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Ultra resolution chemical fingerprinting of dense non-aqueous phase liquids from manufactured gas plants by reversed phase comprehensive two-dimensional gas chromatography

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Ultra resolution chemical fingerprinting of dense non-aqueous phase liquids from manufactured gas plants by reversed phase comprehensive two-dimensional gas chromatography
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   JournalofChromatographyA, 1218 (2011) 4755–4763 ContentslistsavailableatScienceDirect  Journal   of    Chromatography   A  journalhomepage:www.elsevier.com/locate/chroma Ultra   resolution   chemical   fingerprinting   of    dense   non-aqueous   phase   liquidsfrom   manufactured   gas   plants   by   reversed   phase   comprehensivetwo-dimensional   gas   chromatography Laura   A.   McGregor a , ∗ ,   Caroline   Gauchotte-Lindsay a ,   Niamh   Nic   Daéid b ,   Russell   Thomas c ,   Paddy   Daly d ,Robert   M.   Kalin a a DepartmentofCivilEngineering,UniversityofStrathclyde,JohnAndersonBuilding,107Rottenrow,GlasgowG40NG,UK  b CentreforForensicScience,DepartmentofPureandAppliedChemistry,UniversityofStrathclyde,RoyalCollegeBuilding,204GeorgeStreet,Glasgow,UK  c ParsonsBrinckerhoff,QueenVictoriaHouse,RedlandHill,Bristol,UK  d NationalGrid,NationalGridHouse,WarwickTechnologyPark,GallowsHill,Warwick,UK  a   r   t   i   c   l   e   i   n   f   o  Articlehistory: Received14January2011Receivedinrevisedform10May   2011Accepted12May   2011 Available online 20 May 2011 Keywords: ChemicalfingerprintingReversedpolarityGC × GCTOFMSPAHEnvironmentalforensicsDNAPL Manufacturedgasplant a   b   s   t   r   a   c   t Ultra   resolution   chemical   fingerprinting   of    dense   non-aqueous   phase   liquids   (DNAPLs)   from   formermanufactured   gas   plants   (FMGPs)   was   investigated   using   comprehensive   two-dimensional   gas   chro-matography   coupled   with   time   of    flight   mass   spectrometry   (GC   × GCTOFMS).   Reversed   phase   GC × GC(i.e.apolar   primary   column   coupled   to   a   non-polar   secondary   column)   wasfound   to   significantly   improve   theseparation   of    polycyclic   aromatic   hydrocarbons   (PAHs)   and   their   alkylated   homologues.   Sample   extrac-tionand   cleanup   wasperformed   simultaneously   using   accelerated   solvent   extraction   (ASE),   with   recoveryratesbetween   76%   and   97%,   allowing   fast,   efficient   extraction   with   minimal   solvent   consumption.   Princi-pal   component   analysis   (PCA)   of    the   GC ×   GC   data   was   performed   in   an   attempt   to   differentiate   betweentwelve   DNAPLs   based   on   their   chemical   composition.   Correlations   were   discovered   between   DNAPL composition   and   historic   manufacturing   processes   used   at   different   FMGP   sites.   Traditional   chemicalfingerprinting   methods   generally   follow   atiered   approach   with   sample   analysis   on   several   differentinstruments.   We   propose   ultra   resolution   chemical   fingerprinting   as   afast,   accurate   and   precise   methodofobtaining   more   chemical   information   than   traditional   tiered   approaches   while   using   only   a   singleanalytical   technique. © 2011 Elsevier B.V. All rights reserved. 1.Introduction Adensenon-aqueousphaseliquid(DNAPL)isaliquidwhichisbothheavierthanwaterandimmiscibleinwater[1].   Inthiscase,DNAPLreferstocoaltar;acommonsubsurfacecontaminantfoundatformermanufacturedgasplants(FMGPs).CoaltarDNAPLsarecomposedofthousandsoforganicandinorganiccompounds,manyofwhichmay   befoundintracequantities[2].ThecomplexchemicalcompositionofDNAPLshasbeenshowntovarydramaticallywithinasingleFMGPsite,aswellasbetweendifferentsites[3].   AccuratechemicalfingerprintingisrequiredatFMGPsitestoensuremultiplesourcesofcontaminationarenotpresent[4].   Forexample,morerecentspillscouldbedistinguishedfromhistoricalgasworkscontamination.Furthermore,forFMGPssplitintomultiplelandholdings,accuratechemicalfingerprintingcanhelptoidentifyliabilityacrosstheentiresite.Giventhelarge ∗ Correspondingauthor.Tel.:+441415484773;fax:+441415532066. E-mailaddress: l.a.mcgregor@strath.ac.uk(L.A.McGregor). numberofformergasworkssitesintheU.K.andtheintroductionofrecent“polluterspay”legislation,[5]itisreasonabletoassume theremay   bemanyliabilitycasesinthefuture,thusspurringthegrowthoftheenvironmentalforensicsindustryintheU.K.Environmentalforensicchemicalfingerprintingofcomplexsamples,suchascoaltarandcrudeoil,isgenerallyperformedbygaschromatography(GC)incombinationwitheitherflameionisationdetection(GC–FID)ormassspectrometry(GC–MS)withinatieredanalyticalapproach[4,6–8].   However,conventionalGCtechniquesdonothavethecapacitytoresolvethecomplexcompositionof coaltarDNAPLs[9].   Time-consumingandlabour-intensivechemi-calfractionationprocessesaregenerallyrequiredtodividecomplexmixturesintoseveralextractspriortoanalysis[10].TherehavebeenfewreportsonDNAPLcompositioninrecentliterature[3,11,12]andtotheauthors’knowledgethereisnostan- dardisedapproachforanalysisoffreephasecoaltars,certainlynotwithoutextensivesamplefractionation.Brownetal.[3]evaluated thecompositionofDNAPLsfromtendifferentFMGPsitesintheU.S.AindicatingmajordifferencesinPAHcompositionbetweensites.However,thisstudyutilisedGC–MSanalysisafterlengthy 0021-9673/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2011.05.045  4756  L.A.McGregoretal./J.Chromatogr.A 1218 (2011) 4755–4763 fractionationprocesses,sothechemicalinformationobtainedontheDNAPLswaslimitedbyresolutionpowerofthetechnique.Generally,theliteraturefocusesonchallengesinvolvedincharac-terisationandremediationofDNAPLcontaminatedland[2,13–15].Forexample,BirakandMiller[2]statethatfullcharacterisation ofDNAPLsatFMGPsitesisstilllimitedbyanalyticaltechniques.UtilisationofadvancedchromatographictechniquesforchemicalfingerprintingofDNAPLshasthusbeenlong-awaitedtoaidchar-acterisationandallowthemosteffectiveremediationroutestobechosen.Comprehensivetwo-dimensionalgaschromatography(GC × GC)isahigh-resolutionseparationtechnique,devel-opedwiththeintentionofovercominglimitationsassociatedwithconventionalGCtechniques[16].Thecouplingoftwocolumns withdifferentselectivityallowsforatwo-dimensionalsepara-tionofmixtures,acrossaretentionplaneratherthanalongaretentionline[17–21].   AnorderofmagnitudemorecompoundscanbeseparatedbyGC × GCthanwhenusingconventionalGCinstrumentation[22].Generally,along,widebore(0.25–0.32mmi.d.),non-polarcap-illarycolumnisusedinthefirstseparation,whereasashort,narrowbore(0.1–0.2mmi.d.),polarcolumnisinstalledforthesecondseparation;thisisdeemednormalphase.However,reversingthecolumnpolarityhasbeenshowntoprovidebettergroup-typesep-arationincertaincases[23].Theuseofapolar,primarycolumn andnon-polar,secondarycolumnisknownasreversedphase(orreversedpolarity)GC × GC[17].GC × GChasbeenshowntobeespeciallyusefulforenviron-mentalforensicanalysesofcomplexsamples[24,25];themain advantagebeingtheminimisationoreliminationoffractiona-tionprocessespriortoanalysis[16,26].Acomplexsamplecanbe injectedasasingleextracttoprovidefastscreeningoftheentiresample,allowingmanyclassesoforganiccontaminantstobemon-itoredatonce.However,thetechniquehasyettobeappliedtotheanalysisoffreephasecoaltars.ThisworkaimstouseGC × GCTOFMStoresolvetheissuesassociatedwiththeanalysisandsourceapportionmentofcoaltarDNAPLs.ChemicalfingerprintingofenvironmentalsamplesbyconventionalGCtechniquesisdescribedasahighresolutionmethod.Inthisstudy,wedemonstrateanenhancedmethodof chemicalfingerprinting,deemed‘ultraresolution’,bycombiningreversedphaseGC × GCwithstatisticalcomparisonusingprincipalcomponentsanalysis(PCA).Thisprocessgathersmorechemicalinformationpersamplethantraditionaltieredapproachesandhastheadditionalbenefitsofusinganefficientone-stepextractionfollowedbyanalysisonasingleanalyticalinstrument. 2.Experimental  2.1.Samplesandstandards DNAPLsamples(labelled1–12)wereprovidedfromsevendifferentFMGPsitesacrosstheUnitedKingdom.Thegasmanu-facturingprocessesusedateachsitearesummarisedinTable1.DNAPLsamples1–6wereobtainedfromvariouslocationswithinthesamesite(siteA),whileallothersampleswereacquiredfromdifferentsites.Samples1–10wereallobtainedfromsitesthatusedcoalretortstandsforgasproduction,whereassample11wasobtainedfromawoodpreservativesite,wherecoaltarwasdis-tilledtoproducecreosoteoilforcoatingwood[27].Sample12was obtainedfromacarburettedwatergas(CWG)plantwhereamix-tureofhydrogenandcarbonmonoxidewasproducedbypassingsteamthroughheatedcokeratherthanbythecarbonisationofcoalperformedatretortgasworks[28].Thesampleswerestoredat4 ◦ Cpriortoanalysis.  Table1 DescriptionofFMGPsites.Sampleno.Manufacturingprocess(es)Samplinglocation1Verticalcoalretort;potentialtracesofhorizontalretorttarandgasoil(frommicro-simplexgasreformingplantonsite)Borehole2   ”Boreholeneargasholder3   ”Withintartank4 ” Withintartank5 ” Withintartank6 ”   Boreholeneartartank7   HorizontalcoalretortBaseofgasholder8   HorizontalcoalretortWithintartank9   Verticalcoalretort;potentialtracesofcarburettedwatergastarandhorizontalretorttarUnknown10   HorizontalcoalretortUnknown11 Woodpreservationsite;tarprobablyfromadistilledfractionofcreosoteoilSump12Complexmixtureofhorizontalandverticalretorts,watergasandgasoil(fromagasreformingplantonsite)Borehole Allsolventsused( n -hexane,dichloromethane)wereofanalyt-icalgrade,purchasedfromFisherScientific(Loughborough,U.K)andusedwithoutfurtherpurification.AlldeuteratedPAHswereobtainedfromIsotec TM ,Sigma–Aldrich(Gillingham,U.K).AllPAHsandalkylatednaphthaleneswerepurchasedfromSigma–Aldrich.Anhydroussodiumsulphate,silicagel60(bothfromSigma–Aldrich)anddiatomaceousearth(Dionex,Camberley,UK)wereactivatedfor4hat450 ◦ Cpriortouse.Silicagel60wasthendeactivatedby10%water(w/w).AlkylatednaphthaleneswereidentifiedintheDNAPLextractsusingindividuallyprepared200  g/mL(indichloromethane)stan-dardsof1-and2-methylnaphthaleneandthe12C2alkylnaphthaleneisomers.TargetanalytesintheDNAPLextractswerequantifiedusingcalibrationmixturescontaining16PAHs,prioritypollutantsaslistedbytheU.S.EPA[29].The16PAHswerepurchasedas a2000  g/mLstocksolutioninbenzene:dichloromethane(1:1)fromSigma–Aldrich(Gillingham,U.K).A2000  g/mLstocksur-rogatesolutioncontainingdeuteratedPAHs(D8-naphthalene,D10-fluorene,D10-fluorantheneandD12-chrysene)waspreparedtomonitorextractionefficiency.SevencalibrationstandardscontainingthePAHsandsurrogateswerepreparedwithinthecon-centrationrangeof2.5–500  g/mL,eachspikedwith75  Lofa2000  g/mLstocksolutionofD10-phenanthreneasaninternalstandard.Quantificationwasperformedusingtheresponseofspe-cifictargetionspresentinGC × GCchromatograms(targetionsarelistedinTableS1ofsupplementarydata).  2.2.Samplepreparation ExtractionwasperformedusinganASE350AcceleratedSol-ventExtractionsystem(Dionex,Camberley,UK)equippedwith10mLstainlesssteelextractioncells.ThehighseparationcapabilityofGC × GCTOFMSeliminatestherequirementforsamplefrac-tionation,thusasingleextractionusinghexane(includingin-cellcleanupbysilicagel)was   performed.Adry,homogeneousmix   ofDNAPLwaspreparedbygrindingtheDNAPL(approximately0.5g)withsodiumsulphate(NaSO 4 )anddiatomaceousearth(D.E.)ina1:1:1ratio.ThisremovesanywaterpresentintheDNAPLsampleandresultsinafinepowder(ratherthanatar)whichcanbetransferredquantitativelytothe  L.A.McGregoretal./J.Chromatogr.A 1218 (2011) 4755–4763 4757 extractioncells.Toensureaccuratequantification,theDNAPLwasspikedwith600  LofthesurrogatesolutionpriortogrindingwithD.E.andNaSO 4 .Anylossoftargetanalytescouldthenbemonitoredfromthestartofsamplepreparationandstorageofthesampleinthisformalsoallowsanylossoftargetanalytesovertimetobemonitored.Extractioncellswerelinedwith2filterpapers(toensureunwantedparticulatematterdidnotcollectintheextract)andpackedwith3gsilicagel60(10%deactivatedw/w).Approximately0.5gofthegroundDNAPL/surrogatemixturewasaddedtotheextractioncellandtheremainingcellvolumewas   packedwithD.E.Hexanewasusedastheextractingsolventforallextractions.ASEwasperformedat150 ◦ Cand10MPa,usingonedynamic(7min)andtwostatic(5min   each)extractions.Aflushvolumeof150%andpurgetimeof60swereused.Theextractswereconcentratedto1mL    usingaBüchiSyncore ® Analyst(Oldham,U.K).Theextractswerethenmadeuptoexactly10mL    usinghexane.A1mL    aliquotwasthentransferredtoanautosamplervialandspikedwith75  L ofinternalstandardpriortoanalysis.  2.3.GC–MSanalyses AThermoScientific(Hertfordshire,U.K.)TraceUltraGCfittedwithaDSQIImassspectrometerandTriplusautosamplerwas   usedforallGC–MSanalyses.ThecolumnwasaJ&WScientificDB-5fusedsilicacapillarycolumn(30m × 0.25mmi.d. × 0.25  mfilmthick-ness).Allinjectionswereofonemicrolitreandwerecarriedoutusingasplitratioof1:50andinjectionporttemperatureof230 ◦ C.Heliumwasusedasthecarriergas,withaflowrateof1.0mL/min.Allstandardsandextractswereanalysedwiththeoventempera-tureprogrammedat10 ◦ C/minfrom55 ◦ C(maintainedfor2min)to110 ◦ C,3 ◦ C/minto210 ◦ C,thenat8 ◦ C/minto320 ◦ C(maintainedfor15min).  2.4.GC  × GCTOFMSanalyses AllGC × GCTOFMSanalyseswereperformedusingaLeco(St. Joseph,Michigan)timeofflightmassspectrometer,modelPegasus4D,connectedtoanAgilent7890AgaschromatographequippedwithaLecothermalmodulator.TheTOFionsourcewasfixedat200 ◦ Candmassesbetween45and500uwerescannedata200spectra/secondrate.Thedetectorvoltagewassetat1700Vandtheappliedelectronionisationvoltagewassetat70eV.Allstandardsandextractswereanalysedwiththeprimaryoventemperatureprogrammedat10 ◦ C/minfrom55 ◦ C(maintainedfor2min)to110 ◦ C,3 ◦ C/minto210 ◦ C,thenat8 ◦ C/minto310 ◦ C(maintainedfor15min).Thesecondaryovenandmodulatortem-peratureswereprogrammedata20 ◦ Coffsetrelativetotheprimaryoven.Themodulationperiodwas6switha1.3shotpulsetime.Theinjectionporttemperaturewassetto250 ◦ Cusingasplitratioof 1:50.OnemicrolitreofsamplewasinjectedforeachrunusinganMPS2twisterautosampler(Gerstel).Heliumwas   usedasthecarriergas,withaflowrateof1.0mL/min.Thenormalphasecolumnsetcomprisedofanon-polarRxi5-SilMS   (25m × 0.25mmi.d. × 0.25  mfilmthickness)pri-marycolumncoupledtoamid-polarityRxi17(1.2m × 0.1mmi.d. × 0.1  mfilmthickness)secondarycolumn,bothsuppliedbyThamesRestek(Buckinghamshire,U.K.).Thereversedpolaritycol-umn   setcomprisedamid-polarityTR-50MS   suppliedbyThermoScientific(30m × 0.25mmi.d. × 0.25  mfilmthickness)asthepri-marycolumnandanon-polarRtx-5suppliedbyThamesRestek(1.2m × 0.18mmi.d.m × 0.2  mfilmthickness)asthesecondarycolumn,connectedviaaThamesRestekPress-tight ® connector.  2.5.Principalcomponentanalysis VariationsintheDNAPLcompositionwereevaluatedbyprin-cipalcomponentanalysis(PCA)usingMinitab ® 15(MinitabLtd.,Coventry)software.Principalcomponentanalysisisamethodusedtoextractthevariationswithinalargedatasetbyreducingrawsampledataintosmaller,uncorrelatedvariablesknownasprinci-plecomponents[30,31].Scoreplotsoftheprincipalcomponents whichdescribethemostvariationwithinthedataallowrelation-shipsbetweenthesamplestobeevaluated.Peakareasofthetentativelyidentifiedcompoundswereimportedintothestatisticalsoftwareafternormalisation,againstthepeakareaoftheinternalstandard,andcorrectionusingtheexactweightofDNAPLextractedforeachsample. 3.Resultsanddiscussion  3.1.Optimisationofextractionprocedure Theinitialpartofthisstudywasdedicatedtooptimisationof extractionprocedure,withtheaimofextractingallchemicalclassespresentintheDNAPLusingasingleAcceleratedSolventExtraction(ASE)method.Hexanewas   foundtobeasuitableextractionsol-vent,thuseliminatingtheneedforharmful,chlorinatedsolvents.TheASEprocedureutilisedsimultaneousextractionandclean-up,bytheadditionofsilicageltoeachextractioncell,thusfurtherreducingthetotalanalysistimeandsolventconsumption.FractionationofcontaminatedsoilsamplesbyASEhasprevi-ouslybeenachievedbythreeseparateextractionspercellusingsolventsofincreasingpolarity[32].However,thiswasnotpossi- blefortheDNAPLsamplesinvestigatedinthisstudy,astheywerefullyextractedbytheinitial,non-polarsolventdespiteattemptsusinglowtemperatures(40 ◦ C)forthefirstextraction.GC–MSanal-ysisofsuchcomplexsampleswouldgenerallyonlybeperformedafterchemicalfractionation;however,giventheeaseofdissolu-tionoftheDNAPLsitisunlikelythateffectivefractionationcouldbeachievedviaASEwithouttheuseofadditionalcolumnchro-matography.ThehighresolutioncapacityofGC × GCnegatestherequirementforsamplefractionationthusthecombinationofsam-pleextractionandcleanupbyASEprovidesfastscreeningoftheentirecoaltarcomposition.Repeatabilityofthemethodwas   measuredbyextractionof sixreplicatecells,andsubsequentGC–MSanalysis,ofDNAPL sample7.Duetothedifficultiesinvolvedinreplicatingablankcoaltarmatrix,thesurrogaterecoveryvalueswereusedasameasureofrepeatability.FourdeuteratedPAHs(D8-naphthalene,D10-fluorene,D10-fluorantheneandD12-chrysene)werechosenastheyspanarangeofmolecularmasses,from136g/molto240g/mol.Recoveriesbetween76and97%wereobtainedbasedonthedeuteratedsurrogatespikes.Thesevaluesfallwithintheacceptedrangeof70–130%asstatedbytheU.S.EPASW-846Method8000B[33].Re-extractionofsamplecellsconfirmedthat themethodprovidedexhaustiveextractionoftheDNAPL,withonlytheinternalstandardpeakevidentinthechromatogramsofthesecondextracts.Therelativestandarddeviation(RSD)ofsurrogaterecoverywas   foundtobebelow10%foralldeuteratedsurrogates,indicatingsatisfactoryextractionrepeatability.  3.2.ReversedpolarityGC  × GC  ThecolumnsetsandGC × GCparameterswereadjustedtoachievebestpossibleseparationofDNAPLcomponents.NormalphasecolumnsetsaregenerallyusedinGC × GCanalysisofenvi-ronmentalsamples.Duetotherestrictionsinmaximumoperatingtemperatureofmostpolarcolumns,acompromisegenerallyexists  4758  L.A.McGregoretal./J.Chromatogr.A 1218 (2011) 4755–4763 Fig.1. Comparisonoftheseparationcapabilitiesof(a)GC–MS,(b)normalphaseGC × GCand(c)reversedphaseGC × GCusingastandardmixtureofC2alkylnaphthaleneisomers.Thetableprovidesthepeakidentitiesofthenaphthalenesineachfigure(*EtN=ethylnaphthalene,DMN   =dimethylnaphthalene). betweencolumnpolarityandtemperatureprogrammeforthesec-ondaryoven.Thecolumnphasewasreversedtoallowelutionof thehighmolecularweightcompoundspresentinDNAPLswhileremainingwithinthelimitsofthecolumntemperaturerange.AstandardmixtureofC2alkylnaphthaleneswas   usedtocon-firmelutionorderandcomparetheseparatingpowerofthreeGCmethods;GC–MS,normalphaseGC × GCandreversedphaseGC × GC.TheC2alkylnaphthaleneswerechosenforthisstudyasalkylPAHsareoftenusedindiagnosticratiosforsourcedeter-mination[7].   DuetoinsufficientseparationwithconventionalGCtechniques,thealkylPAHsaregenerallycombinedbyalkylationlevel,toprovidediagnosticratiosbasedonquantificationvaluesforthegroupasawhole.Forexample,atypicaldiagnosticratiousingalkylnaphthaleneswouldbeC0N/(C2N+C3N),whereC0NisnaphthaleneandC2NandC3NaretheC2andC3alkylnaph-thalenesrespectively[7].   We   proposethatthehigherresolutionof reversedphaseGC × GCcouldallowenhanceddiagnosticratiostobecalculatedatnoextracostcomparedtonormalphaseGC × GC.Thechromatogramsoftheseparationofamixtureofalkylnaph-thaleneisomersusingGC–MS,normalphaseGC × GCandreversedphaseGC × GCarepresentedinFig.1.TheGC × GCchromatogramsarerepresentedascontourplots;the  x -axisrepresentsthereten-tiontimeintheprimarycolumn,the  y -axisrepresentstheretentiontimeinthesecondcolumnandthecolourgradientrepresentstheintensityofthepeak.NormalphaseGC × GCandGC–MSachievedseparationof7and9peaksrespectively.ReversedphaseGC × GCallowsseparationofthe12C2alkylnaphthalenesinto10peaks,withonly2pairsofthealkylnaphthalenesstillco-eluting(2,6-and2,7-dimethylnaphthaleneand1,3-and1,6-dimethylnaphtha-lene).Interestingly,normalphaseGC × GC,whichisgenerallyusedfortheseparationofcomplexsamples,showedlowerresolutionforthealkylnaphthalenesthanGC–MS.TheenhancedseparationofreversedphaseovernormalphaseGC × GCisfurtherillustratedbychromatogramsoftheC3andC4alkylnaphthalenes(C3NandC4Nrespectively)inFig.2.Normal phaseseparates9peaksoutof34possibleC3Nisomersand14ofthe112possibleC4Nisomers,whilereversedphaseseparates14C3Nand20C4NpeakswithinthesameDNAPLsample.FulltotalionchromatogramsofDNAPLsample1bynormalphaseandreversedphaseGC × GCTOFMScanbefoundinthesupplementarydata(Figs.S2andS3respectively). TheincreasedseparationcapacityofreversedphaseisnotonlylimitedtoalkylPAHs.TheDNAPLsamplesinvestigatedinthisstudywerefoundtocontainawidevarietyofchemicalclasses,includingarangeofalkylatedheterocyclicPAHcompounds.Forexample,alkylbenzothiopheneswereabundantinallDNAPLsamples.Acompar-isonoftheseparatingpowerofthetwo   GC × GCmodesforthebenzothiophenesisshowninFig.3.Thenumberingindicatesthe peaksidentifiedasalkylbenzothiophenesbytheirmassspectra,assomelowintensitypeakscanoftenbemaskedinthecontourplot.Figs.2and3alsoillustratetheorderedstructureofGC × GCcontourplots;chemicalfamilieselutetogetherinaband,allowingstraightforwardidentification.Forexample,theC1alkylnaphthaleneselutetogetheronalinewiththehigheralkylatedhomologuesinsubsequentbands.Thestructuredlayoutofthecontourplotallowspeakstobeassignedquicklywithouttheuseofindividualstandards[34].Thisform oftentativeidentificationwasusedtoassignthemajorchemicalclassesinthechromatogramofDNAPL12(Fig.4)wherethegreatest varietyofcomponentswas   observed.The16U.S.EPApriorityPAHsareidentifiedinFig.4,aswellas theiralkylatedhomologues.TheelutionorderusingreversedphaseGC × GCisnoticeablydifferenttonormalphaseGC × GC.Innormalphase,thealkanesandiso-alkaneselutebeforethePAHsintheseconddimensionduetotheirlowaffinityforthepolarcolumn.Inreversedphase,thealkaneseluteafterthePAHsintheseconddimensionandareshownasabandalongthetopofthecontourplot(Fig.4).  L.A.McGregoretal./J.Chromatogr.A 1218 (2011) 4755–4763 4759 Fig.2. GC × GCcontourplotsofC3andC4alkylnaphthalenesinDNAPL1usingnormalphase,(a)and(b)respectively,andreversedphase,(c)and(d)respectively. Fig.3. GC × GCcontourplotsofC2andC3alkylbenzothiophenesinDNAPL1usingnormalphase,(a)and(b)respectively,andreversedphase,(c)and(d)respectively.Numberingindicatesthepeaksidentifiedasalkylbenzothiopheneisomers.
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