JournalofChromatographyA,834(1999)89101ReviewElectrochemicaldetectionmethodsincapillaryelectrophoresisandapplicationstoinorganicspecies*ThomasKappes,PeterC.HauserUniversityofBasel,DepartmentofChemistry,Spitalstrasse51,CH-4056Basel,SwitzerlandAbstractThethreeelectrochemicaldetectionmethodsincapillaryelectrophoresis,namelyconductometry,amperometryandpotentiometry,arediscussedandcomparedtothemorecommonopticaldetectionmethods.Theprinciplesofeachmethodandtheirimplementationsaredetailedandreportedapplicationstoinorganicspeciesarereviewed.1999ElsevierScienceB.V.Allrightsreserved.Keywords:Electrochemicaldetection；Detection,electrophoresis；Reviews；Inorganicanions；MetalcationsContents1.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,amperometryorpoten-tiometryareanattractivealternative,which,how-Detectionincapillaryelectrophoresis(CE)isever,hasbeenexploredcomparativelylittle.Thisiscommonlycarriedoutusingopticalmeans(absorp-ininterestingcontrasttoion-chromatography,thetionandfluorescence).Electrochemicalmeansintheotherandoldergeneralmethodforiondetermi-nation,whichmainlyemploysconductivitydetection*Correspondingauthor.Fax:141-61-267-1013.andwhereopticalmethodsareseldomused.Perhaps,0021-9673/99/$seefrontmatter1999ElsevierScienceB.V.Allrightsreserved.PII:S0021-9673(98)00685-290T.Kappes,P.C.Hauser/J.Chromatogr.A834(1999)89101thisisevenmoresurprisingwhenoneconsidersthattionandfluorescencemeasurementsarealsore-opticaldetectionismademoredifficultincapillarystrictedtospeciesthatshowtherespectiveprop-electrophoresisbecauseofthesmallercellvolumeserties.Forthisreason,indirectopticalmethodsareandthefactthatmanyionscannotbedetectedoftenusedinwhichthedisplacementofasenseddirectlybyopticalmeans.Theexplanationforthisauxiliaryagentbytheanalytesismonitored(forcedsituationmaybetwofold.Firstly,itisfairlyeasytobytherequirementtohaveoverallchargeneutrality).adaptabsorptiondetectors,aswidelyusedinhigh-Thisapproachmayalsobeusedforelectrochemicalperformanceliquidchromatography(HPLC),fordetectionmethodswhentheanalytespeciescannotcapillaries.ManyCEinstrumentmanufacturersap-besenseddirectly.ChemicalderivatizationofthepeartohavegonethisrouteandhaveincorporatedanalyteinordertoimpartdetectabilityisanotherexistingdetectorsintotheirCEequipment.Secondly,possibleapproach.Neitherofthesemeansisideal,theappliedhighseparationvoltageisanintrinsichowever,asindirectdetectiononlyallowsanarrowinterferenceinelectrochemicaldetection.Intheearlydynamicconcentrationrangeandderivatizationaddsdays,elaborateschemeswereemployedtoovercometothecomplexityofthemethod.Inpractice,thethisdifficulty.Inrecentyears,ithas,however,choiceofthedetectionmethodshouldperhaps,inthebecomeevidentthat,withthecorrectdesignofthefirstinstance,exploitanintrinsicpropertyfordirectsystem,thisdoesnothavetobeaproblem.sensingand,secondly,bebasedontheachievableCommontoallthreevariantsofelectrochemicalandrequireddetectionlimits.Acompromisemaydetectionmeansisthefactthattheyareintrinsicallyhavetobefoundwhenseveralspeciesaretobesimplerthantheopticalmethods.Aermediatephysicalparameter,suchasradiationOtherreviewsonelectrochemicaldetectioninintensityinopticalmethods.Thedetectorhardwarecapillaryelectrophoresisareavailable15.consistsofthreeorfewersmallelectrodesandsomefairlysimpleelectroniccircuitry,whereasforopticaldetection,alightsource,monochromator,optical2.Conductometricdetectiondetectorsandfocussingopticsarenecessary.Inopticalmethods,thecellvolumedirectlyaffectsthe2.1.Principlesignalviatheopticalpathlengthand,forthisreason,thecapillarydiametersalwaysshouldbeaslargeasHeretheabilityofionstoconductchargeinpossible.Forelectrochemicaldetection,thecellsizesolutionisexploited.Thecurrentproducedbetween(samplevolume)hasonlyadirectbearingincon-twoelectrodeswhenapplyingavoltageismeasuredductivitymeasurements.Inamperometry,thesignaland,accordingtoOhmslaw,yieldstheresistanceorisrelatedtotheareaoftheworkingelectrode,thetheconductanceoftheelectrolytesolution.Inordersizeofwhichwillbelimitedbytheavailablesampletosuppresstheinfluenceofanyredoxreactiontakingvolume.Forpotentiometricdetection,thesignalisplaceattheelectrodes,conductometricmeasure-completelyindependentofthesensorsizeand,mentsareusuallycarriedoutbyemployinga.c.therefore,ofthecellvolumeandcapillarydiameter.voltagesatafrequencyoftypically1kHz.IfhigherOpticalmethods,ontheotherhand,havetheadvan-frequenciesareused,itispossibletoemployelec-tageofprovidingcompleteelectricalisolationofthetrodesthatarenotincontactwiththesolution,butdetectorfromtheseparationvoltageappliedtotheattachedoutsidethesamplecell6.Theconduct-capillary.anceofasolution(L)isdependentontheelectrodeConductivitydetectioncanberegardedasaarea(A),theirdistance(l),theconcentration(c)ofuniversalmethod,whileamperometricdetectionisthechargecarriersandtheirmobility(l)intherestrictedtoelectroactivespeciesandpotentiometricelectricfield,accordingtoEq.(1):detectionisnotpossibleforcertainsmallionswithmultiplecharges.VerylowdetectionlimitshavebeenAreportedforamperometricdetection.Opticalabsorp-L5Olc(1)iilT.Kappes,P.C.Hauser/J.Chromatogr.A834(1999)8910191Themobilityofionsisafunctionoftheirsizemethoddidnotfindwideracceptance.ACcon-(radiusofhydratedion)andthenumberofcharges,ductivitydetectionwasintroducedbyEveraertsandincidentally,thisisthesamepropertythatisex-Verheggen8.Intheearlystudies,relativelylargeploitedfortheelectrophoreticseparationoftheions.boreseparationchannelsandcapillarieswereusedConductometricmeasurementsarethereforenotandthedetectorelectrodeswereplacedaheadoftheselectiveand,asstand-alonemethods,theseareterminalelectrolytechamberdirectlyintothesepa-limitedtocircumstanceswheretheoverallsamplerationchannel.ThisarrangementisillustratedinFpositioniswellknown.Thefactthatallions1A.Thetwodetectorelectrodesareideallyarrangedgivearesponseinconductometryisontheotherdirectlyoppositeandperpendiculartotheseparationhandexactlywhatisrequiredfordetectioninachannelinordertoavoidsensinganypotentialseparationmethodforionsand,forthisreason,gradientinthechannel.Withcarefuldesignoftheconductometricdetectioniswidelyusedinion-chro-ACdetectorelectronics,itshouldalsobepossibletomatography7.ThisfeatureontheotherhandalsodiscriminateagainsttheDCfieldorlowfrequencyleadstoaresponsetoanybackgroundion,suchasfluctuationsthereof.Inafirstreportonconductivitythoserequiredasaneluentinion-chromatographyordetectionwithmodernsilicacapillariesbyHuangetaspH-andionicstrengthbufferincapillaryelectro-al.12,twosmallholeswerelaserdrilledintothephoresisandthecounterions(ofoppositecharge)toseparationcapillaryforinsertingthetwodetectortheanalyteion.Forthelatterreason,theconductivityelectrodes.Simplifiedend-columndetectorswereequationhastoincludethesummationterminalllaterintroduced13.Thislatterarrangementcon-cases.Ahighbackgroundconductivitymaycom-sistsofanelectrodemountedinthewall-jetarrange-promisethedetectionlimitfortheanalytespecies.mentdirectlyattheoutletofthecapillaryandaSo-calledsuppresseddetectionmethodsarethereforesecondgroundelectrodelocatedatadistanceintheusedforion-chromatographyinwhichtheback-groundionsareremovedfromthestreambeforedetectiontakesplace.Itisalsonoteworthythatthecellsdimensionsentertheequationviatheelectrodeareaanddistanceand,therefore,thecellvolumeaffectsthemeasuredsignal.2.2.ImplementationsInearlycapillaryzoneelectrophoresisandiso-tachophoresissystems,usewasmadeofso-calledpotentialgradientdetection811.Here,thesolu-tionpotentialinthedetectionregionbroughtaboutbytheapplicationoftheelectricfieldissensedwithasingleelectrodeorapairofinertelectrodes.Thevoltagedropovertheseparationcapillaryisnotuniformiftheconductivityvaries(withsamplezones).Asthisisafunctionoftheconductivity,thiscanbeviewedasaclevermeansofsensingthispropertywithouttheneedforapplyingameasuringsignal.Itmaybeexpected,however,thatsuchanarrangementismorepronetoinherentnoisethanthenormalACmodeofconductivitymeasurements(theFig.1.A.c.conductometricdetection.(A)Anearliersystemwithaddedcomplicationofneedingtoapplyasignaltwodetectorelectrodes(DEs)in-line,electricallyindependentofshouldbeoffseteasilybytheinherentdiscriminationelectrophoreticground(GND).(B)Alatersystemwithasingleagainstelectricalnoise).Perhapsforthisreasonthedetectorelectrodeusedagainstelectrophoreticground.92T.Kappes,P.C.Hauser/J.Chromatogr.A834(1999)89101instrumentwithelectrochemicaldetectionthatap-pearstobeavailablepresentlyincorporatessuchanend-columnconductivitycell14.Detectionlimitsfornormalconductivitydetectionandnon-stackingsampleinjectionarerelatively25high,beingtypically10mol/l.Thesuppresseddetectiontechniqueallowsthedetectionofcon-27centrationsaslowas10mol/l1518.Back-groundbufferionsareremovedbyusingweakacidsFig.2.Conductometricdetectionwithachemicalsuppressor.orbasesthatarerenderednon-ionicwhenincontactElectrophoreticgroundisinthecontainerwiththesuppressantwithanion-exchangermembrane,deliveringprotonssolution.Theconductivitymaybemeasuredattheendoftheorhydroxideions.Toachievethisincapillarycolumnagainstthisground(asshown)orwithaseparatetwo-electrophoresiswithoutexcessivebandbroadening,aelectrodesystem.tubeoftheion-exchangermaterialwithsimilardimensionsisattachedtotheseparationcapillarybuffercontainer,asillustratedinFig.1B.Here,theaheadofthedetectorcell,asillustratedinFig.2.conductivityismeasuredagainsttheelectrophoreticWhilelowerlevelscanbedetermined,theim-ground.Theconductivitysignalwilllargelyariseatplementationismorecomplicatedthannon-sup-thecapillaryoutletwherethedetectorelectrodeispressedconductivitydetectionandposeslimitationslocatedduetothemuchlargercross-sectionoftheonthechoiceofthebufferemployed.fluidaroundthecounterelectrode.ThisgeometryAdifferentapproachtoloweringthedetectionalsoleadstoanimmediatelossoftheele