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JournalofChromatographicScience,Vol.
27,July1989FundamentalsandApplicationsofSupercriticalFluidExtractioninChromatographicScienceJerryW.
KingNorthernRegionalResearchCenter,AgriculturalResearchService,UnitedStatesDepartmentofAgriculture,1815NorthUniversityStreet,Peoria,Illinois61604[Abstract/Theuniquepropertiesofsupercriticalfluidshavepromptedtheiruseforavarietyofapplicationsinthefieldofanalyticaichemistry.
Perhapsthemostwidelyciteduseofthesecompressedfluidshasbeeninthefieldofchromatography,eitherasmobilephaseeluentsorasextractionsolvents.
Thisstudyexaminesthevariousmodesinwhichsupercriticalfluidextraction(SFE)canbeemployedbythechromatographer.
Extraction,solubilization,andfractionationconditionsarepredictedbytheapplicationofwell-knownsolutionthermodynamicprinciples.
Experimentalresultsarereportedfortheremovaloflipidphasesfromnaturalproductsandthecoextractionofpe,sticidemoieties.
Finally,amethodofpredictingtherequiredmobilephasepressuresforsolubilizingandfractionatingoligomericmixturesinsupercriticalfluidchromatographyiscomparedwithliteraturedata.
IntroductionSupercriticalfluids(SF)arefindingwideacceptanceinanumberofanalyticaldisciplinesasuniquesolvationmedia.
Byfarthelargestnumberofapplicationsoccurinthefieldofchromatography,wherethesedensegasesareemployedasex-tractionsolventsandinteractivemobilephases.
Historically,supercriticalfluidchromatography(SFC)hasitsoriginsinthemid-l960s(l-3),whileitsextractionanaloguehasonlyrecentlyseenapplicationinthefieldofanalyticalchemistry.
Intruth,supercriticalfluidextraction(SFE)playsamechanisticroleinSFC,whereitcontributestotheseparationofthesolutesthatareinjectedintothechromatographicsystem.
Themajorityofreportedanalyticalchromatographicapplica-tionsofSFEtodatehavebeenconcernedwiththecouplingofSFE,usingverysmallextractioncells,withcapillary(4,5)andpackedcolumn(6,7)SFCinstrumentstoeffectsequentialSFE-SFCseparationschemes,Examplesoftheseapplicationsarereportedbyothercontributorstothisvolumeandwillnotbediscussedhere.
Thepurposeofthispaperistoprovideamoregeneralguidelinefortheapplicationofsupercriticalfluidstoavarietyofproblemsfacingthechromatographer.
Thesewillincludesamplepreparationpriortochromatography,theselec-tionofphysicalconditionsforextraction,andthespecificationofchromatographicconditionsneededforfractionatingoligomericmixtures.
ThereareanumberofmodesbywhichSFEcantieappliedtothepreparationofsamplesforchromatography.
ThesemethodsareillustratedinFigures1and2,whereananalyteiscontainedinamatrixofinterferingcomponents.
PerhapsthesimplestformofSFEisshowninFigurela,wherebytheanalyteofinterestisseparatedfromtheinterferingmatrixcomponents.
Suchanextractioncaseisrelativelyrareforreasonsthatwillbeexplainedlater.
Infact,SFEisbasicallynotaveryselectiveextractionmethod,exceptincaseswhereselectivesolubilizationofcomponentscanbeeffectedoveraverynarrowpressurerangeorwherethesolutestobeseparateddiffersignificantlyintheirrespectivephysicalproperties(molecularweight,polarity).
Figure1bisperhapsmoregenerallyencounteredinapplyingSFEtosamplematrices.
Heretheanalyteofinterestiscoex-tractedwithanumberofinterferingcomponents.
Initially,thismethodmaynotseemtobeanattractivechoice;however,theuseofanontoxiccompressedgasasanextractionsolventoffersmanyadvantagesoverconventionalliquidorganicsolventsintermsofdisposalandexposureoflaboratorypersonneltotheextractingmedium.
Anexampleofthisapproachinpesticideresidueanalysiswillbecitedlater.
AnextensionofFigurelbisshowninFigurelc.
Heretheanalyteofinterestandtheinterferingcomponentsareextractedbyasupercriticalfluidandtheanalyteissubsequentlyanalyzedbyanappropriateinstrumentaltechnique.
Theanalysismaybeoff-lineoron-line,dependingonthechosenanalyticalmethod.
Severalexamplesofon-lineSFEcoupledwithSFC(8-lo),gaschromatography(11,12),orhigh-performanceliquidchromatog-raphy(13)havebeenreported.
MoreelaboratemethodsareshowninFigure2,whereSFEiscoupledwithsorbenttechnology.
Figure2aproposesSFEextrac-tionofbothanalyteandinterferingcomponentsfollowedbyfractionationoftheanalytefrominterferingsolutes.
Applicationofsorbentcolumnsfortheseseparationsmaybebytraditionalmethodsafterthehighpressureextractionsteporbyswitchingthesupercritical-fluid-derivedextracton-linetoasorbentcol-umnheldatanelevatedpressure.
Retentioncharacteristicsofcompoundsonselectedsorbentsinthepresenceofsupercriticalcarbondioxidehavebeenreportedbytheauthor(14)andmayserveasabasisforchoosingconditionstoisolatespecificanalytes.
Reproduction(photocopying)otedilorialcontentofthispurnalLSprohibitedwithoutpublisher'spermission.
355AnalternativetoFigure2aisillustratedinFigure2b,wherebytheinterferingcoextractedcomponentsfromtheSFEsteparepermanentlyisolatedonthesorbentcartridge.
Suchaschemecanbeeffectedwithasupercriticalfluidmediumthroughoutboththeextractionandisolationsteps.
Theanalytecanthenbedirectlyintroducedintothechoseninstrumentaltechnique.
Suchamethodhasbeenrecentlyreportedforfractionatingcar-bamatepesticidesfromcoextractedlipidcomponents(15).
Finally,itshouldberecognizedthatsupercritical-fluid-basedanalyticaltechniques,suchaschromatography,offertheanalystexcitingpossibilitiesforeliminatingsampleworkuptechniquescompletely.
Theauthorhasreportedseveralexamplesofdirectinjectionofcomplexmatricesintoasupercriticalfluidchro-matograph,therebyeffectingtheseparationoftheanalytefromtheinterferingcomponentswithoutresortingtoanyformalex-tractionstepbeforeanalysis(16).
Suchamethod,Figure2c,makesuseofSFEasaninsitustepduringtheSFCprocess.
AnexampleofFigure2cwillbeprovidedlater.
TheoryManytheoreticalapproachesforpredictingthesolubilityandphaseequilibriaofsolutesinsupercriticalfluidsolventshavebeenreported(17-21).
Thesetheoriesrequireanarrayofphysi-cochemicaldataandconsiderabletimetoyieldinformationthatispertinenttooptimizingextractionconditions.
Suchmethodsareoflimitedvaluetothechromatographerfacedwithday-to-dayanalyticaldecisionsanddonotlendthemselvestopredict-ingtheextractionparametersrequiredforSFEorSFCofstruc-turallycomplexsolutes.
Wehavefoundthataknowledgeoffourbasicparametersofsupercriticalfluidextractionareextremelyhelpfulinunderstandingsolutebehaviorincompressedgasmedia.
Thefirstoftheseparametersisthemiscibilitypressure,whichisthepressureatwhichthesolutestartstodissolveinthesupercriticalfluid.
Thisparameterwastermedthe"thresholdpressure"byGiddings(22)andcorrespondstothecriticallociofmixingbe-tweenthedissolvedsoluteandthesolventgas.
Asnotedbytheauthor(23),themiscibilitypressureistechnique-dependentandAnalytetMatrixComponentsExtractionExtractionExtractionAnalyte(4AnalyteInter&eweAnalytefnterflrence(b)InstrumentalAnalysis63Figure1.
GeneralizedSFmethodsforextractionandanalysis.
JournalofChromatographicScience,Vol.
27,July,989willvaryslightly,dependingonthesensitivityoftheanalyticalmethodthatischosentomonitorthesoluteconcentrationinthesupercriticalfluidphase.
Nonetheless,anapproximateknowledgeofthispressure(orcorrespondingdensity)isveryuseful,foritpermitstheanalysttochooseastartingpressureforsupercritical-fluid-basedfractionationprocesses.
Anotherusefulparameterforspecifyingsupercriticalfluidextractionconditionsisthepressureatwhichthesoluteattainsitsmaximumsolubilityinthecompressedfluid.
ThisconditioncanbeapproximatedbyGiddings'equationwhichrelatesthesolubilityparameterofthegastoitscriticalandreducedstateproperties(3).
Whenthesolubilityparameteroftheextractingfluid(gas)isequivalenttothatofthesolute,maximumsoiubilityshouldbeattained.
Solubilitymaximaforsupercriticalfluid-solutesystemshavebeenrecordedbyanumberofinvestigators(24-27)andcorrelatedbythebasictenetsoftheregularsolutiontheorybyKing(28,29).
Thethirdparameter,thepressureregionbetweenthemisci-bilityandsolubilitymaximumpressures,isthefractionationpressurerangeinwhichasolute'ssolubilitywillrangebetweenzeroanditsmaximumvalueinthesupercriticalgas.
Inthisin-terval,itbecomespossibletoregulatethesolubilityofonesoluterelativetoanotherinthesupercriticalfluid.
Enrichmentofonecomponentoveranotherispossiblebyemployingthevariableofpressure,butitisextremelyrareinSFEexperimentstoisolateonecomponentfromtheotherwithoutresortingtoanauxiliarytechnique(thermalgradients,chromatography,etc.
).
Afrac-tionationbetweensolutesismaximizedinthispressureregionbydifferencesinthephysicalpropertiesofthedissolvedsolutesandbykeepingtheirconcentrationslowinthecompressedfluid.
Finally,aknowledgeofthesolute'sphysicalpropertiesiscriticaltooptimizinganSFE.
ThemeltingpointofthesoluteisaparticularlygermaneparameterinSFE,becausemostsolutesaredissolvedtoagreaterextentinthesupercriticalfluidmediumwhenintheirliquidstate.
IncreasingtheextractiontemperaturemayalsocauseenhancedsolutesolubilityintheSFbecauseofadecreaseinthesolute'scohesiveenergydensity,P,andthereforeitssolubilityparameter,6.
Hence,increasingtheex-Analyte+MatrixComponents/11ExtractionExtractionExtraction111AnalyteAnalyteAnalyteInterflrenceInterfirencelnterfirence1i(4SorbentSorbenti---JiAnalyteInterferenceAnalyte(4iInstrumentalAnalysis(b)Figure2.
GeneralizedSFmethodsforextractionandanalysisinvolvingseparationofinterferingcomponents.
JournalofChromatographicScience,Vol.
27,July1989tractiontemperaturewillnotnecessarilyresultinalowersolutesolubilityinthecriticalfluid.
TheuseofthesolubilityparametertheorycoupledwiththeFlory-Hugginsinteractionparameterconceptexplainsmanyofthephenomenaencounteredinsupercriticalfluidextraction(30-32).
Thisapproachwasfirstutilizedinpolymerchemistrytopredictphasemiscibilityrelationshipsbetweenpolymersdissolvedindensegases,suchasethylene(33,34).
Thedatare-quiredbytheabovetheoriesconsistsofcriticalpropertydataandsoluteorsolventsolubilityparameters.
Suchdataisusuallyavailableorcanbeestimatedfromcorrespondingstatestheory,groupcontributionmethods,ornomographs.
Withtheseparametersinhand,onecanusethefollowingequationtopredictthepressureatwhichmaximumsolutesolubilitywillbeattainedinthesupercriticalfluid:2=x,,+xs=v,(c%-6J2/RT+xsEq.
Iwherexisthetotalinteractionparameter;x,,andxsaretheenthalpicandentropicinteractionparameters,respectively;6,isthesolubilityparameterofthegasasf'(T,P);&isthesolubilityparameterofthesoluteasJ(7',P);7,isthemolarvolumeofthegasasf(T,P)=M,/Q,;M,isthemolecularweightofthegas;andQ,isthedensityofthegas.
Assumingxshasacon-stantvaluedefinedbythelatticecoordinationnumber,themax-imuminsolubilityshouldbeachievedwhen6,equals6,.
Plotsof2versuspressurearehyperbolic,theminimumoccuringatavalueequaltoxs.
Solubilityparametersforthecompressedgasarecalculatedby6,=1.
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ManuscriptreceivedMarch31,1989;revisionreceivedApril27,1989.
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