Recentdevelopmentsin
comprehensivetwo-dimensional
qgaschromatography(GC·GC)I.Introductionandinstrumentalset-up
M.Adahchour,J.Beens,R.J.J.Vreuls,U.A.Th.Brinkman
Wereviewtheliteratureoncomprehensivetwo-dimensionalgaschroma-tography(GC·GC),emphasizingdevelopmentsintheperiod2003–2005.Thereviewopenswithageneralintroduction,theprinciplesofthetechniqueandtheset-upofGC·GCsystems.Italsodiscussestheoreticalaspects,trendsininstrumentation,columncombinations,anddetectiontechniques–notablymassspectrometricdetection.Wedevoteattentiontoawidevarietyofapplicationsandtoanalyticalperformance.ª2006ElsevierLtd.Allrightsreserved.
Keywords:Application;Complexsample;Comprehensivetwo-dimensionalgaschromatography;Detection;GC·GC;Massspectrometry;Modulation;Review
M.Adahchour*,J.Beens,
R.J.J.Vreuls,U.A.Th.BrinkmanFreeUniversity,
DepartmentofAnalytical
Chemistry,
deBoelelaan1083,1081HVAmsterdam,
TheNetherlands
1.Introduction
Some15yearsago,comprehensivetwo-dimensionalgaschromatography(GC·GC)begantoattractattentionandthefull-colour2DchromatogramofanoilsamplepublishedbyJohnPhillipsandhisco-workersin1991[1]wasaneye-openerformanyanalyticalchemists.Overtheyears,quiteanumberofreviewsonGC·GChavebeenpublished.Initially,theymainlydis-cussedtheprinciplesofthetechniqueandthebasictheory–and,ofcourse,theexper-imentalset-up.Next,interfacingtwoGCcolumnsbecameakeytopic,andthefirstfewapplicationswerereported[2–6].Mostofthesewereinthefieldofpetrochemicalanalysisand,forreasonswhichwillbeex-plainedbelow,flameionizationdetectors(FIDs)wereinvariablyusedfordetection[7].Foranappreciationoftheearlydevel-opments,problemsand(partial)solutions,thereadermayconsultreviews[8–10].
Thisreviewispublishedinfourparts.AllreferencesareincludedinPartI.
*qCorrespondingauthor.E-mail:m.adahchour@hotmail.com
Inthepastfewyears,greatstepsfor-wardhavebeenmade(e.g.,indetection,analyteidentificationandquantificationand,specifically,applications).Today,nexttopetrochemicalanalysis[11,12],areassuchasfood,airandenvironmentalanalysisarehighonthelist.Thiswasnicelydemonstratedinanextensivereview[92]inwhich–nexttotechnicaldevel-opments–differenttypesofapplicationwerehighlighted.Inthisreview,whichcoveredtheliteratureuptotheendof2002,closeto100papersonGC·GCwerequoted.Itissomewhatunexpectedtofind,amerethreeyearslater,thatafur-ther150papersdiscussingthenewtech-niquehavebeenpublished.Fromamongstthese,some40werepublishedinJ.Chromatogr.,A1019(2003)and1086(2005),inspecialissuesdevotedtotheFirstandSecondInternationalSymposiumonComprehensiveMultidimensionalGasChromatographyheldinVolendam(TheNetherlands)andAtlanta(GA,USA),respectively.Thesteepincreaseunder-scoresthatGC·GCis,indeed,arapidlyemergingandincreasinglysuccessfultechnique.Itthereforeseemsappropriatetodiscussnoveltrendsininstrumentationandtechniquesandtopresentaselectednumberofapplicationstoindicatetypicaldevelopments.Whileweintendthelistofreferencestobeexhaustiveandwequoteallpublishedpapersinatleastoneofthetablesincludedinthispaper,ouremphasis
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Table1.MainreviewsdevotedtoGC·GCpublishedin2003–2005*Year2003
Title
Comprehensivetwo-dimensionalgaschromatography:apowerfulandversatileanalyticaltool
Applicationofcomprehensivetwo-dimensionalgaschromatographytodrugsanalysisindopingcontrolComprehensivetwo-dimensionalgaschromatography
Areviewofenvironmentaltoxicantanalysisbyusingmultidimensionalandcomprehensive2D-GCOpportunitiesforultra-highresolutionanalysisofessentialoilsusingcomprehensivetwo-dimensionalgaschromatography:areview
Imagebackgroundremovalincomprehensivetwo-dimensionalgaschromatography
Informationtechnologiesforcomprehensivetwo-dimensionalgaschromatographyTheevolutionofcomprehensivetwo-dimensionalchromatography(GC·GC)Trendsinchemometricanalysisofcomprehensivetwo-dimensionalseparationsComprehensivetwo-dimensionalchromatographyinfoodanalysis
Comprehensivetwo-dimensionalgaschromatography–apowerfulandwidelyapplicabletechniqueHyphenationofqMStoGCandGC·GCfortheanalysisofsuspectedallergens
Comprehensivetwo-dimensionalgaschromatography–apowerfulandversatiletechnique
Numberofrefs.11055199053347862883111927
Ref.[92][93][94][95][96][97][98][99][100][101][102][103][104]
2004
2005
*ReviewsbrieflydiscussingGC·GC,butprimarilytreatingrelatedGCtopics,havenotbeenincluded.
willbeontheperiod2003–05.Table1summarizesthemainreviewpapersdevotedtoGC·GCinthatperiod.
Fortherest,weunderstandthatquiteanumberofreaderswillnotbefamiliarwiththeprinciplesandbasicinstrumentationofGC·GCortermssuchasÔmodula-tionÕ,Ôwrap-aroundÕandÔorderedstructuresÕ,whicharefrequentlyusedinthisfield.Fortheirconvenience,thisreviewopenswithanintroductorysectionthatshouldbringthemup-to-date.Moredetailedinformationcanbefoundelsewhere,especially[92].
2.Principlesandinstrumentation
2.1.Generalprinciples
Sinceitsearlydays,GChasbeenrecognizedasatoolofferinghigherpeakcapacitiesthanotherchromato-graphictechniques.Overtheyears,dramaticprogresshasbeenmadeand,today,state-of-the-artone-dimensional(1D)GConcapillarycolumnscan,typically,separatesome100–150peaksinonerun.However,thisdoesnotsufficetoseparatetheindividualconstituentsofmanydifferenttypesofsample,rangingfromoilsorpetrochemicalproductstocontaminatedfishandfood,cigarettesmokeandpollutedair,andtechnicalmixturesofpolychlorinatedbiphenylsortoxaphene.
OnewaytoimprovetheseparationpowerofaGCsystemeffectivelyistocouple,throughaninterface,twoindependentcolumns.Thesuperiorityofsuchtwo-dimensional(2D)GCover1D-GCwasdemonstratedforacapillary-columnsystemsome40yearsago[271]and,overtheyears,so-calledmultidimensionalgaschromatography,abbreviatedasMDGCorGC–GC,hasbeensuccessfullyappliedtoavarietyofproblems[271,273,274].Illustrativeexamples[272]indicatethe
limitationoftheapproach.Inthelargemajorityofcases,theapplicationsareoftheheart-cuttingtype:onlyone,orafew,narrowfraction(s)oftheeluatefromthefirstcolumnis/aretransportedtothesecondcolumnforfurtherseparation.Inotherwords,thetechniquecanberecommended,andisverysuccessful,ifinformationisrequiredaboutonlyafewfractionsthatcontainallanalytesofinterest–typicallyatarget-analysissituation.
However,wheneverscreeningofanentiresampleisthemainaim(i.e.whenunknownsplayamajorrole),MDGCrapidlybecomesanextremelylaboriousandtime-consumingtechnique,withverycarefulfractionation,lengthyre-analysisofallfractions,andreconstructionofthechromatogramsasthemajorproblems.Itwillbeclearthatonecannotfindawayoutbyincreasingthewidthofthefirst-columnfractions:usingwiderfractionswillsimplydestroymoreofthefirst-columnresolution.Thealternativeistosubjectthesampletoacompre-hensive2D-GCseparation:ratherthanbeingseparatedintoafewfractions,theentiresampleisnowseparatedontwodifferentcolumns,thefractionsarekeptnarrowtoensurethatnoinformationgainedduringthefirstseparationislostand–certainlyinGCasopposedtoliquidchromatography(LC)–theinstrumentalset-upisconstructedsoastoensurethatthetotal2Dseparationiscompletedwithintheruntimeofthefirst-dimensionanalysis.Inotherwords,comparedwithMDGC,GC·GCpromisestoyieldsubstantiallyimprovedresolutionforallsampleconstituentswithnolossoftime[105,106].TheschematicofaGC·GCsystemisshowninScheme1(adaptedfrom[92]).Typically,twoGCsepa-rationsbasedondistinctlydifferentseparationmecha-nismsareused,withtheinterface–calledamodulator(seeSection2.3,PartII)–betweenthem.Itsmainfunc-tionsaretocutandtorefocusnarrowadjacentfractions
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Scheme1.Typicalset-upof,andtermsfrequentlyusedinGC·GC,andvisualizationtechniques.
ofthefirst-columneluateand–afterfocusing–tore-leasetheserapidlyintothesecondcolumn.Whilstthisprocessisbeingcompleted,thefirst-dimensionsepara-tionproceeds.Inprinciple,thesecond-dimensionsepa-rationshouldbefinishedbeforetherelease-cum-injectionofthenextfraction[107].ThisisanimportantgoalofallGC·GCstudiesandtheunwantedexceptiontotherule,so-calledwrap-around,willbediscussedbelow.440
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Inmostapplications,samplesarefirstseparatedona(15–30)m·(0.25–0.32)mmID·(0.1–1)lmfilm(df)columncontaininganon-polarstationaryphase.Aftermodulation,eachindividualfractionisinjectedontoamuchshorter,narrowercolumn–withdimensions,typically,(0.5–2)m·0.1mmID·0.1lmdf–con-taininga(medium-)polarorshape-selectivestationaryphase.AswillbediscussedinSection3.3.1(inPartII),it
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Scheme1.(continued)
issometimesadvantageousforseveralreasonstouseaso-calledtwinGC·GCsystem[108].Thisallowsthesimultaneousanalysisofasamplebymeansoftwodif-ferent,independentcolumncombinationshousedinthesameovenandusingasinglemodulator.
Forvisualization,allpresentations,exceptthesomewhatlessdiscerningcontourlines,areusedinfiguresincludedinthisreview.Oneaspectofinterestwithrespecttothefrequentlyapplied2Dcolourplotisthatpropercolourcontrast(i.e.zoomingin/out)tech-niqueshavetobeusedforoptimaldetectionofminoraswellasmajorsampleconstituents.Abrief,butinterest-ing,discussionofthisapproach,anditscombinationwithapexplotting,isavailable[13,92].Twomoreaspectsareofinterest:
Inordertomaintaintheintegrityofthefirst-dimensionseparation,thenarrowfractionssubjectedtomodulationshouldbenowiderthanaboutone-quarter(orr)ofthepeakwidthsinthatdimen-sion,typically5–30s[14](i.e.oneshouldpreferably
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Trendshavethreeorfourmodulationsacrosseachpeak).Consequently,second-dimensionruntimesshouldbeoftheorderof2–8sinotherwords,second-dimensionseparationsinvariablyareessentiallyisothermal.Insomeinstances,thesecondcolumnishousedinasep-arateoventoallowmoreflexibletemperaturecontrol;thisiswhattheset-upinScheme1shows.Sincemeetingthe‘‘modulationcriterion’’oftencausesprob-lems,temperatureprogramminginGC·GCisusuallyatalowerratethanin1D-GC(i.e.2–3°C/min).
Withtheveryfastseparationsofthesecondcolumns,itwillbeclearthatsecond-dimensionpeakswillbeextremelynarrow,typicallydisplaying50–600mswidthsatthebaseline[13,15].Consequently,detectorswillhavetobecorrespondinglyfast,withdata-acquisi-tionratesoftheorderof,preferably,atleast100Hz.Tosummarize,iftheaboveaspectsaredulyconsideredandimplementedintheinstrumentalset-upandseparationprotocol,thethreeessentialdemandsofacomprehensiveseparation[16,109,110,275,276]aremet:
allsampleconstituentsaresubjectedtotwoseparationsinwhichtheirtransportationmechanismdependsondifferentfactors;
anytwosampleconstituentsseparatedinthefirstdimensionwillremainseparatedintheseconddimension;and,
theelutionprofilesfrombothcolumnsarepreserved.2.2.Columncombinations
Inprinciple,allkindsofstationaryphasecanbeusedinthefirstdimensionofaGC·GCsystem.However,generallyspeaking,non-polarphasesarepreferred,especiallyforreasonsoforthogonality[17].Orthogonalityhasbeendefined[111]anddiscussed[112],anditisgenerallyunderstoodthataseparationistrulyorthogonaliftheconstituentdimensionsoperateindependentlyandsynentropyacrossthedimensionsiszero.Minimizingsynentropyorcrossinformationisimportantinmulti-dimensionalseparationsbecausethelargeritbecomes,thelargerbecomesthatpartoftheseparationspacethatisunoccupiedoreveninaccessible[17].
Sincetheseparationmechanismonanon-polarphaseis,inprinciple,basedonanalytevolatilityonly,thisphaseshouldbeselectedforthefirstdimension.
Fortheseconddimension,avarietyofphasescanbeselected,dependingonthedesiredanalyte–stationaryphaseinteraction(s).Inpractice,nearlytheentireplaneofaGC·GCchromatogramisthenavailableforpeakseparation.
Scheme2listsallstationaryphasesmentionedinthisreview.
2.2.1.Orthogonalapproach
AsurveyofcolumncombinationsusedinGC·GCmadeinearly2005showedthatnon-polar·(medium-)polaris442
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byfarthemostpopularoption,withatotalscoreofabout80%.Thisiseasilyunderstandablebecause–apartfromwhatwassaidabove–widespreadinformationisavailableonthebehaviourofahugenumberof(classesof)com-poundsonnon-polarcolumnsinconventional1D-GC,whichcanconvenientlybeusedtooptimizethefirst-dimensionseparation.Here,volatilityisnowtheonlyparameterofinterestand,consequently,aboiling-pointseparationisobtained.Withallothertypesofcolumn,whethermedium-polar,polarorshape-selective,separa-tionwillbeprimarilygovernedbythespecificinterac-tion(s)oftheselectedÔpolarÕcolumn,buttosomedegreealsobyvolatility.Withanon-polarfirst-dimensioncol-umn,analyteswithmutuallycloselysimilarvolatilitieswillelutefromthatcolumnineachindividualnarrowfraction.Becauseoftheveryfastand,thus,isothermalsecond-dimensionseparation,foranalytesofequalvola-tility(i.e.theanalytesineachindividualfraction),therewillbenoboiling-pointcontributioninthatdimension,soonlythespecificinteractionswiththestationaryphasewillgoverntheretention,andtheseparationwillindeedbeorthogonal[18,113,112].
Aswewroteonanearlieroccasion[92],orthogo-nalityis,ofcourse,notagoalinitself.However,onemainbenefitisthatorderedstructuresnowshowupinaGC·GCchromatogramforstructurallyrelatedhomo-logues,congenersandisomers[16,19,20].Structuredchromatogramsareavaluabletoolwhenperforminggroup-typeidentification,aswillbeamplydemonstratedintheApplicationssection(PartsIIIandIV).Contrarytowhatwasgenerallybelievedevenintherecentpast,andimplicitinwhatwesaidabove,thereisnoexclusiveconnectionbetweenorderedstructuresandorthogonal-ity.AswillbediscussedinmoredetailinSection2.2.2below,iftheseparationdimensionsandsampledimensionsareproperlymatched,touseGiddingsÕster-minology[16],‘‘structure’’canalsobeobtainedundernon-orthogonalconditions.
2.2.1.1.Columnselection.Itissomewhatsurprisingthat,evenwithanon-polarfirst-dimensioncolumn,littleattentionhasbeendevotedtosystematicsearchforthe‘‘best’’second-dimensioncolumn.Thatmutuallywidelydivergentoverallselectivitycanbeobtainedduringsuchastudywasdemonstratedfortheseparationof172,3,7,8-substitutedpolychlorinateddibenzo-p-dioxinsanddibenzofurans(PCDD/Fs)and12dioxin-likepolychlorinatedbiphenyls(PCBs)[114].With,inallinstances,a100%methylpolysiloxane(DB-1)firstdimension,eighttypesofsecond-dimensioncolumnweretested.Acompleteseparationofallcongenerswithdif-ferenttoxicequivalencefactor(TEF)valueswasachievedwithtwocolumncombinations,viz.DB-1·VF-23ms(70–90%cyano-containingpolymer)andDB-1·LC-50(50%liquidcrystallinemethylpolysiloxane)).Ontheother
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Commercial code Stationary phase SolgelWax Cyclodex-B HT-8 BPX-5 HT-5 BPX-35 BP-20 BP-21 BP-10 BPX-50
polyethlene glycol in a sol–gel matrix permethylated β -cyclodextrin in OV 1701 8% phenyl-methylpolysiloxane (carborane) 5% phenyl-methylsilphenylene
5% phenyl-methylpolysiloxane (carborane) 35% phenyl-polysilphenylenesiloxane polyethylene glycol
polyethylene glycol (TPA-treated) 14% cyanopropylphenyl-polysiloxane 50% phenyl-methylpolysiloxane (silphenylene)
Producer* SGE SGE SGE SGE SGE SGE SGE SGE SGE SGE J&W J&W
J&W
DB-Wax polyethylene glycol DB-624
6% cyanopropylphenyl-dimethylpolysiloxane
DB-210 trifluoropropylmethylpolysiloxane
DB-1701 14%(cyanopropyl-phenyl)-methylpolysiloxane J&W DB-5 DB-17ht
5% diphenyl-dimethylpolysiloxane 50% phenyl-methylpolysiloxane
J&W J&W J&W J&W
J&W J&W
DB-Petro 100%dimethylpolysiloxaneDB-1
100% methylpolysiloxane
DB-XLB proprietaryDB-DIOXIN
proprietary (44%methyl, 28% phenyl, 20%
cyanopropyl, 8%polyoxyethylene-polysiloxane) 2,3,6-tri-O-methyl β-cyclodextrin
proprietary (70-90% cyano-containing polymer)
Chirasil-DexVF-23ms VF-1msCP-Sil 5 CB CP-Sil 2 CB
Varian/ChrompackVarian/Chrompack
100%methylpolysiloxane Varian/Chrompack100% methylpolysiloxane
Varian/Chrompack
cross-linked siloxane containing a high-molecular-Varian/Chrompack
weight hydrocarbon similar to squalaneScheme2.(Trade)namesandproducersofGCstationaryphasesquotedinthepresentreview.
hand,theuseof007-210(50%trifluoropropylmethyl-polysiloxane)wasfoundnottoaddnoticeablytotheresolutioneffectedbya1D-DGseparation,andwiththeHT-8(8%phenyl-methylpolysiloxane)columnwhichhasastrongboiling-pointbasedselectively,improvedseparationwaslimitedtothe6D3/6F4pair.
Whenanothertypeoffirstdimensioncolumnwasused(andorthogonalitywasslightlysacrificed),viz.aDB-XLBproprietarycolumn),evenbetterresultswereobtained.Inthiscase,botha007-65HT,orVF-23msandLC-50seconddimensioncolumn,acompletesepa-rationofall29prioritycongenerswasachieved.ThisisshowninFig.1,whichalsoillustratesthewidelydif-ferentoverallselectivityoftheseveralcolumnsets.TheDB-XLB·LC-50set,whichprovidesanexcellentgroupseparationbasedonplanarity,gavethebestreal-liferesult(i.e.thebestseparationfrom,also,matrixcon-stituentsinthecaseofmilksamples).Inamorerecentstudybythesamegroup,theGC·GCbehaviourof12classesoforganohalogenatedcompounds–bothchlorinatedandbrominated,andofanaromaticaswellasanon-aromaticcharacter–wasstudiedonfivecolumncombinations(i.e.aDB-1first-dimensioncolumncombinedwith007-210,HT-8,LC-50,007-65HTorVF-23msintheseconddimension[115]).Aninterestingconclusionwasthatdifferentcolumnsetswerefoundtogiveoptimumresultswithregardto,e.g.,between-groupandwithin-groupsepa-rations,resolutionofchlorinatedandbrominatedana-logues,andseparationofplanarfromnon-planargroupsofanalytes.Toquotesomeexamples:
withtheHT-8column,GC·GCseparationismainlybasedonthenumberofhalogensubstituents;
with007-65HT,polychlorinatedn-alkanes(PCAs)andpolybrominateddiphenylethers(PBDEs)areeffectivelyseparatedfromallothercompoundclassestested;and,
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Commercial Stationary phase code 007-65HT 65%phenyl-methylpolysiloxane OV 1701
14% cyanopropylphenyl-methylpolysiloxane
007-210 50%trifluoropropylmethylpolysiloxane 007-1 100%methylpolysiloxane BGB-176SE BGB-172 BGB-1701 BGB-Silaren
20% 2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl β-cyclodextrin diluted in SE-52
25% 2,3,6-tert.-butyldimethylsilyl β-cyclodextrin diluted in PS086 (15% phenyl-methylpolysiloxane) 14% cyanopropylphenyl-dimethylpolysiloxaneSildiphenylether-diphenylpolysiloxane
Producer* Quadrex Quadrex
Quadrex Quadrex BGB BGB BGB BGB
Restek Restek Restek
Restek Restek Supelco Supelco Sigma–Aldrich MeGA J&K
Rtx-500 diphenyl/dimethylpolysiloxane Rtx-200 trifluoropropylmethylpolysiloxane Stabilwax polyethylene glycol
XTI-5 95%dimethyl-diphenylpolysiloxane RTX-1 100%dimethylpolysiloxaneSupelcoWax-10 polyethylene glycol
SPB-5 5%diphenyl-dimethylpolysiloxane SP-2340 100%biscyanopropylpolysiloxane EtTBS-β-CD LC-50
25% 2,3-diethyl-6-tert.-butyldimethylsilylated β-cyclodextrin diluted in PS086
50% liquid crystalline-methylpolysiloxane
* SGE International, Ringwood, Australia; J&WScientific, Folsom, CA, USA; Varian, Middelburg, TheNetherlands; Quadrex, New Haven, CT, USA; BGB Analytik, Aldiswil, Switzerland;Restek Corp., Bellefonte, PA, USA; Supelco, Bellefonte, PA, USA; Sigma-Aldrich, Brussels, Belgium; Varian-Chrompack, Middelburg, The Netherlands;MeGA, Milan, Italy; J&K Environmental, Sydney, NS, Canada.
Scheme2.(continued)
DB-1·VF-23msyieldsexcellentwithin-classsepara-tions,especiallyofnon-aromaticcompounds,suchasPCAs,toxapheneandorganochlorinepesticides.Animpressiveexampleoftheseparationofthreeclassesofcompounds–toxaphene(atechnicalmixturemainlycomprisingchlorobornanesandchlorocamph-enes),PCA-60(atechnicalmixtureofpolychlorinatedn-alkaneswithachlorinationdegreeof60wt.%),andpolychlorinatedterphenyls(PCTs;a1:1mixtureofthetechnicalproductsAroclor5442andAroclor5460)–isshowninFig.2.MoredetailedinformationonthecompositionoftoxapheneandthePCAmixtureispre-sentedinSection3.2(PartIII).
Morerecently,theGC·GCbehaviourofavarietyoftestcompoundswasstudiedonarangeoffirst-dimen-sioncolumnsofdifferentpolarityandwithBPX-5orBP-20asthesecond-dimensioncolumn[112].Thepolarityofthefirst-dimensioncolumnwassystemati-callyalteredbycouplingdifferentlengthsofBPX-5andBP-20toaconstanttotallengthof20m.Theobser-444
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vationsmadeinthepapersquotedabovewereessen-tiallyconfirmed,butthemoresystematicnatureofthepresentstudy[112]gaveresultsthatcanbemoreeasilyinterpreted.Theauthorsfoundthat‘‘correlated’’phaseswithsimilarpolarityinbothdimensionsresultedinanessentiallyone-dimensionalseparation,withtheana-lytesdistributedalongadiagonalinthe2Dplane.Theuseoftheseparationspacewasminimal.Conversely,maximalusewasachievedwhenthefirstandseconddimensionsweremostdissimilar.Theauthorspro-posedtheuseofasimpleestimator(i.e.theamountofseparationspaceusedforretainedsolutescomparedwiththevoidspace)toexpresstheorthogonalityofaseparation.
Whileitwillbeclearthatmuchcanindeedbegainedintermsofbetween-andwithin-groupseparationbyselectingthepropercolumncombination,itisgoodtokeepinmindthat,withreal-lifeapplications,thesepa-rationofthetargetclassesofanalytesfrommatrixinterferencesalsoplaysarole–and,frequently,amore
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B– DB-XLBB-XLB×007-65HT3281123157114105771184F14D11261671561695D16F16D16D25F15F21896D36F36F46F27F17D18F7F28D105060705F21895F15D18090100110C– DB-XLB×VF-232181123114771181054F11261674D11561571696F36F26D36F47F27F17D16F16D16D28F8D05087660708090100110D– DB-XLB×LC-507F26F47D18F5434F14D16D36F36F15D16D16D26F21691897F18D5F2281771181051261565F1101231671141575060708090100110Figure1.GC·GC–lECDchromatogramsofamixtureof10CDFs(red,fromtetra4Ftoocta8F),7CDDs(green,fromtetra4Dtoocta8D)and12dioxin-likeCBs(orange,CB81–189)foraDB-XLBcolumncombinedwithsecond-dimensioncolumnsasindicatedintheframes.Temper-atureprogrammeforfirstandsecondcolumns:90°C(2min),at30°C/minto170°C(5min),thenat1.5°C/minto290°C(10min).Finaltemper-atureforLC-50was270°Cbecauseoftemperaturelimits.InletpressureforDB-XLB·LC-50,39psi,forothercombinations,45psi.Modulationperiod,8s[114].importantone.Typicalexamplescanbefound[21,96,114,116–119].Theoreticalstudiesonthedevelopmentof2Dresolutionmodelsand/ororderedstructurestendtooverlookthisaspect.
2.2.1.2.Columndimensions.Asmentionedabove,awell-established1D-GCprocedureisoftenselectedasthebasisofanorthogonalGC·GCseparation.However,thetemperatureprogrammeisfrequentlymaderelativelyslowtomakethefirst-dimensionanalytepeakssome-whatwiderand,thus,tomakeiteasiertoachievetherequired3–4-foldmodulationofeachpeak,theso-calledmodulationcriterion.Eventhoughthisisnotahugedisadvantage,itisofinteresttotrytodecreaseruntimesbyproperlyadaptingthedimensionsofeitheroneorbothGCcolumns.Apreliminarystudyinthisarea[110]showedthatadramaticimprovementcan,indeed,beobtained.Withaconventionalfirst-dimensioncolumncombinedwitha(0.5m·0.05mmID·0.05lmdf)insteadofthemoreusual(1–2m·0.1mmID·0.1lmdf)second-dimensioncolumn,thetimeofanalysiswasreducedfrom100minto27min.Thisalmostfour-foldreductionwasclosetowhatwasexpectedonthebasisofthetwo-foldsmallerboreofthesecond-dimensioncolumn.Theoverallresolutiondidnotappeartosufferfromthechange:thepeakcapacitywasfoundtoin-creasefrom6000to9000.
Theaboveisagratifyingfirstresult.However,thequotedstudywaslimitedand,inaddition,veryfew50-lmIDcolumnsarecommerciallyavailabletoday.Moreover,thesignificantlyincreasedpeakcapacitysuggeststhat,nexttothecolumnIDeffect,otherphe-nomena–possiblyrelatedtopressureandpressureprofiles–mayplayarole.Thistopichasrecentlybeen
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Figure2.GC·GC–lECDchromatogramonDB-1·007-65HTofPCAs(PCA-60),PCTs(Aroclors5442+5460)andtechnicaltoxaphene[115].
discussedinapaperthatshows,onthebasisofvarioussetsofcalculations,thatonecolumnshouldbeoperatedclosetoitsoptimumflowconditions,andasub-optimumseparationontheothercolumnaccepted[120].
Aninterestingconclusionwasthatcombiningafirst-dimensioncolumnwitha0.25–0.32mmIDwithasecondonefeaturinganIDof0.15–0.18mmturnsouttobeagoodchoice.However,herealso,someslowingdownofthetemperature-programmingrateisneededtofulfilthemodulationcriterion.AsimplermodelwasdevelopedbyHarynuketal.[121].Itcalculatestheflowratesinthecolumnsandpredictsappropriatedelay-loopdimensionsforagivenset-up.Toquoteanexample,forasystemwitha30m·0.25mmIDfirstcolumn,two0.12m·0.10mmIDtrappingcapillaries,anda1.55m·0.15mmIDsecondcolumn,themodelsug-gestsusinga1.1m·0.25mmIDloopinordertohaveanaveragelinearvelocityof30cm/sinthefirstcolumnandadeadtimeof2.5sintheloopat35°C.SuchamodelalsopredictsthepressureprogrammingratesthatcanbeusedtogeneratespecificflowratesinanyGC·GCsystem.
AnotherchallengingissuewasdiscussedbyShellieetal.[122],whostudiedtheeffectsofpressuredroponabsoluteretentionmatchinginGC·GC.Ifpeakidentitieshavetobeassignedtospecificpeakpositionsinthe2Dseparationspace,andthisnecessitatesmatchingpeakretentionsinGC·GC–FIDandGC·GC–ToFMS,theatmosphericvs.vacuumoutletconditionsconfoundthistask.Theauthorsshowedthatbyutilizingasupplementarygassupply,providedtoaT-unionbetweenthesecond-dimensioncolumnoutletandtheMSinterface,2Dchromatogramscanbegeneratedthatareessentiallyexactlymatched.For18testanalytesrepresentingvariousclassesofcom-446
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pounds,theaverageabsoluteretention-timedifferencewas42msintheseconddimension,and3.7sinthefirstdimension.
2.2.2.Non-orthogonalapproach
DespitethevariousadvantagesofusingorthogonalGC·GCconditionsmentionedinSection2.2.1,non-orthogonalapproachset-upswerealsousedinalimitednumberofearlystudies.AnumberofmorerecentapplicationsarelistedinTable2,whichshowsthattheapproachhasbeensuccessfulinwidelydifferentareas.OneexampleconcernstheseparationofplanarCBsfromothercongeners[22],inwhichapolarLC-50·non-polarBPX-5columncombinationgavearemarkablysuccessfuloverallresult(seealsoSection3.2,PartIII),butwithoutanyorderedstructure.Earlier,onewouldhavesaidthatthisisnorealsurprisebecausetheorthogonalityprinciplehasbeenviolated.However,wenowknowthat,alsowithnon-orthogonalseparations,orderedstructuresmaybepresent–andevenprominent.Asanillustration,Fig.3displaysboththewell-knownconventional(non-polar·polar)anda‘‘reversed’’(i.e.polar·medium-polar)separationofadieseloil[123].Whilethecompletelyreversedorderofseparationofthealkanes,mono-aromaticanddi-aro-maticcompoundscanbesaidtobeaccordingtoexpectations,thedifferentlyorganized,butobviouslyordered,structureissomethingofasurprise.Thevariousgroupsofcompoundsseemtobepackedtightertogether(aswasalsofoundinotherstudies[124,125];seebelow).Underoptimizedexperimentalconditions–pri-marilywithregardtotemperaturecontrol–notonlycanthealkanesbeseparatedfromthecycloalkanes,butde-tailedseparationswithineachclasscanalsobeachieved[123].TheresultspresentedinFig.3wereobtainedby
TrendsinAnalyticalChemistry,Vol.25,No.5,2006Trends
Table2.Examplesofnon-orthogonal(reversed-type)GC·GCcolumncombinations(period,2003–2005)*Application
RoastedcoffeebeansRoastedcoffeebeansFAMEsinfatsandoilFAMEsinmilkfatFAMEsinlipidsFlavoursinfoodFoodextractsPlanarCBsÔPhillipsmixÕ
Cigarettesmokecondensate:acidicfractionVOCsindieselfuel
Aminoacidsinwine,beerandhoney
*Columncombination[m·mmID·lmdf]
[30·0.25·0.25]SupelcoWax-10·[1·0.1·0.1]SPB-5[30·0.25·0.25]SolgelWax·[1·0.1·0.1]BPX-5
[30·0.25·0.25]SupelcoWax-10·[1·0.1·0.1]SPB-5[100·0.25·0.25]CP7420·[1·0.1·0.1]HP-1
[30·0.25·0.25]SupelcoWax-10·[1·0.1·0.1]SPB-5[30·0.25·0.25]BP21·[1·0.1·0.1]BPX-35[25·0.32·0.25]BP21·[1·0.1·0.1]BPX-35[10·0.15·0.1]LC-50·[0.4·0.1·0.1]BPX-5[30·0.25·0.25]SP-2331·[2·0.1·0.1]Rtx-5[50·0.25·0.25]CEC-Wax·[2·0.1·0.1]DB-17ht[60·0.25·0.25]DB-Wax·[3·0.1·0.4]DB-1701
[5·0.25·1.4]SP-2331·[5.2·0.25·0.1]DB-WaxorDB-210[30·0.25·0.25]SolgelWax·[0.4·0.1·0.1]BP1
Ref.[126][124][130][127][131][123][108][22][113][175][128][129]
Enantioselectivecolumncombinations,whichalsobelonginthiscategory,arediscussedinSection2.2.3andTable3.
Figure3.GC·GC–FIDchromatogramsofadieseloilobtainedontwodifferentcolumnsets:
(top)non-polar·polarand(bottom)polar·medium-polar[123].
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usingasecondoven(40°Chighertemperature).Withoutthatoven,within-andbetween-groupseparationswereevenmoredetailed.
Furtherworkindicatedthatmoreimportantadvan-tagesaregainedwhenpolarand/orionogeniccompoundshavetobestudied[123,126–129,175].Fig.4,tobediscussedinmoredetailinSection3.3(PartIII),clearlyshowstheimprovedchromatographicbehaviourandimprovedpeakshapeofanalyteclasses,suchasaldehydes,ketonesandalcoholsand,specifically,carboxylicacids,whichcouldhardlyberecognizedwithanorthogonalapproach.Inaddition,inthiscase,themodulationtimecouldbelimitedto2–3swithoutwrap-aroundoccurring.Consequently,thefirst-dimensionseparationcouldberunclosertoitsoptimalconditionswithoutlosingthedesiredfourmodulationsacrossapeak.Anothertypicalexampleinvolvestheanalysisofthevolatilefractionofcoffeebeans[126].Anon-orthogo-nalSupelcoWax-10·SPB-5columnsetgaveaneffi-cientseparationandthechromatogramswerehighlystructured.Fig.5showspartofaGC·GC–FIDchro-matogramofroastedArabicacoffeebeans.Thepyra-zinesclearlyformaspecificpatterninthe2Dplane.Pyrazine-ringsystemswiththesamedegreeofalkyl-carbonsubstitution(e.g.,dimethylpyrazineandethyl-pyrazine)tendtoalignthemselveswithsecond-dimen-sionelutiontimewhichgraduallydecreasesastheGCoventemperatureincreases.Thispermitsdetailedcharacterizationofthepyrazinefraction,whichplaysamajorroleinthecoffee-aromaprofile.Itshouldbeno-tedthatnotallspotsshowingupintheellipsescanbeattributedtopyrazinesub-groups,butonlythosenumbered1–28.Sampledimensionalityand/orsample
Figure4.GC·GC–FIDof40oliveoilstandardsontwocolumnsets:(top)non-orthogonalBP21·BP-35and(bottom)orthogonalDB-1·BP20.Insertsdemonstratepeakshapesof(a)3-methylbutanoicacidandthreealcohols,(b)1-hexanol,(c)cis-3-hexenoland(d)trans-2-hexenol[123].
448
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Figure5.PartofGC·GC–FIDchromatogramofroastedArabicacoffeebeansusingaSupelcoWax-10·SPB-5columnset[126].Formoredetails,seetext.
complexityobscurethe(degreeof)orderedstructures.Onlythenumberedspotsinthethreeellipsesarepyr-azines(theothersare,e.g.,thiophenes,ketonesandpyridines).Whenthequotednon-orthogonalandanorthogonalapproachwerecomparedforthesamesample[124],theformerwasfoundtobesuperior:thenumberofpeakswashigherandalargerpartoftheseparationspacewasused(alsoseeSection3.3.5,PartIII).
2.2.3.EnantioselectiveanalysisinGC·GC
Anotherfieldofinterestistheuseofenantioselective(cyclodextrin)phases[141](Table3).Shellieetal.[23,25]successfullyseparatedenantiomericpairsofmonoterpenehydrocarbonsandoxygenatedmonoter-penesandalsodeterminedtheenantiomericcompositionofthemainmonoterpenesinAustralianteatreeoil(Fig.6).Theauthorsusedbothanon-orthogonal(cyclodextrin·BP-20)andanorthogonal(DB-5·cyclodextrin)approach.Sincetheseparationofpairsofenantiomersrequireslongruntimes(andhencelongcolumns),theformerapproachispreferredwhenorderedstructuresarenotofinterest.If‘‘structure’’isrequired,thenthelattercolumncombinationgenerallygivesbet-terresults.However,suchanachiral·chiralapproachismuchmoredemandingbecauseadequateenantiomerresolutionofalltargetpairshastobeeffectedduringtheveryfastsecond-dimensionseparation.Becauseofthis,theauthors[23]usedanarrow-bore(100lmID)non-polarfirst-dimensioncolumnconnectedtoawider-bore(250lmID)second-dimensionenantioselectivecolumnwithvacuum-outletconditions.Suchconditionscausediffusioncoefficientsandanalytevolatilitytobecomehigher;second-dimensionretentiontimeswerefour-foldlowerthanwithambientpressureoutletconditions.Retentiontimeswereasshortas8sfor(±)-limonene,withadequateenantioseparation(Rsca.1)ona1-mcyclodextrinderivative-coatedcapillarycolumn.Asanapplication,bergamotessentialoilwasanalysed.
Asimilarachiral·chiralstrategywasusedbyWuetal.[132]toanalysevolatileoilsintraditionalChinesemedicines.However,a(60m·0.25mm·0.25lm)polarSolgelWaxfirst-dimensioncolumnanda(3m·0.1mm·0.1lm)Cyclodex-Bcolumnintheseconddimensionwereneededtoachieveadequateseparation.ThiscolumnsetgavebetterresultsthanDB-Wax·DB-1701andDB-Petro·DB-1701combina-tionsof,roughly,thesamedimensions.
Intwomorerecentstudies,two20–30-mcolumnshadtobecombinedtoprovidea‘‘chiral’’first-dimensionset-upwhichyieldsspecificseparationsof,e.g.,2-and3-methylpositionalisomersofbutanol,butylacetateandbutanoicacidanditsethylester,inredwine[138],andtheenantiomersoftheimportantflavourslinaloolandfuraneolinstrawberries[139].MoredetailsontheseapplicationswillbepresentedinSections3.3.3and3.3.5,respectively(PartIII).
QuiteanumberofpapershasbeendevotedtotheseparationofchiralCBsbymeansofGC·GC.ThistopicwillalsobediscussedindetailinSection3.2.2(PartIII).
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Table3.EnantioselectivecolumncombinationsusedinGC·GCApplication
Monoterpenesinessentialoils
EssentialoilsinChinesemedicines
Terpenesinzedoaryvolatileoil[133,134]EnantiomericpairsofCBsEnantiomericpairsofCBsEnantiomericpairsofCBsEnantiomericpairsofCBs
2-and3-methyl-substitutedisomersinredwineMonoterpenesinAustralianteatreeoil
IsomersoffuraneolandlinaloolinstrawberryStudyofinterconversionbehaviourinGC·GC
Columncombination(m·mmID·lmdf)
[10·0.1·0.1]DB-5·[1·0.1·0.1]EtTBS-b-CDor[1·0.25·0.25]EtTBS-b-CD
[60·0.25·0.25]SolgelWax·[3·0.1·0.1]Cylodex-B[60·0.25·0.25]SolgelWax·[3·0.1·0.1]Cylodex-B[10·0.10·0.1]Chirasil-DexCB·[1·0.1·0.1]LC-50
[25·0.25·0.25]Chirasil-Dex·[0.9·0.1·0.1]LC-50orSupelcoWax-10,or·[1·0.1·0.1]VF-23ms
[30·0.25·0.25]Chirasil-Dex,BGB-172orBGB-176SE·[1or2·0.1·0.1]HT-8,BPX-50orSupelcoWax-10
[10·0.10·0.1]Chirasil-DexCB·[1.4·0.15·0.1]LC-50,or·[1.5·0.15·0.1]VF-23ms
[24·0.25·0.25]EtTBS-b-CD+[30·0.25·0.25]CycloSil-B·[1·0.1·0.1]BPX-50
25·0.25·0.25EtTBS-b-CD·0.8·0.1·0.1BP-20
[20·0.25·0.25]EtTBS-b-CD+[26·0.25·0.25]CycloSil-B·[1·0.1·0.1]BPX-50
[30·0.25·0.25]CP-Chirasil-DexCB·[0.8,2or4·0.1·0.1]BP20
Ref.[23][132][24][135][136][137][138][25][139][140]
Figure6.Enantio-GC·GC–FIDofahigh-pHdistillationofaflushgrowth(youngleaves)sampleofteatreeessentialoil.Peaknumbers:1.a-thujene;3.sabinene;4.b-pinene;8.p-cymene;9.limonene;11.c-terpinene;12.trans-sabinenehydrate;14.cis-sabinenehydrate;18.terpinen-4-ol;19.a-terpineol;U.unidentifiedcomponent.Individualisomersindicatedby(+)or(À)sign,ora,bwherecorrectassignmentofisomerswasnotconfirmed[25].
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