbonsSeawater(200)–60min,900rpmMS0.
1–1ngL-1–[286]24organicpollutantsWater(100)10gNaCl12h,800rpm,21°CMS0.
1–5ngL-190–110[264]HSSEPBDEsWater(80)Add30%NaCl14h,95°ClECD0.
1–2pgmL-185–90[262]VolatilesCoeepowder(0.
050)–60min,50°CMS––[265]Halophenols,haloanisolesCork–60min,100°CMS3–30ngg-170–115[287]MycotoxinsFungi–60min,25°CMS––[273]SesquiterpenehydrocarbonsFungi–30min,25°CMS––[274]SevouraneUrine(1)Add1.
5mL10MH2SO460min,100°CMS(SIM)1lgL-1–[288]aForalmostallapplications:stirbar:10–30mm,24–47lLPDMS;post-treatment:(wash)+dry;desorption:splitless,250–280°CbIMS,ion-mobilityspectroscopyReviewChromatographiaSupplementVol.
69,2009S61natedwithawater-immiscibleorganicsolvent,whichispresentinthemem-branepores.
Inanotherapproach,non-poroussiliconerubberisusedasthemembranematerial.
Inbothcases,themembraneseparatestwoaqueousphasesandthesamplepH(donorchannel)isadjustedtoensurethattheanalytesofinterestarenotchargedandareeasilyextractedintothemembraneliquidorthesiliconepolymerlm.
TheacceptorphasehastheproperpHtoeectioni-zationoftheanalytesimmediatelyaftertheirpassingthemembranetopreventback-extraction.
Withthesiliconemem-branesonecanalsoaddanorganicsol-venttotheacceptorphasetoimprovethetrappingofneutralcompounds.
Thethirdmodeofmembraneextractionusesaporousmembranewithanorganicsolvent,bothinthemembraneporesandintheacceptorchannel.
Bothat-sheetandhollow-bremembraneunitscanbeapplied.
Withthistechnique,micropo-rousmembraneLLE(MMLLE),largerextractionsurfacescanbeachievedwiththehollowbres,whichleadstoim-provedextractioneciency.
Counter-currentdonor/acceptorsolventowisusuallyappliedinordertocreateopti-mumconditions[21,291].
MMLLEdif-fersfromtheothertwointhatitcanbecomparedtoasingleLLEstepratherthantoLLEplusaback-extraction.
Acommoncharacteristicofallthreetech-niquesisthatselectivityisobtainedbe-causesamplecomponentswhichdonotreadilydissolveinthemembraneliquid,areretainedinthedonorchannel.
Whenusingastagnantacceptorphaseandaowingdonorphase(themostcommonwayofmembraneextraction),thedonorphaseow-ratewillhaveadistinctinuenceonthemembrane-extractionperformance.
Iflowdetectionlimitsarerequiredandtherearenosample-volumelimitations(e.
g.
,withnaturalwaters),thebestoptionistousealargesampleandapplyarelativelyhighdonorow-rateof,of-ten,1–2mLmin-1[292].
Ifsamplevol-umeisalimitingfactor,suchasforplasma,thesampleiseitherkeptstag-nantinthedonorchannelorpumpedatalowow-rateoftypically25–50lLmin-1[293].
Alternatively,asamplecanbepassedthroughthemembranedeviceseveraltimestoobtainabetterrecovery.
Alsoformembraneextractions,therearesomepracticallimitationsandas-pectsworthtakingintoaccount.
Aproblemistheincompletetransferofanalytesfromthemembranetotheacceptorphaseduringthesampleprep-arationprocess.
Thisleadstoadecreaseintherecoveryand,moreseriously,tocarry-overeectsforsequentialanalyses.
Thoroughrinsingoftheacceptorchan-nelisthereforeessential.
Ingeneral,ifanalytesareeasilyextractedintothemembrane,theyalsoshowlargecarry-overeectsobviouslybecausetheyhaveahighanityforthemembranematerialandarenotreadilyreleasedintotheacceptorphase.
SinceforMMLLEthereisnodistinctionbetweenthemembranesolventandtheacceptorphase,therearenoproblemsofslowmasstransfertotheacceptorphaseorseriouscarry-overef-fectswiththistechnique.
Leakageofthemembraneliquidadverselyaectstheextractionperformanceandshouldbeavoidedasmuchaspossible.
Mem-branesimpregnatedwithnon-polarsol-ventswhichareinsolubleinwater,aregenerallystableforseveralweekswith-outanyregeneration.
Obviously,withsiliconemembranesthereisnoleakageofthemembranematerialandtheyare,indeed,quitestable.
Thecontinuoususeofasinglesiliconemembraneforaper-iodofmorethan2monthshasbeenre-ported[294].
Theapplicationofmembranesforon-linesamplepreparationwasatrendinthe1990s,wherethecouplingtoanLCsystemismoststraightforward:transferring(partof)theacceptorphasetoaninjectionloopandinjectingitisinprinciplesucient.
InordertocoupleanSLMandacapillaryGCsystemon-line,purewaterisusedastheacceptorphase.
Theanalytesaretrappedonapolymersorbent,whichisdriedwithnitrogenFig.
20.
Chromatogramsofchlorophenolsandsurrogatestandardsinhumanurinesample[267].
DCP,dichlorophenol;TrCP,trichlorophenol;TeCP,tetrachlorophenol;PCP,pentachlo-rophenolS62ChromatographiaSupplementVol.
69,2009Reviewpriortodesorptionwithanorganicsol-vent,e.
g.
,ethylacetate.
On-lineinjectiontoaGCisperformedviaLVI(alsoseesectiononSPE).
MoresuitablefordirectcouplingtoGCistheuseofanentirelyorganicacceptorphase,whichhasbeenperformedwithsiliconemembranes[295]andMMLLE[296,297].
Anotherauto-matedtechniqueofmembraneextrac-tionismembrane-assistedsolventextraction(MASE),whichwasrstde-scribedbyHauserandPopp[298].
Theextractioncellconsistsofaconventional20-mLheadspacevialwithamembraneinsert.
Membranebagsaremadefromdensepolypropylene,attachedtoastainless-steelfunnelandxedwithaPTFEring.
Thefunnelissuspendedintheopeningofthevial,whichisclosedwithacrimpcap.
Thevialcontainsanaqueoussample,typically15mL,andthebag100–800lLorganicsolvent.
AfteragitationanaliquotoftheorganicsolventisanalysedbyLVI–GC.
AnautomateddeviceismanufacturedbyGerstel.
Themembranetechniquesmentionedsofarareallcharacterizedbyliquiddonoraswellasacceptorphases.
How-ever,forbestcompatibilitywithGCagaseousacceptorphaseisthemoreconvenient.
Thisistheapproachusedinmembraneextractionwithasorbentinterface(MESI)[300].
Themembraneisapolymerichollowbre,andtheana-lytesareextractedfromthesurroundingliquidorgaseoussample(seeFig.
21fordierentcongurations).
Agasinsidethehollowbretransportstheanalytemoleculesintoacoldsorbenttubewheretheyaretrapped.
Next,theanalytesarethermallydesorbedfromthesorbentandguidedintotheGC.
Onecanalsouseacatalyticreactiontotraptheextractedanalytesdirectlyinthegasphase[301].
Inanintegratedinstrumentset-up,theGCcarriergaspassesthroughthemembranebreandthesorbenttrap[300].
However,onecanalsousethetechniqueo-line,e.
g.
,ineldsampling.
ThesorbenttrapisthenlaterconnectedtotheGCanddesorbedinaseparatestep[302,303].
ToquoteanexampleofMESI,Brownetal.
[304]describedthemonitoringoftrihalomethanesindrink-ingwater.
Thewaterwassampledataowrateof2.
5mLmin-1.
Analyteswereextractedinaheliumgasstreamof30mLmin-1andtrappedonTenax.
Next,thetrapwasheatedandtheana-lytesweretransferredtoaGC–ECDsystem.
LODsoftrihalomethaneswere0.
1–1lgL-1.
ApplicationsAlistofselectedappli-cationsfortheisolationofarangeofcompoundsfromavarietyofmatricesisshowninTable11.
ThislistisrestrictedtoGCapplicationsonly.
Anequallylong,ifnotlonger,listcouldalsobecompiledforLC.
ItwasstatedabovethatSLMcanbecombinedwithGC;however,norecentapplicationsarere-ported.
MASE,MMLLEandMIMS(membraneintroductionmassspec-trometry)aremainlyusedforenviron-mental(air,water),andfoodandbeverage(juice,wine)samples;anexampleofeachofthesetechniquesisbrieydiscussedbelow.
Rodiletal.
[305]determinedPAHsinwaterandbever-agesbymeansofMASEcombinedwithLVI–GC–MS.
A20-mLheadspacevialwaslledwith15mLofariverwater,applejuice,orredwinesample.
Apolypropylenemembranebagcontain-ing400lLofethylacetate,washunginthesampleandthevialclosed.
After60minofagitation,100lLoftheethylacetateextractwereanalysedbyPTV–GC–MS(SIM).
TheLODswere3–40ngL-1.
On-lineMMLLE–GC–MSofPAHsinredwinewasreportedbyHyo¨tyla¨inenetal.
[296].
TheMMLLEunitconsistedoftwoPTFEblocks,bothwith11-lLgrooves.
Thegrooveswereseparatedbyaporouspolypropylenemembranewettedwiththeacceptorsol-vent,toluene.
Extractionatadonorowrateof0.
2mLmin-1took40min.
TheacceptorphasewaspumpedtoaloopinaGCtransfervalve.
ThewholecontentoftheloopwasinjectedintotheGCtoensuretransferoftheentireextract.
TheLODsofanalytessuchasquinalphosandisoproturonforMSdetection(scanmode)wereintherangeof0.
03–0.
4lgL-1.
Figure22showsthechro-Fig.
21.
DierentcongurationsforMESI[299]ReviewChromatographiaSupplementVol.
69,2009S63Table11.
SelectedapplicationsofmembraneextractioncombinedwithGCAnalytesMatrix(mL,g)Pre-treatmentMembraneAcceptor(lL)Extractiontime(min)DetectorLODRecovery(%)Ref.
MASEPCBsRiverwater,whitewine,applejuice(15)–PPbagCyclohex(800)30MS(SIM)2–10ngL-188–114[307]PAHsRiverwater,redwine,applejuice(15)10%MeOHPPbagEtOAc(400)60MS(SIM)3–40ngL-170–136[305]PesticidesWastewater,bacterialculture(15)–PPbagCyclohex(1,000)30MS(SIM)2–50ngL-1–[308]VOCsRiverwater(15)–PPbagCyclopen(100)30ECD5–50ngL-140–96[309]PhenolsGroundwater(15)Sat.
NaCl,pH2PPbagEtOAc(800)60MS(SIM)9–600ngL-110–98[310]MESITrihalomethanesWaterSiliconeatsheetHelium0.
9ECD0.
1–1lgL-1110–128[304]VOCsWaterPDMS-BAPCaatsheetHydrogen0.
5TCD25–90ngL-1–[311]BiogenicemissionsEucalyptusleavesPDMSatsheetHeliumMS[312,313]MIMS(S)VOCsAir,water–PDMS-coatedPPhollowbreHelium–MS30–540lgL-1–[314]VolatilesMicrobiol.
system–Siliconeat-sheet,siliconehollowbreVacuum–MS––[315]BTEXWater–SiliconehollowbreVacuum0.
16ToFMS0.
03–1ngL-1–[306]Alcohols,organicacids,aromasBeer––Vacuum8MS0.
3–30mgL-1–[316]MMLLEPBDEsWater(100,owing)–PPhollowbreUndecane(10,stagnant)60MS(SIM)0.
3–1ngL-185–110[317]PAHsSediment(0.
010),Soil(0.
005)SHWE(30min,300°C)PPhollowbreCyclohex(30,stagnant)30MS(SIM)0.
1–1lgg-1–[291]PAHsSoil(–)SHWE(50min,300°C)PTFEatsheet,PPatsheetIsooctane(125,stagnant)50FID0.
7–2lgg-1–[21]PesticidesWine(6,owing)DilutePPatsheetCyclohex(150,stagnant)30FID1–370lgL-19–35[318]PesticidesWine(8,owing)PPatsheetTol(11,stagnant)40MS0.
03–0.
4lgL-1–[296]aBAPC,bisphenolApolycarbonateS64ChromatographiaSupplementVol.
69,2009Reviewmatogramsofablankwine,aspikedredwineandapositiveredwine.
DirectcombinationofmembraneextractionwithMS,sowithoutaGCinbetween,ispossible.
ContinuousBTEXscreeningbymeansofMIMSwasdescribedbyOseretal.
[306].
Aconstantowofwaterwaspumpedthroughasiliconemembranetube.
Asthesamplepassesacrosstheinnersurfaceofthemem-brane,theanalytesdiusethroughthemembraneandevaporateintotheMSionsource.
LODsobtainedbyToFMSwere0.
03–1ngL-1.
Single-DropMicro-ExtractionIn1996,LiuandDasgupta[319],andJeannotandCantwell[320]introducedtheconceptofusingasmalldropforsamplepreparation,so-calledsingle-dropmicro-extraction(SDME),whichcombinesanalyteextractionandpre-concentrationpriortoinstrumentalanalysis.
ForreviewsonSDME,thereadershouldconsultrefs.
[321–325].
LiuandDasguptareporteda'drop-in-drop'congurationinwhicha1.
3-lLorganicdrop,suspendedinalargeraqueousdrop,extractstheanalyteofinterest.
Thesystemhastheadvantagesoflowconsumptionoforganicsolventandthefacilityofautomatedbackwash.
JeannotandCantwellintroducedatechniquewherean8-lLdropoforganicsolventcontaininganinternalstandardisleftsuspendedattheendofaPTFErodimmersedinastirredaqueoussam-plesolution.
Aftersampling,therodiswithdrawnfromthesolutionand,withthehelpofamicro-syringe,analiquotofthedropisinjectedintoaGCsystem.
Asamoreconvenientalternative,micro-extractioncanbeperformedbysus-pendinga1-lLdropdirectlyfromthetipofamicrosyringeneedleimmersedinastirredaqueoussample.
Afterextrac-tion,themicrodropisretractedbackintotheneedleand,next,transferredtotheGC[326,327].
Figure23showstheschematicofanSDMEsystem.
Sincedropletinstabilityathighstirringspeedscancauseproblems,whilesuchhighspeedsareusuallybenecialbecauseFig.
22.
MMLLE–GC–FIDanalysisof(a)blankwine,(b)MMLLEextractofaspikedredwine(c=0.
05mgL-1)and(c)MMLLEextractofanItalianredwinecontainingtetradifon.
Peakidentication:1=Aldicarb;2=Diphenylamine(ISTD);3=Simazine;4=Atrazine;5=Lindane;6=Terbuthylazine;7=Metoxuron;8=Metobromuron;9=Vinclozolin;10=Isoproturon;11=Chlortoluron;12=Metazachlor;13=Quinalphos;14=Procymi-done;15=EndosulfanI;16=EndosulfanII;17=Endosulfansulphate;18=Tetradifon[296]Fig.
23.
SchematicofanSDMEsystem[325]ReviewChromatographiaSupplementVol.
69,2009S65theyenhanceextraction,theuseofamodiedtipdesignwasrecommendedinrecentwork[328].
ThesimilarityofSDMEandSPMEoperationssuggeststhatautosamplersthatcanbeusedforSPMEshouldalsoworkwithSDME.
Firstresultsusinga2-lLdropofhexadecaneforBTEXanalysis[333]usingaCombiPAL(CTC,Zwingen,Switzerland)autosampler,andastandard10-lLmicrosyringecon-rmedthissupposition.
Asinglemagnetmixerwasusedtopermittemperature-controlledextractionswhilestirringthesample.
Inordertoimprovetheextractioneciency,HeandLee[327]developeddynamicLPME(withPfor'phase'be-causethereisno'Dfordrop'congu-ration).
Withthistechnique,extractionoccursbywithdrawinganaqueoussampleintoamicrosyringealreadycon-taininganorganicsolvent.
Afteradwelltimeofafewsecondstoallowextractionoftheanalytesintoathinlmoforganicsolventadheringtothewallofthebarrelasthebulkofthesolventiswithdrawnbackup,theaqueousphaseispushedout.
Thecyclehastoberepeatedquiteanumberoftimes(20inthequotedexample)beforetheanalyte-enrichedorganicphaseissubjectedtoGCanaly-sis.
Insubsequentstudies,aprogram-mablesyringepumpwasusedtoautomatetherepetitivesamplewith-drawal/expellingprocess.
Incontinuous-owmicro-extraction(CFME),whichevolvedfromconven-tionalSDME[329],anaqueoussampleispumpedcontinuouslyintoaca.
0.
5-mLglasschamberviaapieceofPEEKtubingwhichservesforbothsampledeliveryandtheintroductionoftheor-ganicsolvent.
Oncetheglasschamberislledwiththeaqueoussample,there-quiredvolumeoftheextractantisintroducedthroughaninjectorandmoved,togetherwiththesamplesolu-tion,towardstheglasschamber.
WhenitreachestheendofthePEEKtubing,amicrodropisformedwhichisvirtuallyimmobilizedneartheoutletofthetub-ing.
Sincetheaqueoussamplesolutioniscontinuouslypumpedaroundthedropofextractant,highenrichmentfactorscanbeobtained.
Afterapresettimeofextraction,thedropiswithdrawnwithamicrosyringeandtransferredtotheinjectorofaGCsystem.
Anotherrecentadditiontothelistofdrop-typeextractiontechniquesishead-spaceSDME(HS-SDME)[330].
ThetechniqueisrathersimilartoHS-SPME,theonlydierencebeingthatthebreusedinSPMEisreplacedbyaliquidmicrodrop.
Inthethree-phasesystem,aqueous-phasemasstransferistherate-determiningstep,andahighstirringspeedisthereforeindicated.
ComparedwithHS-SPME,HS-SDMEappearstohavesimilarcapabilitiesintermsofprecisionandspeedofanalysis;however,itoerstwodistinctadvantages.
Firstly,intuitively,thechoiceofsolventsiswider,ifnotvirtuallyunlimited,ascomparedtothelimitednumberofphasescurrentlyavailableforSPME.
Solventscanhaveboilingpointbeloworabovethecompoundsofinterestandcancoverawiderangeofpolarities.
Sec-ondly,thecostofsolventisnegligiblecomparedtothatofcommerciallyavailableSPMEbres.
However,theuseofSDMEforheadspaceanalysisseemsrelativelydicult,becausesolventswithrelativelylowvapourpressureswouldbepreferred.
Yet,themostsuitablesolventsforGCwouldhaverelativelyhighva-pourpressures.
Thedicultywiththelattersolventsisclear:theywouldevaporatetooquicklyintheheadspaceduringextraction.
Thus,inreality,thechoiceofsuitablesolventsisfairlylim-ited.
Intherecentliterature,severalat-temptstoimprovetheevaporationsituationbymeansofsemi-orfullyautomateddynamicHS-SDMEwerereported[331,332].
Oneinterestingsolutionmaybetheuseofthesamesolventassamplesolventanddropofextractant[333].
Theoreticalconsiderationsconcern-ingthenatureanddynamiccharacteris-ticsofthevariousmicro-extractionprocesses,anddiscussionsoftheinu-enceofvariousparameters—e.
g.
,dropsize,samplingtime,solventselection,saltaddition,dwelltime—arepresentedinseveralofthereviewsandpaperscitedabove,notablyin[322].
ApplicationsIntheliterature,some50applicationsofSDME-typesamplepreparationcombinedwithGChavebeenreported.
Themainapplicationareasareenvironmental,bioandfoodanalysis,andawidevarietyofanalyteshasbeendetermined(Table12).
Severalselectedapplicationsarebrieydiscussedbelow.
Inaninterestingstudy,HS-SDMEandsimultaneousderivatizationwereappliedforthedeterminationofacetoneinhumanbloodasadiabetesbio-marker[334].
A1-mLbloodsamplewasintroducedinaheadspacevial.
Deriva-tizationandextractionofacetonewereperformedbyusing2lLn-decanecontainingPFBHA,atanextractiontemperatureof25°Candanextractiontimeof4min.
Analyterecoverywas88%andtheLODforMSdetectionwas2nM.
Inanotherstudy,OPPsweredeterminedinorangejuice[335].
5%NaClwasaddedto5mLoforangejuiceforsaltingouttheanalytesofinterest.
SDMEwasperformedbyimmersingthesyringeneedleinthesample,exposinga1.
6-lLdropoftol-ueneduring15min(stirringat400rpm).
Withanalyterecoveriesof76–108%,theLODsforFPDdetectionwerebelow5lgL-1.
AthirdexampleshowsthatevenSDMEcanbeminia-turized[336].
Inso-calleddrop-to-dropsolventmicro-extraction(DDSME),theextractionofmethoxyacetophenoneisomersfromwaterwasperformedina100-lLvialcontainingonedrop(7lL)ofwater.
A0.
5-lLdropoftoluenewasexposedtothesamplefor5-minextraction(stirringat360rpmandroomtemperature).
TheextractantwasdirectlyinjectedintoaGC–MSsystemandLODsof1ngmL-1wereobtainedforallisomers.
SinceSDMEisstronglyrelatedtoSPME,thetwotechniquesarefrequentlycompared.
Toquoteanexample,Palitetal.
[337]studiedtheuseofSDMEandSPMEfortheanalysisofchemicalwar-fareagentssuchasdimethylmethyl-phosphonate,sesquimustardandSarininwater.
UnderoptimizedSDMEcon-ditions,LODswithMSdetectionwereintherangeof10–75lgL-1.
SDMEwasfoundtoextractanalytesofdiversestructure,whileSPMEwasnoteectiveinthecaseofpolaranalytes.
TheauthorsalsopreferredSDMEwithregardto,e.
g.
,cost,timeofanalysisandversatility.
S66ChromatographiaSupplementVol.
69,2009ReviewTable12.
SelectedapplicationsofSDMEandLPMEcombinedwithGCAnalytesSample(mLorg)Pre-treatmentSolvent(lL)ExtractionDetectorLOD(lgL-1,mgkg-1)Recovery(%)Ref.
SDMEOPPsOrangejuice(5)NaCl5%w/vTol(1.
6)15minFPD1.
0–1.
676–108[335]OPPsWater(2),juice(2)pH5–6Tol(1.
5)20minFPD0.
2–0.
677–114[339]FungicidesWater,wine(5)Xyl(1.
6)15minlECD0.
0006–0.
00180–102[340]AnisaldehydeUrine,serum(20)Tol(0.
5)25°C,5minMS2–582–98[341]ChemicalwarfareagentsWater(1.
8)DCM–Tetra(3:1)(1)30minMS10–75[337]DialkylphthalateestersFoodsimulant(10)DCM–Hex–Tol(7:3:0.
5)(2)50°C,25minFID0.
03–0.
4[342]AmphetaminesUrine(2)pH10.
5,lterTCM(2)8minPDHID15–50[343]MethoxyacetophenoneWater(7lL)Tol(0.
5)Roomtemp.
,5minMS1[336]AminoacidsUrine(1)Deriv.
,50mgNaCl,200rpm,2minTCM–tol(3:1)(1.
5)5minMS0.
3–6092–101[344]SolventresiduesEdibleoils(4–5)Benzylalcohol(2)60°C,6–15minFID,ECD0.
11–0.
37(FID)0.
001–0.
05(ECD)[338]HS-SDMEOrganotinsWater(5),sediment(0.
5)Deriv.
Dec(2)1minMS[345]BTEXWater(1.
5)Hexadecane(1)23°C,6minFID0.
7–5[346]BTEXEngineoil(0.
5)Hexadecane(1)50°C,3minFID[347]CancerbiomarkersHumanblood(1)60°C,240min,1,300rpmDec+PFBHA(2)40°C,6min,dropderiv.
MS0.
1–0.
2nM86–90[348]AlcoholsBeer(5)Ethyleneglycol(1)60°C,15minFID0.
004–0.
05[349]LPMEBisphenolAWater(10)1mL1MNaOH,deriv.
Tol(4)Roomtemp.
,90minMS0.
002[350]HS-LPMEVolatilesolventsPharmaceuticalproduct(0.
5)5mL10%NaClOctanol(3)20minFID4–400mgg-1[351]AcetoneBlood(1)40°C,10min,1,100rpm,deriv.
Tol(2)40°C,50sMS6nM87[352]FattyacidsBloodplasma(0.
5)0.
3gNaCl,pH1.
0,diluteto1mLButylphthalate(2)60°C,45minFID20–8070–87[353]AlcoholsBeer(2)60°C,10min,1,500rpmOctanol(0.
8)60°C,9.
5minMS1–10090–114[331]ReviewChromatographiaSupplementVol.
69,2009S67MichulecandWardencki[338]usedSDME–GC–ECDand–FIDtodeter-mine(chlorinated)hydrocarbonsolventresiduesinedibleandpharmaceuticaloils.
SDMEwasfoundtobeasrapidandpreciseasSPME.
Ontheotherhand,thelinearrangewasmuchnar-rower,andtheLODswerehigherthanforSPMEprocedures.
However,theLODseasilymettherequirementsforthequotedapplications.
Insuchcases,itisaclearadvantagethatSDMErequiresnospecialequipment.
HeadspaceandPurge-and-TrapHeadspacetechniquesarewellsuitedforsamplepreparationpriortotheGCdeterminationofvolatilesinliquidand(semi-)solidsamples.
Insteadofdirectsampling,agasphaseinequilibriumwiththesamplematerialissampledandanalysed.
Inmostinstances,aconsiderableenrichmentoftheanalytescanalsobeobtainedinthegasphase,whichimprovesanalytedetectability.
Moreover,becauseonlythegasphaseinequilibriumwiththesampleisinjected,contaminationissuesareabsent,evenforvery'dirty'samples.
Thepracticabilityofthemethoddrewmuchattentionaftertherstpublica-tionin1958[354],andinstrumentsforfullyautomatedheadspacesamplingincombinationwithGCweremarketedsoonafterbyPerkinElmer(Shelton,CT,USA).
Today,thereishardlyanadequatelyequippedlaboratoryintheenvironmental,foodordrugsareawhichiswithoutaheadspaceinstru-ment.
Thestateoftheartofheadspaceanalysisisdocumentedinbookchap-tersandreviews,whichalsodiscussawidevarietyofapplications(see,e.
g.
,[355–360]).
Themainvariableisthedistributionconstantofananalytebetweenthegasphaseandtheliquidorsolidphase;themoretheequilibriumisshiftedtothegasphase,themoresensitivetheanalytecanbedeter-mined.
Thedistributionconstant,initsturn,primarilydependsonthevapourpressureoftheanalyteandtheactiv-itycoecientoftheanalyteinthematrix.
Therearetwoexperimentalap-proachesinheadspaceanalysis.
Ifthesampleisinequilibriumwiththegasphaseinaclosedvessel,thenthemethodofanalysisisreferredtoasstatichead-space,orHS.
Ifacarriergasispassedover,orthrough,thesampleandtheextractedvolatilecompoundsaccumu-latedinacryogenicorsorbenttrap,thenthemethodisgenerallyreferredtoasdynamicheadspace,gas-phasestrippingorpurge-and-trap,withP&Tasthecommonacronym.
HSAnalysisInHSanalysis,thevolatilesinthesam-plematerialareequilibratedwithagasphaseabovethesampleinaclosedvial.
Afterapredeterminedequilibrationtime,partofthegasphaseis(automat-edly)withdrawnfromthevessel,andinjectedintoaGCsystem.
Forcom-poundswhich,becauseoflowdistribu-tionconstants,largelyremainintheliquidorsolidmatrix,anobviouswaytoenhancetheanalyteconcentrationinthegasphaseistoincreasetheirvapourpressurebyincreasingtheequilibrationtemperatureortodecreasetheactivitycoecientby,e.
g.
,increasingtheionicstrengthofthesolution('saltingout').
Inliquids,analytediusiongenerallyisfastenoughforequilibriumtobereachedinashorttimeandmanyHSsystemshavestirringfacilitiestoaidthis.
In(semi-)solids,however,diusionisoftenveryslowandproceduressuchasgrindingofthesampleareusedtospeeduptheanalysis.
Afterequilibriumhasbeenestab-lishedinthecarefullythermostatedvial,thegasphaseissampledusingasyringeformanualproceduresorautomaticallyusingcommerciallyavailablepneumaticheadspaceanalysers.
Pneumaticsam-plingensuresthatboththepressureandvolumeoftheheadspacesampledareidenticalforallsamplesandstandards.
Aconstantpressureisobtainedbypressurizingtheheadspacevialswithaninertgastoapressureatleastequaltothecolumninletpressure.
Thesampleistheneitherexpandeddirectlyintothecolumnortoasampleloopofathermostatedgas-samplingvalve.
In-steadofrstllingaloop,apressurizedheadspacegascanalsobeexpandeddi-rectlyintotheGCcolumnbyusingaso-calledbalancedsamplingsystem[357,361].
Anotherproceduretocollectthestaticheadspacefromasampleistheuseofasorbent.
Theadsorbentisallowedtostayintheheadspaceforaspecicperi-odoftimeandataconstanttempera-ture.
Afterequilibriumhasbeenreached,(analiquotof)thesolidsorbentistransferredtoathermaldesorber.
Inthepastthisprocedurewasoftenperformedusingsmallpaperbags('teabags')lledwithTenaxoranotherpolymersorbent.
Today,anSPMEbreistypicallyused(HS-SPME;seesectiononSPME).
However,onehastobeawarethat,withthistechnique,thedistributionisbe-tweenthebreandthematrix.
Conse-quently,eventhoughraisingthetemperatureincreasestheanalytecon-centrationintheheadspace,itreducesthedepositiononthebrebecausethevapourconcentrationoftheanalytein-creasesabovethesample,butalsoabovethebre.
HS-SPMEcanthereforegiveaselectivitywhichmarkedlydiersfromthatofHSanalysis:HSwillfavourthevolatileanalytes,butHS-SPMEthelessvolatilecompounds.
Finally,oneshouldkeepinmindtheoverridingimportanceofrigorouslycontrollingthetemperaturebothduringanalysis,fromsampletosample,andfromsampletostandard,inordertoensurereliablequanticationandadequaterepeatability/reproducibility.
MeetingthesedemandsisfacilitatedbyusingautomatedHSsamplers.
P&TAnalysisInP&Tanalysis,asampleiscontinu-ouslypurgedwithaninertgas(com-monlyhelium)andvolatilesaretransportedfromthesampletoatrapwithsucientlyhighretentionpower(e.
g.
,Tenax,activatedcarbonorsilica)fortheanalytestobecollectedwithouttheriskofbreakthrough.
Afterpurging,thetrapisheatedandthetrappedvola-tilesarereleasedontoaGCcolumn,usuallyviaacoldtrap(Fig.
24).
P&T—which,inprinciple,enablesquantitativeanalyteisolation—isaneectivewayofachievingmuchbetteranalytedetect-S68ChromatographiaSupplementVol.
69,2009Reviewabilitythanequilibrium-typeHS:underfavourableconditionslow-andsub-ngL-1LODscanbeobtainedformanyVOCs.
ThekeyparametersinP&Toptimizationarepurgetime,owrateandtemperature.
Extendingthepurgetimewill,generallyspeaking,enhancetherecoveryoftheanalytesofinterest.
However,highlyvolatilecompoundsmaybe(partly)lostifpurgetimesaretooprolongedand/orthetrapdisplaysinsucientretention.
Asforthepurgetemperature,sincelessvolatileand/ormorewater-solubleanalyteswillbere-movedonlypartlyevenunderoptimizedconditions,carefulcontrolofthetem-peratureofthesamplevesselisrequiredforprecisequantication.
Fortherest,forobviousreasonselevatedtempera-tureswillenhanceanalyterecovery.
However,thedisadvantageisthatmorewatervapourwillbecarriedoverintothetrapandtheGCanalyticalsystem.
Actually,watermanagementisaseriousprobleminP&T(muchmorethaninHSsamplingwherethegasvolumesarerel-ativelysmall)becausealargeamountofwatervapourfromtheliquidsamplematrixisalsotransportedbytheinertgas.
Sincecoldtraps,whicharefre-quentlyusedtocollecttheanalytes,easilybecomeblockedthroughthelargeamountofvapour,itisimportanttoremovethemoisturefromthepurgegasbeforeitentersthecoldtrap.
Inorganicdesiccants,watercondensers,pre-sepa-rationonacolumnpackedwithTenaxoranothersuchsorbent,orselectivepermeationthroughapolymeric(oftenaNaon)membraneareallusedtothisend.
However,eachofthesealternativesunfortunately,hasspecicdisadvantageswhichinvariablycausetheuncontrolla-blelossofparticularclassesofanalytes.
Fordetails,thereadershouldconsulttheliterature[362].
VendorsofHSandP&TsystemsarePerkinElmerwhichmarketstheLSC2000andLSC3000,Tekmar(Mason,OH,USA)withtheTekmar-3000,Stra-tumPTCandVelocityXPT,andQuma(Wuppertal,Germany)withtheQHSS20/40/100/111.
ApplicationsOvertheyears,alargenumberofmutuallydivergentapplica-tionshavebeenpublishedwhichuseHSorP&Tforsamplepreparation.
AselectionofrecentcontributionstothiseldissummarizedinTables13and14,respectively.
Inaninterestingstudy,Cudjoeetal.
[363]identiedpheromonesinladybugsthatcanaectthebouquetandtasteofwine,usingHS–GC–MSintheSIMmode.
Forthisanalysis,veladybugswereplacedinaheadspacevialthatwasequilibratedfor20minat95°C.
Theheadspacegaswastransferredbybal-ancedsamplingwithaninjectiontimeof30s.
Hippodemiaconvergensposedthehighestthreattowineproductionduetothehighlevelsofmethoxypyrazinesfoundinthem.
Inanotherpaper,P&Tsamplingwasusedtodeterminevolatilesinfruits[364].
15mLoffruitpulpwereequilibratedat80°Candsubjectedtoa35-minpurgewithhelium.
TheextractedvolatilesweretrappedonamixtureofTenax/silica/charcoalkeptat30°C.
Afterpurging,thetrapwasheatedto180°C,totransfertheanalytestoaGC–MSsystem.
Ingeneralitwasconcludedthatinthetotalvolatileprole,thecompoundsbelongingtotheterpeneandalcoholclassesdecreaseduringmatura-tionofthefruitfromthehalf-ripetotheripestage.
Inenvironmentalanalysis,Hu-ybrechtsetal.
[365]determined27VOCsinmarinewater.
P&Tofa60-mLsample(45°C,20min)wasusedtotraptheanalytesonamultibedsorbent.
Afterdesorptionat275°C,theanalyteswererefocusedonacryotrap(-150°C),and,next,rapidlydesorbedat260°C.
LODsforGC–MS(SIM)analysiswere0.
2–7ngL-1for23ofthetargetVOCs.
Fordichloromethane,chloroform,benzeneand1,4-dichlorobenzene,theLODswere20–40ngL-1.
Finally,Rooseetal.
[366]determinedVOCsineelsamplesbymeansofon-lineP&T–GC–MS.
15gofsamplewerehomogenizedwithablenderandtransferredtoasamplevialcon-taining25mLofwater.
Thevolatileswereforcedoutbypurgingthesamplefor34minat70°C.
Thetrappedana-lytesweredesorbedinthebackushmodeintothecryofocusingmoduleand,next,releasedbyrapidlyheatingthismodulefrom-120to200°C.
Analyticalperformancewasfullysatisfactorywithanalyterecoveriesof80–99%andLODsof0.
003–0.
2ngg-1(whenusingfull-scanMS).
AtypicalchromatogramisshowninFig.
25.
ConclusionsEssentiallyallmodernreviewersemphasizethatsampletreatmentisakeyaspectoftrace-levelorganicanalysisandthatitisoftenthemosttime-consumingandleastsophisticatedstep.
Itisalsorecognizedthat,eventhoughstate-of-the-artinstrumentalchromatographictechniquesaresucientlymaturetoen-ablehyphenationwithpowerful(usuallyMS-based)detectorsthatprovidehighinformationdensity,samplepreparationisstillnecessaryinmostinstances.
ThisFig.
24.
SchematicofP&Twithcryogenictrapping.
(a)Samplepurgeandcollectionofthestrippedvolatilesinatrapand(b)desorptionfromthetrapandintroductionintothegaschromatograph.
IG,inertpurgegas;CG,carriergas;TB,adsorbenttube;SV,samplevessel;CT,cryogenictrap;SP,split(optional);CC,capillarycolumn.
[361]ReviewChromatographiaSupplementVol.
69,2009S69istrue,notonlybecausemanysolidandsemi-solidmatricescannotbehandleddirectlyanyway,butalsobecause(1)analyteenrichmentisrequiredtoreachconcentrationlevelsinthenalextractthatpermitreliablecompoundidenti-cationandquantication,and(2)removalofinterferingsampleconstitu-ents(e.
g.
,fat,proteins,sulphur,gritorstronglyadsorbingmaterials)isoftenneededtomaintaintheperformanceoftheanalyticalset-upoverpro-longedperiodsoftime.
Anothercon-clusion,frequentlytobereadbetweenthelines—i.
e.
,intheapplicationswhicharediscussedandininformationprovidedinthetableswhichareincluded—isthatforalargemajorityofallchallenginganalyticalproblemsdetectionisdonewithanMSinstru-ment,withToFMSandion-trapMS/MSgraduallycomingintotheirownnexttoquadrupoleMS.
Onemajorexceptionistheuseofselectiveand,moreso,highlysensitiveECDdetectionfor,specically,aromaticorganohalogenmicro-contaminants.
Tophrasethingsdierently,manyworkersstatethat,sincethereisanobviousneedforfaster,morecost-eectiveandenvironmentallyfriendlyanalyticalmethods,thereisalsoaclearneedtoimprovetheperformanceprovidedbytheclassicalmethodsofsamplepreparation.
Inthepasttwodecades,severaltensofnewlydesignedand,also,upgradedoldermethodshavebeenreportedandtheprogressmadeinthisareaiscontinuallybeingreviewed.
Onestrikinggeneralobservationisthat,despitetheimprovedperformanceofthe(GC)separationplus(MS)detectionstepeectedinthepast10orsoyears,samplepreparationis,inmanyin-stances,asextensivetodayasitwasinthe1990s.
Thisisespeciallyremarkablebecause,inthesameperiodoftime,comprehensive2D-GC,orGC9GC,withitsconsiderablyimprovedoverallchromatographicresolution,hasarrivedonthescenetofacilitatetheanalysisofhighlycomplexsamples[390].
Theobviousconclusionisthatmuchofthestepsforwardmadeintheeldsofsamplepreparationandinstrumentalanalysishavebeenusednottosimplifytheprocedures,buttoenhancethequalityoftheinformation.
Toouropinion,conclusionssuchasthosegivenabove,aremorerelevantthanadetaileddiscussionofthechar-acteristicsoftheindividualsample-preparationtechniques.
Moreover,aninterestingcomparisonofmanyofthetechniquesincludedinthepresentreviewhasrecentlybeengivenbyHyo¨tyla¨inenandRiekkola[391].
Nevertheless,somebriefcommentsshouldbepresentedalsohere.
Asregardssolidandsemi-solidsam-ples,PLEisapromisingtechnique,andfeaturesshortextractiontimesandlowsolventconsumption.
SFEandPLEshareseveralbenecialcharacteristicsbut,becausePLEcanbeusedwithallconventionalsolvents,itsapplicationrangeisdistinctlywiderthanthatofSFEwith(modied)CO2.
SFEmoreoverhasamatrix-dependentextractionmecha-nismandoptimizationisratherdemanding.
Ontheotherhand,SFEtypicallyisthemethodofchoiceforthermolabilecompounds.
WithMAE,propersolventselectionisthekeytoasuccessful—and,oftenrapid—extraction;hexane–acetone(1:1)hasbeenshowntobeafairlyideal'gen-eralpurpose'mixture.
Thetechniqueof-ferslittleselectivityandclean-upafterextractionisneededinmostinstances.
AlmostallMAEapplicationsinvolveo-lineproceduressinceoperationofthetechniqueaspartofadynamicsystemisdicult.
ThebenecialroleofultrasoundassistanceinUSE,butalsotoacceleratedigestion,sampledissolutionorenhancereactionkinetics,iswelldocumented[80,81].
Inmanyinstances,USEandUSleachingareecientalternativestomoreTable13.
SelectedapplicationsofHScombinedwithGCAnalytesSample(gormL)Pre-treatmentEquilibrationtime(min)Temperature(°C)SamplingTransferline(°C)DetectorLODRef.
BTEXOliveoil(10)–2595Loop/110°C/3mL120MS3–9ngmL-1[367]BTEXWater(15)2.
2gKCl,300lL5MHNO32070Loop/110°C/3mL120MS–[368]VOXsLandllleachates(5)–1575Loop/110°C/1mL110MS0.
05ngmL-1[248]VolatilesBacterialbiodegradation–2080Syringe/81°C/0.
4mL–MS–[369]ResidualsolventsPharmaceuticaldrugs(0.
2)–6080Loop/85°C/1mL85FID0.
3–8lgmL-1[370]AldehydesWodka(5)Deriv.
3070Balancedpressure/0.
5min90ECD0.
02–4lgL-1[371]TATPPost-explosiondebris–3090Syringe/1mL–MS0.
1ng[372]EpichlorohydrinDrinkingwater(5)300gNaClL-12280Loop–ECD40lgL-1[373]PheromonesLadybugs(5)–2090Balancedpressure/0.
5min95MS(SIM)–[363]S70ChromatographiaSupplementVol.
69,2009ReviewTable14.
SelectedapplicationsofP&TcombinedwithGCAnalytesSample(gormL)Pre-treatmentTemp.
(°C)Purgetime(min)Purgeow(mLmin-1)AnalytetrapDesorptiontemp.
(°C)Desorptiontime(min)CryotrapDetectorLOD(ngg-1,ngmL-1)Ref.
VOCsMarineorganisms(10–15)Ultra-turrax,ultrasonicbath703420Vocarb4000250–-120°CMS0.
003–0.
2[374,375]VOCsSediments(30)Dilutewithwater703020Vocarb4000250–-120°CMS0.
003–0.
2[376]VOCsWater(60)–452050TenaxTA,Carboxen1000and100127515-150°CMS(SIM)0.
001–0.
03[377,378]VOCsWater(13)–251135Tenax2253–MS(SIM)2–115[379]VHOCsSoil(5)SLE25940TenaxGC,silica,activatedcarbon2604–AED3–40[380]VHOCsWater(5),beverages(5)–30940TenaxGC,silica,activatedcarbon2604–AED0.
05–0.
5[381]VHOCsWater(10)–251010–––-100°CMS(SIM)–[382]VolatilesGrapemust(2)–301530Tenax18010–MS–[383]VolatilesSpondiassp.
(15)–803540Tenax/silica/charcoal18020–MS–[364]TrihalomethanesDrinkingwater(45)–651530Tenax220––ECD0.
2–0.
8[304]MTBEWater(44)–4010TenaxTA22010–MS–[384]EpichlorohydrinDrinkingwater(5)300gNaClL-1807060Tenax/silica/carbon/mol.
sieve1801–ECD0.
01[373]Chloroform,trichloroaceticacid,trichloroethanolBlood(1),urine(0.
3–0.
5)–401665Vocarb30002502-110°CMS(SIM)0.
300–2[385]1,2-Dichloroethane,1,4-dichlorobenzene,naphthaleneHoney(10)Pre-heating,40°C404040Tenax1806–MS0.
05–0.
8[386]2,4,6-TrichloroanisolCork,wine(25)USEorLLE,conc.
inwater251040CarbopackB,Carboxen1000and10012405–AED0.
03,5[387]EstersCider(5)–203050Tenax23010–MS(SIM)5–120[388]Benzene,tolueneHumanmilk(5)–3011HPBTEXtrap2208-150°CMS(SIM)-[389]ReviewChromatographiaSupplementVol.
69,2009S71conventionalapproaches,andquanti-cationisfullysatisfactoryifaprobede-viceratherthananultrasoundbathisused.
Becauseofthelowoveralltemper-atureduringtheoperation,analytether-molabilityisnoseriousproblem.
Theinherentadvantageofdynamic/continu-oussystemsmeritsmoreattention.
USEisoftencomparedwithMAE.
Itissim-plerand,sometimes,alsofasterthanthattechnique.
Ontheotherhand,USEisconsideredlessrobustandparticlesizecanbeacriticalfactor.
MSPDisatechniquedesignedtosimultaneouslydisruptanddisperseasampleoveraproperlyselectedsolidsupport.
Thecombinationofextractionandclean-up,shortextractiontimes,smallsamplesizeanduseoflittlesorbentandsolvent(s)aremainadvantages.
TheverysimplicityofMSPDexplainswhyadditionaltreatmentwillusuallybere-quiredpriortoGCanalysis.
If,however,suchtreatmentcomprisesthree,fourorevenmoresteps(Table6),onemayseriouslydoubtthecost-,andtime-,eectivenessoftheapproach.
DTDisarecentlyintroducedsample-preparationtechniquewhichhasbeenappliedalreadytoavarietyofdicultmatricesandcanbefullyautomated,althoughatconsiderablecost.
Theex-tractsarerathercleanandrewardingresultsareobtainedforverysmallsam-plessuchasafewpollen[163]orsmallpiecesofcheese[167].
Themainlimita-tionsarethedeterminationofthermo-labileandveryhigh-boilingcompounds.
Fortheanalysisofaqueous/liquidsamples,SPEisnodoubtthemoste-cientandexibletechnique.
Thisisalsofrequentlyindicatedbyotherreviewers.
IfcombinedwithGCanalysis,non-selectivesorbentsarepreferredbecausecollectingawide(polarity)rangeofanalytesismoreimportantthancreatingselectivity.
Inotherwords,usingacommercialcopolymersorbentis,gen-erallyspeaking,abetterapproachthandesigninganotherMISPEmaterial.
AvarietyofSPEformatsforo-line,on-lineandsemi-orfullyautomatedoper-ationis(commercially)availableandforminiaturized(ca.
1mL),conventional-size(ca.
100mL)andlarge-scale(1Landover)applications.
Comparedwithother—frequentlyequilibrium-type—techniques,amuchlargeranalyteenrichmentcanusuallybeachievedwithexhaustivelyextractingSPE.
Fromamongtheequilibriumtechniques,SPMEandSBSEareprobablybestknown.
Onemainadvantageisthattheyarebothsolvent-free.
Ontheotherhand,forafairnumberofapplications,reachingequilibriumconditionsistime-consuming.
ThisisespeciallytrueforSBSE,whichhastheadditionaldisad-vantagethatquitesomemanualhan-dlingisrequiredandautomationisessentiallyimpossible.
Generallyspeak-ing,thismakesSPME—forwhichfullyautomatedsystemsarecommerciallyavailable—amuchmoreattractiveop-tion,eventhoughitsapplicationrangeisrelativelylimited[258].
Recentlyintro-ducedSDMEisaninexpensiveequilib-rium-typealternative,with'drop-size'extractionvolumesasanattractivefea-ture.
Unfortunately,theprolongedextractiontimesneededtoreachequi-libriummaycausedropdissolution.
Ifsampleagitationisusedtoenhanceextraction,properprocedureshavetobeusedtopreventdropdislodgement.
Insummary,SDMEisnotwithoutitstechnicalproblems.
Thereareseveralmorepointswhichbrieyrequireourattention.
Forexam-ple,fromamongthegoalsmentionedintheintroductorytextofthissection,environmentalfriendlinessisrepeatedlyemphasizedinthepublishedliteratureandsolvent-freetechniquesarethereforerecommended.
Ontheotherhand,de-spitealltheemphasisfrequentlygiventohighsamplethroughput,speedisoftengiveninsucientattention.
Inaddition,designingsample-preparationmethodsthatareeasilycoupledon-linetotheGC–MSsystemusuallyhasnohighpriorityandthesubstantialgainthatcanbeeectedbyinjectingtheentire(on-line)insteadofaminoraliquotof(o-line)sampleextractisoftenoverlooked.
Theobviousdisadvantagesofequilib-riummethods—i.
e.
,theriskoflowanalyterecoveryandtheproblemoflonganalyte-extractiontimesiftheapplica-tionrangeofthemethodisundulyex-panded—usuallyareinsucientlyconsidered.
Onthepositiveside,severalofthemorerecentlydevelopedmethods,notablyDTDandSDME—andalsoSPME—enableminiaturizationorare,inessence,micromethods.
ItisalsoFig.
25.
P&T–GC–MSchromatogramof15gofeelfromtheriverScheldt:9=Chloroform;10=1,1,1-Trichloroethane;13=Benzene;20=Toluene;23=Tetrachloroethene;27=Chlorobenzene;29=Ethylbenzene;32=o-Xylene;33=Styrene;34=Bromoform[366]S72ChromatographiaSupplementVol.
69,2009ReviewgoodthatreviewerssuchasSmith[392]andKristensson[393]emphasizethatderivatizationand/oranalytelabellingshouldbeavoidedwheneverpossible.
Theadditional,oftenmulti-step,proce-duresadverselyaectsamplethroughputandcostofanalysis.
Artefactsareoftencreatedandtheapplicationisnotalwaysvalidatedattheultra-tracelevel.
WithmanyLC–MStechniquesbeingavailabletostudytheintactanalytes—adistinctadvantagewhenidenticationisapri-marygoal—derivatizationisanaccept-ableapproachonlyincasessuchas,forexample,themethylationoffattyacidsandtransestericationoflipids,thesi-lylationofselectedsteroidsortheacyl-ationofamines.
Oneaspectthatisnotalwaysgivendueattentionisdistinguishingtarget-compoundmonitoringandprolingen-tiresamples(see,e.
g.
,[391]).
Inthefor-mercase,inwhichthesearchislimitedtospecic,pre-identiedcompounds,properoptimizationofthesamplepreparationtocreateasuitablyselectiveproceduremaybeuseful,althoughitwilloftenbesuperuousbecauseoftheselectivityinherentintheGC–MSpartoftheanalysis.
Inthemuchmorechal-lengingprolingsituation,inwhichallconstituentsofasampleareregardedasanalytes,non-selectiveand(closeto)exhaustiveanalyteextractionarekeyis-sues.
[Ifnecessary,astraightforwardLC-typefractionationmaybeincludedasarststep.
]EquilibriummethodssuchasSBSE,SPMEandMMLLEshouldnotbeselectedforsuchstudies,specicallynotbecausetheextractionbehaviouroftheunknowncompoundscannotbepredicted.
Instead,robustnon-selectiveSPEshouldbeused.
Similarly,withvolatileorganiccompounds,P&Tisamorepowerful—i.
e.
,muchmoresensi-tive,andautomatable—techniquethanHS-SPME,althoughonemayarguethatthedierenceisnottoolargeinthiscasebecausethefocusonvolatileanalytescreatesasituationinbetweentargetmonitoringandproling.
Finally,oneshouldtakeintoaccountthatthereisanincreasinguseofGC9GCinsteadofGC.
Thissignicantlyhelpstounravelthecompositionofmanyfood,shandbiotaaswellassoil,sedimentandaero-solsamples:applyingthecomprehensivetechniqueshouldbeseriouslyconsideredwheneverprolingofsuchsamplesisrequired.
Insummary,thedevelopmentsde-scribedinthischapterdemonstratethatintheeldofsamplepreparation,avarietyofapproachroutesiscontinuallybeingopened,optimizedand,next,oftenmodied.
Theyservemanydierentpurposessuchas,e.
g.
,simplifyingtheoverallanalyticalprocedureand/orenhancingitsperformance,increasingsamplethroughput,facilitatinganalyteidenticationorenablingmorereliablequantication.
Or,asayoungscientistwrotein2005[393]:Actually,asisincreasinglybeingsaidbyexpertsintheeld,wearerapidlycreatingconditionsinwhichitisnotperformingtheanalysesandhandingintheresults,butthesubsequentdatahandlinganddatainter-pretationwhichwillbecomethestumblingblock.
Inotherwords,whilestillworkingonsolvingtheanalyticalproblemsofthepresentgeneration,thoseofthenextgen-erationarealreadyloomingonthehorizon.
Thisstatementisstillvalidtodayor,inotherwords,theeortsofthe''nextgeneration''arestillurgentlyrequired.
GlossaryAcetAcetoneAEDAtomicemissiondetectorASEAcceleratedsolventextractionATDAutomatedthermaldesorptionBenzBenzeneBSTFAN,O-bis(Trimethylsilyl)triuoroacetamideBTEXBenzene,toluene,ethylbenzene,andxylenesButOAcButylacetateCFMEContinuous-owmicro-extractionConc.
ConcentrateCTMEcis/transMethylesterCyclohexCyclohexaneCyclopenCyclopentaneDCMDichloromethaneDDSMEDrop-to-dropsolventmicro-extractionDecDecaneDeriv.
DerivatizationDI-SPMEDirect-immersionsolid-phasemicro-extractionDMAEDynamicmicrowave-assistedextractionDMIDicult/dirtymatrixintroductionDSIDicultsampleintroductionDTDDirectthermaldesorptionDUSEDynamicultrasound-assistedextractionECDElectron-capturedetectorEPAEnvironmentalProtectionAgencyEtAcAceticacidEtOAcEthylacetateEtOHEthanolFAMEFattyacidmethylesterFIDFlameionizationdetectorFMAEFocusedmicrowave-assistedextractionFPDFlamephotometricdetectorGCGaschromatographyGC9GCComprehensivetwo-dimensionalgaschromatographyGPCGelpermeationchromatographyHepHeptaneHexHexaneHexOAcHexylacetateHRMSHigh-resolutionmassspectrometryHSHeadspaceHS-LPMEHeadspaceliquid-phasemicro-extractionHS-SDMEHeadspacesingle-dropmicro-extractionHS-SPMEHeadspacesolid-phasemicro-extractionHSSEHeadspacesorptiveextractionIAEImmunoanityextractionIASPEImmunoanity-basedsolid-phaseextractionIMSIonmobilityspectrometryIRInfraredISTDInternalstandardKo/wOctanol–waterdistributioncoecientLCColumnliquidchromatographyLODLimitofdetectionLLELiquid–liquidextractionLPMELiquid-phasemicro-extractionLVILarge-volumeinjectionMAEMicrowave-assistedextractionMASEMembrane-assistedsolventextractionMeCNAcetonitrileMeOHMethanolMESIMembrane-extractionsorbentinterfaceMIMSMembrane-introductionmassspectrometryMIPMolecularlyimprintedpolymerMISPEMolecularlyimprintedsolid-phaseextractionMMLLEMicroporousmembraneliquid–liquidextractionReviewChromatographiaSupplementVol.
69,2009S73MSMassspectrometerMSPDMatrixsolid-phasedispersionMTBEMethyltert-butyletherNPDNitrogenphosphorusdetectorNPLCNormal-phaseliquidchromatographyOCPOrganochlorinepesticideOPPOrganophosphoruspesticidesP&TPurge&trapPAHPolycyclicaromatichydrocarbonsPBDEPolybrominateddiphenyletherPCBPolychlorinatedbiphenylPCDD/FPolychlorinateddibenzo-p-dioxin/furanePDHIDPulsed-dischargeheliumionizationdetectorPDMSPolydimethylsiloxanePEEKPolyetheretherketonePenPentanePFBHAO-(2,3,4,5,6-Pentafuorobenzyl)hydroxylaminehydrochloridePFEPressurizeduidextractionPFPDPulsedamephotometricdetectorPHWEPressurizedhot-waterextractionPIDPhotoionizationdetectorPLEPressurizedliquidextractionPMAEPressurizedmicrowave-assistedextractionPOPPersistentorganicpollutantPPPolypropylenePTFEPolytetrauoroethylene(Teon)PTVProgrammedtemperatuevaporizerQqQTriplequadrupoleRAMRestricted-accessmediaRSDRelativestandarddeviationSat.
SaturatedSBSEStir-barsorptiveextractionSCDSulphurchemiluminescencedetectorSDSteamdistillationSDBStyrene–divinylbenzeneSDMESingle-dropmicro-extractionSEPSample-enrichmentprobeSFESupercriticaluidextractionSHWESubcriticalhot-waterextractionSIMSingleionmonitoringSLESolid–liquidextractionSLMSupportedliquidmembraneSPESolid-phaseextractionSPDESolid-phasedynamicextractionSPMESolid-phasemicro-extractionSVOCSemi-volatileorganiccompoundTATPTriacetonetriperoxideTCDThermalconductivitydetectorTCMChloroformTDThermaldesorptionTetraTetrachloromethaneTICTotalioncurrentTMSHTrimethylsulphoniumhydroxideToFMSTime-of-ightmassspectrometryTolTolueneTSDThermionicspecicdetectorUSEUltrasound-assistedextractionVHOCVolatilehalogenatedorganiccompoundVOCVolatileorganiccompoundVOXVolatileorganichalogensXylXyleneOpenAccessThisarticleisdistributedunderthetermsoftheCreativeCommonsAttributionNoncommercialLicensewhichpermitsanynoncommercialuse,distribution,andreproductioninanymedium,pro-videdtheoriginalauthor(s)andsourcearecredited.
References1.
RichterBE,EzzellJL,FelixD,RobertsKA,LaterDW(1995)AmLab27:242.
RamosL,KristensonEM,BrinkmanUATh(2002)JChromatogrA975:33.
SmithRM(2002)JChromatogrA975:314.
MendiolaJA,HerreroM,CifuentesA,IbanezE(2006)JChromatogrA1152:2345.
Hyo¨tyla¨inenT(2007)JChromatogrA1153:146.
EzzellJL,RichterBE,FelixWD,BlackSR,MeikleJE(1995)LC-GCInt13:247.
Bjo¨rklundE,NilssonT,BwadtS(2000)TrendsAnalChem19:4348.
DingWH,FannJCH(2000)JChro-matogrA866:799.
WindalI,MillerDJ,dePauwE,Haw-thorneSB(2000)AnalChem72:391610.
WaksmundzkaM,PetruczynikA,Dra-ganA,WianowskaD,DawidowickAL,SowaI(2004)JChromatogrB800:18211.
KlejdusB,VacekJ,AdamV,ZehnalekJ,KizekR,TrnkovaL,KubanV(2004)JChromatogrB806:10112.
RichterBE,JonesBA,EzzellJL,PorterNL,AvdalovicN,PohlC(1996)AnalChem68:103313.
RichterBE(2000)JChromatogrA874:21714.
VematsuM,FranckEV(1980)JPhysChemRefData9:219115.
RichterP,SepulvedaB,OlivaR,Cal-deronK,SeguelR(2003)JChromatogrA994:16916.
RodilR,PoppP(2006)JChromatogrA1124:8217.
GarridoFrenichA,MartnezJL,Vidal,CruzSiciliaAD,GonzalezRodrguezMJ,PlazaBolanosP(2006)AnalChimActa558:4218.
RamosL,VreulsJJ,BrinkmanUATh(2000)JChromatogrA891:27519.
BautzH,PolzerJ,StieglitzL(1998)JChromatogrA815:23120.
Lu¨thjeK,Hyo¨tyla¨inenT,Rautiainen-Ra¨ma¨M,RiekkolaM-L(2005)Analyst130:5221.
KuosmanenK,Hyo¨tyla¨inenT,Harto-nenK,Jo¨nssonJA,RiekkolaM-L(2003)AnalBioanalChem375:38922.
SuchanP,PulkrabovaJ,HajsˇlovaJ,KocourekV(2004)AnalChimActa520:19323.
Dabrowski,Giergielewicz-Mo_zajskaH,BiziukM,GacaJ,NamiesnikJ(2002)JChromatogrA957:5924.
WibergK,SporringS,HaglundP,Bjo¨rklundE(2007)JChromatogrA1138:5525.
RamosJJ,DietzC,GonzalezMJ,Ra-mosL(2007)JChromatogrA1152:25426.
LiguoriL,HeggstadK,HoveHT,Jul-shamnK(2006)AnalChimActa573–574:18127.
Concha-GranaE,Turnes-CarouMI,Muniategui-LorenzoS,Lopez-MahaP,Fernandez-FernandezE,Prada-Rodr-guezD(2004)JChromatogrA1047:14728.
Barriada-PereiraM,Gonzalez-CastroMJ,Muniategui-LorenzoS,Lopez-MahaP,Prada-RodrguezD,Fernan-dez-FernandezE(2007)Talanta71:13429.
Daz-CruzMS,BarceloD(2006)JChromatogrA1132:2130.
DagnacT,BristeauS,JeannotR,MouvetC,BaranN(2005)JChroma-togrA1067:22531.
CanosaP,Perez-PalaciosD,Garrido-LopezA,TenaMT,RodrguezI,RubE,CelaR(2007)JChromatogrA1161:10532.
VandeWegheH,VanermenG,Gemo-etsJ,LookmanR,BertelsD(2006)JChromatogrA1137:9133.
CurrenMSS,KingJW(2001)JAgricFoodChem49:217534.
O¨zelMZ,Go¨gu¨sF,HamiltonJF,LewisAC(2005)AnalBioanalChem382:11535.
DengCh,JiJ,WangX,ZhangX(2005)JSepSci28:123736.
Go¨gu¨sF,O¨zelMZ,LewisAC(2006)FlavourFragrJ21:12237.
O¨zelMZ,KaymazH(2004)AnalBio-analChem379:112738.
DengCh,WangA,ShenS,FuD,ChenJ,ZhangX(2005)JPharmBiomedAnal38:32639.
EikaniMH,GolmohammadF,Rows-hanzamirS(2007)JFoodEng80:73540.
EricssonM,Colmsjo¨A(2003)AnalChem75:171341.
DeanJR(1998)Extractionmethodsforenvironmentalanalysis.
Wiley,NewYorkS74ChromatographiaSupplementVol.
69,2009Review42.
CamelV(2000)TrendsAnalChem19:22943.
LetellierM,BudzinskiH(1999)Analu-sis27:25944.
Lopez-AvilaV,BenedictoJ(1996)TrendsAnalChem15:33445.
SaimN,DeanJR,AbdullahMdP,ZakariaZ(1997)JChromatogrA791:36146.
Lopez-AvilaV(2000)Microwave-as-sistedextraction.
In:WilsonID(ed)Encyclopediaofseparationscience.
AcademicPress,London,p138947.
SparrEskilssonC,Bjo¨rklundE(2000)JChromatogrA902:22748.
HummertK,VetterW,LuckasB(1996)Chromatographia42:30049.
Du¨ringR-A,Ga¨thSt(2000)FreseniusJAnalChem368:68450.
DeanJR,BarnadasIJ,FowlisIA(1995)AnalProc32:30551.
LopezdeSabandoO,GomezdeBalu-geraZ,GoicoleaMA,RodrguezE,SampedroMC,BarrioRJ(2002)Chro-matographia55:66752.
EricssonM,Colmsjo¨A(2002)JChro-matogrA964:1153.
YoungJC(1995)JAgricFoodChem43:290454.
LuquedeCastroMD,Jimenez-Car-monaMM,Fernandez-PerezV(1999)TrendsAnalChem18:70855.
HastyE,ReveszR(1995)AmLab66(27):66–7456.
EricssonM,Colmsjo¨A(2000)JChro-matogrA877:14157.
Lopez-AvilaV,YoungR,TeplitskyN(1996)JAssocOAnalChemInt79:14258.
RamilCriadoM,RodrguezPereiroI,CelaTorrijosR(2004)Talanta63:53359.
PareraJ,SantosFJ,GalceranMT(2004)JChromatogrA1046:1960.
BayenS,LeeHK,ObbardJP(2004)JChromatogrA1035:29161.
RegueiroJ,LlompartM,Garca-JaresC,CelaR(2006)JChromatogrA1137:162.
Yusa`V,PardoO,PastorA,delaGuardiaM(2006)AnalChimActa557:30463.
GarcaM,RodrguezI,CelaR(2007)JChromatogrA1152:28064.
ShuYY,TeySY,WuDKS(2003)AnalChimActa495:9965.
FuentesE,BaezME,ReyesD(2006)AnalChimActa578:12266.
Barriada-PereiraM,Gonzalez-CastroMJ,Muniategui-LorenzoS,Lopez-MahaP,Prada-RodrguezD,Fernan-dez-FernandezE(2007)Talanta71:134567.
PapadakisEN,VryzasZ,Papadopou-lou-MourkidouE(2006)JChromatogrA1127:668.
Esteve-TurrillasFA,AmanCS,PastorA,delaGuardiaM(2004)AnalChimActa522:7369.
SanusiA,GuilletV,MonturyM(2004)JChromatogrA1046:3570.
BasheerCh,ObbardJPh,LeeHK(2005)JChromatogrA1068:22171.
ZhuX,SuQ,CaiJ,YangJ(2006)AnalChimActa579:8872.
Daz-VazquezLM,GarcaO,VelazquezZ,MarreroI,RosarioO(2005)JChromatogrB825:1173.
LatorreA,LacorteS,BarceloD,Mon-turyM(2005)JChromatogrA1065:25174.
MoralesS,CanosaP,RodrguezI,RubE,CelaR(2005)JChromatogrA1082:12875.
BartolomeL,CortazarE,RaposoJC,UsobiagaA,ZuloagaO,EtxebarriaN,FernandezLA(2005)JChromatogrA1068:22976.
GfrererM,LankmayrE(2005)AnalChimActa533:20377.
LuiR,ZhouJL,WildingA(2004)JChromatogrA1038:1978.
GatidouG,ZhouJL,ThomaidisNS(2004)JChromatogrA1046:4179.
BendichoC,LavillaI(2000)Ultrasoundextractions.
In:WilsonID(ed)Ency-clopediaofseparationscience.
AcademicPress,London,p144880.
Luque-GarcaJL,LuquedeCastroMD(2003)TrendsAnalChem22:4181.
Priego-CapoteF,LuquedeCastroMD(2004)TrendsAnalChem23:64482.
SanchezC,EricssonM,CarlssonH,Colmsjo¨A,DyremarkE(2002)JChro-matogrA957:22783.
SanchezC,EricssonM,CarlssonH,Colmsjo¨A(2003)JChromatogrA993:10384.
Moralez-MunozS,VreulsRJJ,deCas-troMD(2005)JChromatogrA1086:12285.
Moralez-MunozS,LuquedeCastroMD(2005)JChromatogrA1066:186.
Caballo-LopezA,LuquedeCastroMD(2003)JChromatogrA998:5187.
GoncalvesC,AlpenduradaMF(2005)Talanta65:117988.
USEPA(2000)Ultrasonicextraction,testmethodsforevaluatingsolidwaste,Method3550C,Rev.
3,USEnviron-mentalProtectionAgency,WashingtonDC,USA,November89.
ZhouJ,XueX,LiY,ZhangJ,WuL,ChenL,ZhaoJ(2007)JSepSci30:191290.
Luque-GarcaJL,LuquedeCastroMD(2004)JChromatogrA1034:23791.
SanchezAvilaN,PriegoCapoteF,Lu-quedeCastroMD(2007)JChromatogrA1165:15892.
MoretI,PiazzaR,BenettiM,GambaroA,BarbanteC,CesconP(2001)Che-mosphere43:55993.
BanjooDR,NelsonPK(2005)JChro-matogrA1066:994.
Sanchez-BruneteC,MiguelE,TadeoJL(2002)JChromatogrA976:31995.
BarroR,Garcia-JaresC,LlompartM,BollainMH,CelaR(2006)JChroma-togrA1111:196.
AlissandrakisE,DafereraD,TarantilisPA,PolissiouM,HarizanisPC(2003)FoodChem82:57597.
Cabredo-PinillosS,Cedron-FernandezT,Gonzalez-BriongosM,Puente-Pasc-ualL,Saenz-BarrioC(2006)Talanta69:112398.
ZuoY,ZhangL,WuJ,FritzJW,MedeirosS,RegoCh(2004)AnalChimActa526:3599.
ShenH-Y(2005)Talanta66:734100.
Priego-LopezE,LuquedeCastroMD(2003)JChromatogrA1018:1101.
SmithRM,HawthorneSB(1997)Supercriticaluidsinchromatographyandextraction.
Elsevier,Amsterdam102.
ZougaghM,ValcarcelM,RosA(2004)TrendsAnalChem23:399103.
PourmortazaviSM,HajimirsadeghiSS(2007)JChromatogrA1163:2104.
SaengcharoenratCh,GuyerDE(2004)JFoodEng63:33105.
RissatoSR,GalhianeMS,KnollFRN,AponBM(2004)JChromatogrA1048:153106.
GarrigosMC,MarnML,CantoA,SanchezA(2004)JChromatogrA1061:211107.
PooleCF,PooleSK(1996)AnalCom-mun33:11H108.
TehraniJ(1993)AmLab2:40109.
McNallyMEP(1995)AnalChem67:308A110.
TaylorLT(1995)AnalChem68:364A111.
HawthorneSB,MillerDJ(1987)JChromatogr403:63112.
VedaramanN,SrinivasakannanC,BrunnerG,RamabrahmanBV,RaoPG(2005)JSupercritFluids34:27113.
SantoyoS,lloraR,JaimeL,IbanezE,SenoransFJ,RegleroG(2006)EurFoodResTechnol222:565114.
Hyo¨tyla¨inenT,RiekkolaM-L(2004)AnalBioanalChem378:1962115.
RamosL,RamosJJ,BrinkmanUATh(2005)AnalBioanalChem381:119116.
AguileraA,BrontosM,RodrguezM,ValverdeA(2003)JAgricFoodChem51:5616117.
PoutskaJ,HoladovaK,HajsˇlovaJ(2003)EurFoodResTechnol216:68118.
AntunesP,GilO,Bernardo-GilMG(2003)JSupercritFluids25:135119.
ChuangJC,HartK,ChangJS,BomanLE,VanEmonJM,ReedAW(2001)AnalChimActa444:87120.
PourmortazaviSM,GhadiriM,Ha-jimirsadeghiSS(2005)JFoodComposAnal18:439121.
KarasekP,PlanetaJ,Varad'ovaOstraE,MikesˇovaM,GoliasJ,RothM,Ve-jrostaJ(2003)JChromatogrA1002:13122.
MichielinEMZ,BrescianiLFV,Dan-ielskiL,YunesRA,FerreiraSRS(2005)JSupercritFluids33:131123.
SmelcerovicA,LepojevicZ,DjordjevicS(2004)ChemEngTechnol27:1327124.
Ashraf-KhorassaniM,UdeM,Doane-WeidemanT,TamczakT,TaylorLT(2002)JAgricFoodChem50:1822125.
KoT-F,WengY-M,ChiouRY-Y(2002)JAgricFoodChem50:5343126.
BarkerSA,LongAR,ShortCR(1989)JChromatogrA475:353127.
KristensonEM,RamosL,BrinkmanUATh(2006)TrendsAnalChem25:96128.
BarkerSA(2007)JBiochemBiophysMeth70:151ReviewChromatographiaSupplementVol.
69,2009S75129.
BogialliS,DiCorciaA(2007)JBiochemBiophysMeth70:163130.
HercegovaA,Do¨mo¨to¨rovaM,Mati-sovaE(2007)JChromatogrA1153:54131.
KristensonEM,HaverkateEGJ,Sloo-tenCJ,RamosL,VreulsRJJ,BrinkmanUATh(2001)JChromatogrA917:277132.
BlascoC,FontG,PicoY(2002)JChromatogrA970:201133.
Gomez-ArizaJL,BujalanceM,GiraldezI,VelascoA,MoralesE(2002)JChro-matogrA946:209134.
RamosL,EljarratE,HernandezLM,RiveraJ,GonzalezMJ(1999)Chemo-sphere38:2577135.
ChuX-G,HuX-Z,YaoH-Y(2005)JChromatogrA1063:201136.
RamosJ-J,GonzalezM-J,RamosL(2004)JSepSci27:595137.
KristensonEM,ShahmiriS,SlootenCJ,VreulsRJJ,BrinkmanUATh(2004)Chromatographia59:1138.
NavarroM,PicoY,MarnR,ManesJ(2002)JChromatogrA968:209139.
SolerC,ManesJ,PicoY(2004)JChromatogrA1048:41140.
PenaMT,CasaisMC,MejutoMC,CelaR(2007)JChromatogrA1165:32141.
MartnezA,RamilM,MontesR,Her-nanzD,RubE,RodrguezI,Cela-TorrijosR(2005)JChromatogrA1072:83142.
ValsamakiVI,BotiVI,SakkasVA,Al-banisTA(2006)AnalChimActa573–574:195143.
MatosLinoC,FerreiraAzzoliniCB,ValenteNunesDS,RochaSilvaJM,NoronhadaSilveiraMI(1998)JChro-matogrB716:147144.
FerrerC,GomezMJ,Garca-ReyesJF,FerrerI,ThurmanEM,Fernandez-AlbaAR(2005)JChromatogrA1069:183145.
CarroAM,LorenzoRA,FernandezF,RodilR,CelaR(2005)JChromatogrA1071:93146.
AlberoB,Sanchez-BruneteC,TadeoJL(2003)JAgricFoodChem51:6915147.
Sanchez-BruneteC,AlberoB,MiguelE,TadeoJL(2002)JAssocOAnalChem85:128148.
MorzyckaB(2002)JChromatogrA982:267149.
TadeoJL,Sanchez-BruneteC(2003)Chromatographia57:793150.
FrenichAG,BolanosPP,Martnez-Vi-dalJL(2007)JChromatogrA1153:194151.
Kubala-DrincicH,BazulicD,Sapunar-PostruznikJ,GrubelicM,StuhneG(2003)JAgricFoodChem51:871152.
EstebanJL,Matrnez-CastroI,MoralesR,FabrellasB,SanzJ(1996)Chroma-tographia43:63153.
CuppettCM,FindeisPM,KlotzJC,WoodsLA,StreinTG(1999)LCGCNAm17:532154.
Pu¨ttmannW(1991)JChromatogr552:325155.
GerardL,ElieM,LandaisP(1994)JApplPyrol29:137156.
GargAK,PhilipRP(1994)OrgGeo-chem21:383157.
LarterSR,SenftleJT(1985)Nature318:277158.
vanLieshoutMPM,JanssenH-G,Cra-mersCA,vandenBosGA(1997)JChromatogrA764:73159.
vanLieshoutMPM,JanssenH-G,Cra-mersCA,HetemMJJ,SchalkHJP(1996)JHighResolutChromatogr19:193160.
HaysMD,LavrichRJ(2007)TrendsAnalChem26:88161.
FoxRL(1999)AnalChem71:109R162.
HoSSH,YuJ(2004)JChromatogrA1059:121163.
deKoningJA,BlokkerP,Ju¨ngelP,AlkemaG,BrinkmanUATh(2002)Chromatographia56:185164.
http://www.
gerstel.
com/ALEX_eng.
pdf165.
http://www.
atasgl.
com166.
Ju¨ngelP,deKoningS,BrinkmanUATh,MelcherE(2002)JChromatogrA953:199167.
Go¨gu¨sF,O¨zelMZ,LewisAC(2006)JSepSci29:1217168.
WelthagenW,Schnelle-KreisJ,Zim-mermannR(2003)JChromatogrA1019:233169.
Schnelle-KreisJ,SklorzM,PetersA,CyrysJ,ZimmermannR(2005)AtmosEnviron39:7702170.
Schnelle-KreisJ,WelthagenW,SklorzM,ZimmermannR(2005)JSepSci28:1648–1657171.
VogtL,Gro¨gerT,ZimmermannR(2007)JChromatogrA1150:2172.
AmiravA,DaganS(1997)JMassSpectrom3:105173.
JingH,AmiravA(1997)AnalChem69:1426174.
http://www.
varianinc.
com175.
DeKoningS,LachG,Linkerha¨gnerM,Lo¨scherR,TablackPH,BrinkmanUATh(2003)JChromatogrA1008:247176.
PatelK,FussellRJ,GoodallDM,KeelyBJ(2003)Analyst128:1228177.
AnastassiadesM,LehotaySJ,Stajnba-herD,SchenckFJ(2003)JAOACInt86:412178.
PatelK,FussellRJ,GoodallDM,KeelyBJ(2004)FoodAdditContam21:658179.
CˇajkaT,MasˇtovskaK,LehotaySJ,HajsˇlovaJ(2005)JSepSci28:1048180.
BlokkerP,PelR,AkotoL,BrinkmanUATh(2002)JChromatogrA959:191181.
AkotoL,PelR,IrthH,BrinkmanUATh,VreulsRJJ(2005)JAnalApplPyrol73:69182.
EstebanJL,Martinez-CastroI,SanzJ(1993)JChromatogrA657:155183.
HaysMD,SmithND,KinseyJ,DongY,KariherP(2003)JAerosolSci34:1061184.
SanzJ,SoriaAC,Garca-VallejoMC(2004)JChromatogrA1024:139185.
Perez-CoelloMS,SanzJ,CabezudoMD(1997)JChromatogrA778:427186.
GarcaMA,SanzJ(2001)JChromatogrA918:189187.
ZuninP,BoggiaR,LanteriS,LeardiR,DeAndreisR,EvangelistiF(2004)JChromatogrA1023:271188.
deKoningS,KaalE,JanssenH-G,VanPlaterinkCh,BrinkmanUATh(2008)JChromatogrA1186:228189.
Go¨gu¨sF,O¨zelMZ,LewisAC(2007)Talanta73:321190.
HelmigD,BauerA,Mu¨llerJ,KleinW(1990)AtmosEnviron24A:179191.
FalkovichAH,RudichY(2001)Envi-ronSciTechnol35:2326192.
WaddellR,DaleDE,MonagleM,SmithSA(2005)JChromatogrA1062:125193.
SigmanME,MaCh-Y(1999)AnalChem71:4119194.
NakamuraS,TakinoM,DaishimaS(2001)JChromatogrA912:319195.
MielleP,SouchaudM,LandyP,Gui-chardE(2006)SensActuatorsB116:161196.
O¨zelMZ,Go¨gu¨sF,LewisAC(2006)AnalChimActa566:172197.
O¨zelMZ,Go¨gu¨sF,LewisAC(2006)JChromatogrA1114:164198.
MolHGJ,JanssenH-GM,CramersCA,VreulsJJ,BrinkmanUATh(1995)JChromatogrA703:277199.
HankemeierTh,BrinkmanUATh(2000)In:NiessenWMA(ed)Currentpracticeofgaschromatography–massspectrometry.
MarcelDekker,NewYork,pp155200.
LouterAJH,VreulsJJ,BrinkmanUATh(1999)JChromatogrA842:391201.
HankemeierTh,LouterAJH,RinkemaFD,BrinkmanUATh(1995)Chroma-tographia40:119202.
HankemeierTH,vanLeeuwenSPJ,VreulsJJ,BrinkmanUATh(1998)JChromatogrA811:117203.
HankemeierTh,KokSJ,VreulsRJJ,BrinkmanUATh(1999)JChromatogrA841:75204.
HankemeierTh(2000)PhDThesis,Chap.
3.
2,FreeUniversity,Amsterdam205.
BrossaL,MarceRM,BorrullF,Pocu-rullE(2003)JChromatogrA998:41206.
JahrD(1998)Chromatographia47:49207.
CaroE,MarceRM,BorrullF,CormackPAG,SherringtonDC(2006)TrendsAnalChem25:143208.
TamayoFG,TurielE,Martn-EstebanA(2007)JChromatogrA1152:32209.
ShiY,ZhangJ-H,ShiD,JiangM,ZhuY-X,MeiS-R,ZhouY-K,DaiK,LuB(2006)JPharmBiomedAnal42:549210.
AnderssonLI,PapricaA,ArvidssonT(1997)Chromatographia46:57211.
HarveySD(2005)JSepSci28:1221212.
Dallu¨geJ,HankemeierTh,VreulsRJJ,BrinkmanUATh(1999)JChromatogrA830:377213.
HankemeierTh,HooijschuurE,VreulsRJJ,BrinkmanUATh(1998)JHighResolutChromatogr21:341214.
MolHGJ,HankemeierTh,BrinkmanUATh(1999)LCGCInt12:108215.
HankemeierTh,RozenbrandJ,Adahc-hourM,VreulsJJ,BrinkmanUATh(1998)Chromatographia48:273216.
deKoningS,VanLieshoutM,JanssenH-G,BrinkmanUATh(2000)JMicro-colSep12:153S76ChromatographiaSupplementVol.
69,2009Review217.
PocurullE,AguilarC,BorrullF,MarceRM(1998)JChromatogrA818:85218.
VermaKK,LouterAJH,JainA,Pocu-rullE,VreulsJJ,BrinkmanUATh(1997)Chromatographia44:372219.
HankemeierTh,SteketeePC,VreulsJJ(1999)FreseniusJAnalChem364:106220.
WeigelS,BesterK,Hu¨hnerfussH(2001)JChromatogrA912:151221.
TrˇskaJ(1995)Chromatographia40:712222.
BeinerK,PoppP,WennrichR(2002)JChromatogrA968:171223.
ZhuX,YangJ,SuQ,CaiJ,GaoY(2005)JChromatogrA1092:161224.
ArthurCL,PawliszynJ(1990)AnalChem62:2145225.
deAlpenduradaMF(2000)JChroma-togrA889:3226.
LordH,PawliszynJ(2000)JChroma-togrA885:153227.
DietzCh,SanzJ,CamaraC(2006)JChromatogrA1103:183228.
PawliszynJ(ed)(1997)Solid-phasemicroextraction–theoryandpractice.
Wiley,NewYork229.
PawilszynJ(ed)(1999)Applicationsofsolid-phasemicroextraction.
RoyalSocietyofChemistry,Cambridge230.
SheppersWercinskiSA(1999)Solid-phasemicroextraction:apracticalguide.
MarcelDekker,NewYork231.
LipinskiJ(2001)FreseniusJAnalChem369:57232.
ChaiM,PawliszynJ(1995)EnvironSciTechnol29:693233.
MotlaghS,PawliszynJ(1993)AnalChimActa284:265234.
MesterZ,SturgeonR,PawliszynJ(2001)SpectrochimActaB56:233235.
StashenkoEE,MartnezJR(2004)TrendsAnalChem23:553236.
UglandHG,KroghM,RasmussenKE(1999)JPharmBiomedAnal19:463237.
GmeinerG,KrassnigC,SchmidE,TauschH(1998)JChromatogrB705:132238.
NameraA,YashikiM,LiuJ,OkajimaK,HaraK,ImamuraT,KojimaT(2000)ForensSciInt109:215239.
SetkovaL,RisticevicS,PawliszynJ(2007)JChromatogrA1147:213240.
http://www.
ctc.
ch241.
PizarroC,Perez-del-NotarioN,Gon-zalez-SaizJM(2007)JChromatogrA1166:1242.
HutchinsonJP,SetkovaL,PawliszynJ(2007)JChromatogrA1149:127243.
HookGL,KimmGL,HallT,SmithPhA(2002)TrendsAnalChem21:534244.
KozielJA,NovakI(2002)TrendsAnalChem21:840245.
AugustoF,ValenteALP(2002)TrendsAnalChem21:428246.
AulakhJS,MalikAK,KaurV,Schmitt-KopplinP(2005)CritRevAnalChem35:71247.
PenaRM,BarcielaJ,HerreroC,Gar-ca-MartnS(2005)Talanta67:129248.
FlorezMenendezJC,FernandezSan-chezML,FernandezMartnezE,San-chezUraJE,Sanz-MedelA(2004)Talanta63:809249.
BurgerBV,MarxB,leRouxM,BurgerWJG(2006)JChromatogrA1121:259250.
PurcaroG,MorrisonP,MoretS,ConteLS,MarriottPhJ(2007)JChromatogrA1161:284251.
SanchezA,MillanS,SampedroMC,UncetaN,RodrguezE,GoicoleaMA,BarrioRJ(2008)JChromatogrA1177:170252.
ShuYY,WangSS,TardifM,HuangY(2003)JChromatogrA1008:1253.
HuangY,YangY-C,ShuYY(2007)JChromatogrA1140:35254.
ChenY-I,SuY-S,JenJ-F(2002)JChromatogrA976:349255.
AlvesRF,NascimentoAMD,NogueiraJMF(2005)AnalChimActa546:11256.
BurbankHM,QianMC(2005)JChro-matogrA1066:149257.
BaltussenE,SandraP,DavidF,Cra-mersC(1999)JMicrocolSep11:737258.
DavidF,SandraP(2007)JChromatogrA1152:54259.
PicoY,FernandezM,RuizMJ,FontG,BiochemJ(2007)BiophysMeth70:117260.
KawaguchiM,ItoR,SaitoK,Nakaz-awaH(2006)JPharmBiomedAnal40:500261.
FontanalsN,MarceRM,BorrullF(2007)JChromatogrA1152:14262.
MontesR,RodrguezI,RubE,CelaR(2007)JChromatogrA1143:41263.
LeonVM,Llorca-PorcelJ,AlvarezB,CobolloMA,MunozS,ValorI(2006)AnalChimActa558:261264.
HuertasC,MorilloJ,UseroJ,Gracia-ManarilloI(2007)Talanta72:1149265.
BicchiC,CorderoC,LibertoE,RubioloP,SgorbiniB,DavidF,SandraP(2005)JChromatogrA1094:9266.
KawaguchiM,IshiiY,SakuiN,Oka-nouchiN,ItoR,InoueK,SaitoK,NakazawaH(2004)JChromatogrA1049:1267.
KawaguchiM,IshiiY,SakuiN,OkanouchiN,ItoR,SaitoK,Nakaza-waH(2005)AnalChimActa533:57268.
KawaguchiM,InoueK,YoshimuraM,ItoR,SakuiN,NakazawaH(2004)AnalChimActa505:217269.
KawaguchiM,ItoR,SakuiN,Oka-nouchiN,SaitoK,NakazawaH(2006)JChromatogrA1105:140270.
BicchiC,CorderoC,IoriC,RubioloP,SandraP(2000)JHighResolutChro-matogr23:539271.
KreckM,SchrarrerA,BilkeS,MosandlA(2002)FlavFragrJ17:32272.
BicchiC,IoriC,RubioloP,SandraP(2002)JAgricFoodChem50:449273.
DemyttenaereJCR,MorinaRM,San-draP(2003)JChromatogrA985:127274.
DemyttenaereJCR,MorinaRM,deKimpeN,SandraP(2004)JChroma-togrA1027:147275.
SandraP,TienpontB,VercammenJ,TredouxA,SandraT,DavidF(2001)JChromatogrA928:117276.
BenijtsT,VercammenJ,DamsR,PhamTuanH,LambertW,SandraP(2001)JChromatogrB755:137277.
PoppP,KeilP,MonteroL,Ru¨ckertM(2005)JChromatogrA1071:155278.
Llorca-PorcelJ,Martnez-SanchezG,AlvarezB,CobolloMA,ValorI(2006)AnalChimActa569:113279.
OchiaiN,SasamotoK,KandaH,Na-kamuraS(2006)JChromatogrA1130:83280.
SandraP,TienpontB,DavidF(2003)JChromatogrA1000:299281.
LiuW,HuY,ZhaoJ,XuY,GuanY(2005)JChromatogrA1095:1282.
ZalacainA,MartnJ,AlonsoGL,Sali-nasMR(2007)Talanta71:1610283.
DezJ,DomnguezC,GuillenDA,VeasR,BarrosoCG(2004)JChromatogrA1025:263284.
KawaguchiM,InoueK,YoshimuraM,SakuiN,OkanouchiN,ItoR,Yo-shimuraY,NakazawaH(2004)JChromatogrA1041:19285.
LokhnauthJK,SnowNH(2006)JChromatogrA1105:33286.
RoyG,VuilleminR,GuyomarchJ(2005)Talanta66:540287.
LorenzoC,ZalacainA,AlonsoGL,SalinasMR(2006)JChromatogrA1114:250288.
AccorsiA,MorroneB,BenzoM,Gan-diniC,RaGB,ViolanteFS(2005)JChromatogrA1071:131289.
Jo¨nssonJA,MathiassonL(2000)JChromatogrA902:205290.
HyltonK,MitraS(2007)JChromatogrA1152:199291.
KuosmanenK,Hyo¨tyla¨inenT,Harto-nenK,RiekkolaM-L(2003)Analyst128:434292.
KnutssonM,NilveG,MathiassonL,Jo¨nssonJA,SundinP(1996)AnalLett29:1619293.
PalmarsdottirS,ThordarsonE,EdholmLE,MathiassonL,Jo¨nssonJA(1997)AnalChem69:1732294.
MelcherRG,BakkeDW,HughesGH(1992)AnalChem64:2258295.
MelcherRG,BouyoucosSA(1990)ProcContrQual1:63296.
Hyo¨tyla¨inenT,Lu¨thjeK,Rautiainen-Ra¨ma¨M,RiekkolaM-L(2004)JChromatogrA1056:267297.
Hyo¨tyla¨inenT(2008)JChromatogrA1186:39298.
HauserB,PoppP(2001)JSepSci24:551299.
YangMJ,HarmsS,LuoYZ,PawliszynJ(1994)AnalChem66:1339300.
PrattKF,PawliszynJ(1992)AnalChem64:2101301.
ShenY,PawliszynJ(2001)JSepSci24:623302.
MatzG,KibelkaG,DahlJ,LennemanF(1999)JChromatogrA830:365303.
SanJuanA,GuoX,MitraS(2001)JSepSci24:599304.
BrownMA,MillerS,EmmertG(2007)AnalChimActa592:154305.
RodilR,SchellinM,PoppP(2007)JChromatogrA1163:288306.
OserH,CoggiolaMJ,YoungSE,CrosleyDR,HaferV,GristG(2007)Chemosphere67:1701ReviewChromatographiaSupplementVol.
69,2009S77307.
SchellinM,PoppP(2003)JChromatogrA1020:153308.
HauserB,SchellinM,PoppP(2004)AnalChem76:6029309.
SchellinM,PoppP(2006)JChromatogrA1103:211310.
SchellinM,PoppP(2005)JChromatogrA1072:37311.
CiucanuI,ChiriacA(2002)JSepSci25:447312.
WangL,LordH,MoreheadR,DormanF,PawliszynJ(2002)JAgricFoodChem50:6281313.
LiuX,PawliszynR,WangL,PawliszynJ(2004)Analyst129:55314.
AllenTM,CisperME,PhHHemberger,ChWWilkerson(2001)IntJMassSpectrom212:197315.
LloydD,ThomasKL,CowieG,Tam-mamJD,WilliamsAG(2002)JMicro-biolMeth48:289316.
TarkiainenV,KotiahoT,MatillaI,Virkaja¨rviI,AristidouA,KetolaRA(2005)Talanta65:1254317.
FontanalsN,BarriT,Bergstro¨mS,Jo¨nssonJA(2006)JChromatogrA1133:41318.
Hyo¨tyla¨inenT,Tuutija¨rviT,Kuosma-nenK,RiekkolaM-L(2002)AnalBio-analChem372:732319.
LiuH,DasguptaPK(1996)AnalChem68:1817320.
JeannotMA,CantwellFF(1996)AnalChem68:2236321.
XuL,BasheerCh,LeeHK(2006)JChromatogrA1152:184322.
WardenckiW,CuryoJ,NamiesnikJ(2007)JBiochemBiophysMeth70:275323.
LambropoulouDA,AlbanisTA(2007)JBiochemBiophysMeth70:195324.
PsillakisE,KalogerakisN(2003)TrendsAnalChem22:565325.
PsillakisE,KalogerakisN(2002)TrendsAnalChem21:53326.
JeannotMA,CantwellFF(1997)AnalChem69:235327.
HeY,LeeHK(1997)AnalChem69:4634328.
AhmadiF,AssadiY,MilaniHosseiniSMR,RezaeeM(2006)JChromatogrA1101:307329.
LiuY,HashiY,LinJ-M(2007)AnalChimActa585:294330.
TheisAL,WaldackAJ,HansenSM,JeannotMA(2001)AnalChem73:5651331.
SarajiM(2005)JChromatogrA1062:15332.
OuyangG,ZhaoW,PawliszynJ(2005)AnalChem77:8122333.
WoodDC,MillerJM,ChristI(2004)LCGCEur17:573334.
DongL,ShenX,DengCh(2006)AnalChimActa569:91335.
ZhaoE,HanL,JiangS,WangQ,ZhouZ(2006)JChromatogrA1114:269336.
WuH-F,YenJ-H,ChinCh-Ch(2006)AnalChem78:1707337.
PalitM,PardasaniD,GuptaAK,Du-beyDK(2005)AnalChem77:711338.
MichulecM,WardenckiW(2006)Chromatographia41:191339.
XiaoQ,HuB,YuCh,XiaL,JiangZ(2006)Talanta69:848340.
LiuY,ZhaoE,ZhouZ(2006)AnalLett39:2333341.
LiuB-M,MalikP,WuH-F(2004)Ra-pidCommunMassSpectrom18:1059342.
BattleR,NernC(2004)JChromatogrA1045:29343.
CasariC,AndrewsARJ(2001)ForensicSciInt120:165344.
FiamegosYC,NanosChG,StalikasCD(2004)JChromatogrB813:89345.
ColombiniV,Bancon-MontignyCh,YangL,MaxwellP,SturgeonRE,MesterZ(2004)Talanta63:555346.
PrzyjaznyA,KokosaJM(2002)JChromatogrA977:143347.
KokosaJM,PrzyjaznyA(2003)JChromatogrA983:205348.
LiN,DengCh,YinX,YaoN,ShenX,ZhangX(2005)AnalBiochem342:318349.
TankeviciuteA,KazlauskasR,Vicka-ckaiteV(2001)Analyst126:1674350.
KawaguchiM,ItoR,EndoN,Oka-nouchiN,SakuiN,SaitoK,NakazawaH(2006)JChromatogrA1110:1351.
WangX,JiangT,YuanJ,ChengCh,LiuJ,ShiJ,ZhaoR(2006)AnalBioanalChem385:1082352.
DengCh,LiN,WangX,ZhangX,ZengJ(2005)RapidCommunMassSpectrom19:647353.
TanL,ZhaoXP,LiuXQ,JuHX,LiJS(2005)Chromatographia62:305354.
BovijnL,PirotteJ,BergerA(1958)GasChromatography,DestyDH(ed),But-terworths,London,p310355.
KolbB,EttreLS(2006)Statichead-space–gaschromatography:theoryandpractice,2ndedn.
Wiley,Chichester356.
SnowN,SlackGC(2002)TrendsAnalChem21:608357.
KolbB(2000)Headspacegaschroma-tography.
In:WilsonID(ed)Encyclo-pediaofseparationscience.
AcademicPress,London,p489358.
ChaintreauA(2000)Sampleprepara-tion,headspacetechniques.
In:MeyersRA(ed)Encyclopediaofanalyticalchemistry.
Wiley,Chichester,p4229359.
DewulfJ,vanLangenhoveH(2002)TrendsAnalChem21:637360.
RooseP,BrinkmanUATh(2005)TrendsAnalChem24:897361.
KolbB(1999)JChromatogrA842:163362.
PooleC(2003)Theessenceofchroma-tography.
Elsevier,Amsterdam,TheNetherlands,Chapter3.
7.
3363.
CudjoeE,WiederkehrTB,BrindleID(2005)Analyst130:152364.
NarainN,deSousaGalvaoM,SuelyMadrugaM(2007)FoodChem102:726365.
HuybrechtsT,DewulfJ,MoermanO,vanLangenhoveH(2000)JChromatogrA893:367366.
RooseP,BrinkmanUATh(1998)Ana-lyst123:2167367.
PenaF,CardenasS,GallegoM,Val-carcelM(2004)JChromatogrA1052:137368.
SerranoA,GallegoM(2004)JChro-matogrA1045:181369.
SakataSK,TaniguchiS,RodriguesDF,UranoME,Wandermu¨renMN,Pelliz-ariVH,ComassetoJV(2004)JChro-matogrA1084:67370.
OteroR,CarreraG,DulsatJF,Fabre-gasJL,ClaramuntJ(2004)JChroma-togrA1057:193371.
SowinskiP,WardenckiW,PartykaM(2005)AnalChimActa539:17372.
StambouliA,ElBouriA,BouayounT,BellimamMA(2004)ForensicSciInt146S:S191373.
LucentiniL,FerrettiE,VeschettiE,Si-bioV,CittiG,OttavianiM(2005)MicrochemJ80:89374.
RooseP,BrinkmanUATh(2000)MarPollBull40:1167375.
RooseP,BrinkmanUATh(1998)JChromatogrA799:233376.
RooseP,DewulfJ,BrinkmanUATh,vanLangenhoveH(2001)WaterRes35:1478377.
HuybrechtsT,DewulfJ,vanLangenh-oveH(2004)WaterRes38:3241378.
HuybrechtsT,DewulfJ,vanLangenh-oveH(2005)EnvironPoll133:255379.
MartnezE,LacorteS,LlobetI,VianaP,BarceloD(2002)JChromatogrA959:181380.
CampilloN,VinasP,Lopez-GarcaI,AguinagaN,Hernandez-CordobaM(2004)Talanta64:584381.
CampilloN,VinasP,Lopez-GarcaI,AguinagaN,Hernandez-CordobaM(2004)JChromatogrA1035:1382.
ZoccolilloL,AmendolaL,CafaroC,InsognaS(2005)JChromatogrA1077:181383.
MamedeMEO,PastoreGM(2006)FoodChem96:586384.
TanabeA,TsuchidaY,IbarakiT,KawataK,YasuharaA,ShibamotoT(2005)JChromatogrA1066:159385.
JohnsDO,DillsRL,MorganMS(2005)JChromatogrB817:255386.
TananakiCh,ZotouA,ThrasyvoulouA(2005)JChromatogrA1083:146387.
CampilloN,AguinagaN,VinasP,Lo-pez-GarcaI,Hernandez-CordobaM(2004)JChromatogrA1061:85388.
MadreraRR,GarcaNP,GarcaHeviaA,VallesBS(2005)JChromatogrA1069:245389.
FabiettiF,AmbruzziA,DelizeM,SprechiniMR(2004)EnvironInt30:397390.
AdahchourM,BeensJ,BrinkmanUATh(2008)JChromatogrA1186:67391.
Hyo¨tyla¨inenT,RiekkolaM-L(2007)TrendsAnalChem26:788392.
SmithRM(2003)JChromatogrA1000:3393.
KristenssonM(2005)PhDThesis,Chap.
1.
3,FreeUniversity,AmsterdamS78ChromatographiaSupplementVol.
69,2009Review