外文翻译--毛细管电泳电化学检测方法 中文版.pdf

JournalofChromatographyA,834199989–101ReviewElectrochemicaldetectionmethodsincapillaryelectrophoresisandapplicationstoinorganicspeciesThomasKappes,PeterC.HauserUniversityofBasel,DepartmentofChemistry,Spitalstrasse51,CH4056Basel,SwitzerlandAbstractThethreeelectrochemicaldetectionmethodsincapillaryelectrophoresis,namelyconductometry,amperometryandpotentiometry,arediscussedandcomparedtothemorecommonopticaldetectionmethods.Theprinciplesofeachmethodandtheirimplementationsaredetailedandreportedapplicationstoinorganicspeciesarereviewed.1999ElsevierScienceB.V.Allrightsreserved.KeywordsElectrochemicaldetectionDetection,electrophoresisReviewsInorganicanionsMetalcationsContents1.Introduction............................................................................................................................................................................892.Conductometricdetection........................................................................................................................................................902.1.Principle.........................................................................................................................................................................902.2.Implementations.............................................................................................................................................................913.Amperometricdetection...........................................................................................................................................................923.1.Principle.........................................................................................................................................................................923.2.Implementations.............................................................................................................................................................934.Potentiometricdetection..........................................................................................................................................................954.1.Principle.........................................................................................................................................................................954.2.Implementations.............................................................................................................................................................955.Applicationstoinorganicspecies..............................................................................................................................................996.Conclusions............................................................................................................................................................................100References..................................................................................................................................................................................1001.Introductionformofconductometry,amperometryorpotentiometryareanattractivealternative,which,howDetectionincapillaryelectrophoresisCEisever,hasbeenexploredcomparativelylittle.Thisiscommonlycarriedoutusingopticalmeansabsorpininterestingcontrasttoionchromatography,thetionandfluorescence.Electrochemicalmeansintheotherandoldergeneralmethodforiondetermination,whichmainlyemploysconductivitydetectionCorrespondingauthor.Fax141612671013.andwhereopticalmethodsareseldomused.Perhaps,00219673/99/–seefrontmatter1999ElsevierScienceB.V.Allrightsreserved.PIIS002196739800685290T.Kappes,P.C.Hauser/J.Chromatogr.A834199989–101thisisevenmoresurprisingwhenoneconsidersthattionandfluorescencemeasurementsarealsoreopticaldetectionismademoredifficultincapillarystrictedtospeciesthatshowtherespectivepropelectrophoresisbecauseofthesmallercellvolumeserties.Forthisreason,indirectopticalmethodsareandthefactthatmanyionscannotbedetectedoftenusedinwhichthedisplacementofasenseddirectlybyopticalmeans.Theexplanationforthisauxiliaryagentbytheanalytesismonitoredforcedsituationmaybetwofold.Firstly,itisfairlyeasytobytherequirementtohaveoverallchargeneutrality.adaptabsorptiondetectors,aswidelyusedinhighThisapproachmayalsobeusedforelectrochemicalperformanceliquidchromatographyHPLC,fordetectionmethodswhentheanalytespeciescannotcapillaries.ManyCEinstrumentmanufacturersapbesenseddirectly.ChemicalderivatizationofthepeartohavegonethisrouteandhaveincorporatedanalyteinordertoimpartdetectabilityisanotherexistingdetectorsintotheirCEequipment.Secondly,possibleapproach.Neitherofthesemeansisideal,theappliedhighseparationvoltageisanintrinsichowever,asindirectdetectiononlyallowsanarrowinterferenceinelectrochemicaldetection.Intheearlydynamicconcentrationrangeandderivatizationaddsdays,elaborateschemeswereemployedtoovercometothecomplexityofthemethod.Inpractice,thethisdifficulty.Inrecentyears,ithas,however,choiceofthedetectionmethodshouldperhaps,inthebecomeevidentthat,withthecorrectdesignofthefirstinstance,exploitanintrinsicpropertyfordirectsystem,thisdoesnothavetobeaproblem.sensingand,secondly,bebasedontheachievableCommontoallthreevariantsofelectrochemicalandrequireddetectionlimits.Acompromisemaydetectionmeansisthefactthattheyareintrinsicallyhavetobefoundwhenseveralspeciesaretobesimplerthantheopticalmethods.Anelectricalsignaldeterminedasnotallmayhavethesamesetofisobtaineddirectlywithouttheinvolvementofandetectableproperties.intermediatephysicalparameter,suchasradiationOtherreviewsonelectrochemicaldetectioninintensityinopticalmethods.Thedetectorhardwarecapillaryelectrophoresisareavailable1–5.consistsofthreeorfewersmallelectrodesandsomefairlysimpleelectroniccircuitry,whereasforopticaldetection,alightsource,monochromator,optical2.Conductometricdetectiondetectorsandfocussingopticsarenecessary.Inopticalmethods,thecellvolumedirectlyaffectsthe2.1.Principlesignalviatheopticalpathlengthand,forthisreason,thecapillarydiametersalwaysshouldbeaslargeasHeretheabilityofionstoconductchargeinpossible.Forelectrochemicaldetection,thecellsizesolutionisexploited.Thecurrentproducedbetweensamplevolumehasonlyadirectbearingincontwoelectrodeswhenapplyingavoltageismeasuredductivitymeasurements.Inamperometry,thesignaland,accordingtoOhmslaw,yieldstheresistanceorisrelatedtotheareaoftheworkingelectrode,thetheconductanceoftheelectrolytesolution.Inordersizeofwhichwillbelimitedbytheavailablesampletosuppresstheinfluenceofanyredoxreactiontakingvolume.Forpotentiometricdetection,thesignalisplaceattheelectrodes,conductometricmeasurecompletelyindependentofthesensorsizeand,mentsareusuallycarriedoutbyemployinga.c.therefore,ofthecellvolumeandcapillarydiameter.voltagesatafrequencyoftypically1kHz.IfhigherOpticalmethods,ontheotherhand,havetheadvanfrequenciesareused,itispossibletoemployelectageofprovidingcompleteelectricalisolationofthetrodesthatarenotincontactwiththesolution,butdetectorfromtheseparationvoltageappliedtotheattachedoutsidethesamplecell6.Theconductcapillary.anceofasolutionLisdependentontheelectrodeConductivitydetectioncanberegardedasaareaA,theirdistancel,theconcentrationcofuniversalmethod,whileamperometricdetectionisthechargecarriersandtheirmobilitylintherestrictedtoelectroactivespeciesandpotentiometricelectricfield,accordingtoEq.1detectionisnotpossibleforcertainsmallionswithmultiplecharges.VerylowdetectionlimitshavebeenAreportedforamperometricdetection.OpticalabsorpL5Olc1iilT.Kappes,P.C.Hauser/J.Chromatogr.A834199989–10191Themobilityofionsisafunctionoftheirsizemethoddidnotfindwideracceptance.ACconradiusofhydratedionandthenumberofcharges,ductivitydetectionwasintroducedbyEveraertsandincidentally,thisisthesamepropertythatisexVerheggen8.Intheearlystudies,relativelylargeploitedfortheelectrophoreticseparationoftheions.boreseparationchannelsandcapillarieswereusedConductometricmeasurementsarethereforenotandthedetectorelectrodeswereplacedaheadoftheselectiveand,asstandalonemethods,theseareterminalelectrolytechamberdirectlyintothesepalimitedtocircumstanceswheretheoverallsamplerationchannel.ThisarrangementisillustratedinFig.compositioniswellknown.Thefactthatallions1A.Thetwodetectorelectrodesareideallyarrangedgivearesponseinconductometryisontheotherdirectlyoppositeandperpendiculartotheseparationhandexactlywhatisrequiredfordetectioninachannelinordertoavoidsensinganypotentialseparationmethodforionsand,forthisreason,gradientinthechannel.WithcarefuldesignoftheconductometricdetectioniswidelyusedinionchroACdetectorelectronics,itshouldalsobepossibletomatography7.ThisfeatureontheotherhandalsodiscriminateagainsttheDCfieldorlowfrequencyleadstoaresponsetoanybackgroundion,suchasfluctuationsthereof.InafirstreportonconductivitythoserequiredasaneluentinionchromatographyordetectionwithmodernsilicacapillariesbyHuangetaspHandionicstrengthbufferincapillaryelectroal.12,twosmallholeswerelaserdrilledintothephoresisandthecounterionsofoppositechargetoseparationcapillaryforinsertingthetwodetectortheanalyteion.Forthelatterreason,theconductivityelectrodes.Simplifiedendcolumndetectorswereequationhastoincludethesummationterminalllaterintroduced13.Thislatterarrangementconcases.Ahighbackgroundconductivitymaycomsistsofanelectrodemountedinthewalljetarrangepromisethedetectionlimitfortheanalytespecies.mentdirectlyattheoutletofthecapillaryandaSocalledsuppresseddetectionmethodsarethereforesecondgroundelectrodelocatedatadistanceintheusedforionchromatographyinwhichthebackgroundionsareremovedfromthestreambeforedetectiontakesplace.Itisalsonoteworthythatthecellsdimensionsentertheequationviatheelectrodeareaanddistanceand,therefore,thecellvolumeaffectsthemeasuredsignal.2.2.ImplementationsInearlycapillaryzoneelectrophoresisandisotachophoresissystems,usewasmadeofsocalledpotentialgradientdetection8–11.Here,thesolutionpotentialinthedetectionregionbroughtaboutbytheapplicationoftheelectricfieldissensedwithasingleelectrodeorapairofinertelectrodes.Thevoltagedropovertheseparationcapillaryisnotuniformiftheconductivityvarieswithsamplezones.Asthisisafunctionoftheconductivity,thiscanbeviewedasaclevermeansofsensingthispropertywithouttheneedforapplyingameasuringsignal.Itmaybeexpected,however,thatsuchanarrangementismorepronetoinherentnoisethanthenormalACmodeofconductivitymeasurementstheFig.1.A.c.conductometricdetection.AAnearliersystemwithaddedcomplicationofneedingtoapplyasignaltwodetectorelectrodesDEsinline,electricallyindependentofshouldbeoffseteasilybytheinherentdiscriminationelectrophoreticgroundGND.BAlatersystemwithasingleagainstelectricalnoise.Perhapsforthisreasonthedetectorelectrodeusedagainstelectrophoreticground.92T.Kappes,P.C.Hauser/J.Chromatogr.A834199989–101instrumentwithelectrochemicaldetectionthatappearstobeavailablepresentlyincorporatessuchanendcolumnconductivitycell14.Detectionlimitsfornormalconductivitydetectionandnonstackingsampleinjectionarerelatively25high,beingtypically10mol/l.Thesuppresseddetectiontechniqueallowsthedetectionofcon27centrationsaslowas10mol/l15–18.BackgroundbufferionsareremovedbyusingweakacidsFig.2.Conductometricdetectionwithachemicalsuppressor.orbasesthatarerenderednonionicwhenincontactElectrophoreticgroundisinthecontainerwiththesuppressantwithanionexchangermembrane,deliveringprotonssolution.Theconductivitymaybemeasuredattheendoftheorhydroxideions.Toachievethisincapillarycolumnagainstthisgroundasshownorwithaseparatetwoelectrophoresiswithoutexcessivebandbroadening,aelectrodesystem.tubeoftheionexchangermaterialwithsimilardimensionsisattachedtotheseparationcapillarybuffercontainer,asillustratedinFig.1B.Here,theaheadofthedetectorcell,asillustratedinFig.2.conductivityismeasuredagainsttheelectrophoreticWhilelowerlevelscanbedetermined,theimground.Theconductivitysignalwilllargelyariseatplementationismorecomplicatedthannonsupthecapillaryoutletwherethedetectorelectrodeispressedconductivitydetectionandposeslimitationslocatedduetothemuchlargercrosssectionoftheonthechoiceofthebufferemployed.fluidaroundthecounterelectrode.ThisgeometryAdifferentapproachtoloweringthedetectionalsoleadstoanimmediatelossoftheelectricfieldlimitistheuseofsamplestackingmethods,andoutsidethecapillaryend.Theonlycommercialconcentrationsbelow1ppbhavebeendetermined14,19,20.AnelectropherogramforalowppbstandardmixtureisgiveninFig.3toillustratethistechnique.Samplestackingis,however,onlypossiblewithsamplesoflowionicstrengthandtheuseofinternalstandardizationmaybenecessarytoobtainadequateprecision.Acontactlessconductivitydetectorcellhasalsorecentlybeendescribed.Twotubularelectrodesareplacedoverthecapillaryandcouplingtothedetectionvolumeisachievedcapacitivelybyapplyingana.c.fieldof40kHz21.Theconstructionofthiscellisverysimpleandallowscombinationwithaseconddetector.Limitsofdetectionappeartobecomparabletonormalendcolumndetection.3.Amperometricdetection3.1.PrincipleAmperometricdetectionreliesonoxidationorreductionoftheanalytespeciesonaworkingelectrode.ThemethodisthereforenotasuniversalasFig.3.Electropherogramusingconductivitydetectionforananionconductivitydetection,asonlyelectroactivespeciesstandardmixtureoflowconcentrationsdeterminedbysamplestackingreproducedwithpermissionfromRef.14.areaccessible.Ontheotherhand,verylowdetection