Liquid-phase microextraction–gas chromatography–mass spectrometry for the determination

JournalofChromatographyA,1148(2007)

145–151

Liquid-phasemicroextraction–gaschromatography–massspectrometryforthedeterminationofbromate,iodate,

bromideandiodideinhigh-chloridematrix

KishanReddy-Noone,ArchanaJain,KrishnaK.Verma

DepartmentofChemistry,RaniDurgavatiUniversity,Jabalpur482001,MadhyaPradesh,IndiaReceived24December2006;receivedinrevisedform5March2007;accepted7March2007

Availableonline16March2007

Abstract

Inthedeterminationofbromateandiodate,anyfreebromideandiodidepresentwasquantitativelyremovedbyanionexchangewithsilverchlorideexploitingthedifferencesinsilversaltssolubilityproduct,beingAgCl,1.8×10 10,AgBr,5.0×10 13,AgI,8.3×10 17,AgBrO3,5.5×10 5andAgIO3,3.1×10 8.Theoxyhalideswerereducedwithascorbicacidtohalidesandconvertedto4-bromo-2,6-dimethylanilineand4-iodo-2,6-dimethylanilinebytheirreactionwith2-iodosobenzoateinthepresenceof2,6-dimethylanilineatpH6.4and2–3,respectively.Singledropmicroextraction(SDME)ofthehaloanilinesin2 loftolueneandinjectionofthewholeextractintoGC–MS,orliquid-phasemicroextraction(LPME)into50 loftolueneandinjectionof2 lofextract,resultedinasensitivemethodforbromateandiodate.Thelattermethodofextractionhasbeenfoundmorerobust,sensitiveandtogivebetterextractioninshorterperiodthanSDME.Totalbromine/iodinewasdeterminedwithoutanytreatmentwithsilverchloride.Highconcentrationofchlorideinthematrixdidnotinterfere.Arectilinearcalibrationgraphwasobtainedfor0.05 g–25mgl 1ofbromate/bromideandiodate/iodide,thelimitofdetectionwere20ngl 1ofbromate,15ngl 1ofiodate,20ngl 1ofbromideand10ngl 1ofiodide(byLPMEin50 loftoluene).Themethodhasbeenappliedtoseawaterandtablesalt.Fromthepooleddata,theaveragerecoveryofspikedoxyhalide/halidetorealsampleswasinrange96.7–105.7%withRSDinrange1.6–6.5%.

©2007PublishedbyElsevierB.V.

Keywords:Bromate;Iodate;Iodide;Bromide;High-chloridematrix;Liquid-phasemicroextraction;Singledropmicroextraction;Gaschromatography–massspectrometry

1.Introduction

Ontheoccurrenceofhighamountsinenvironmentalsamples,bromiderepresentsahazardtopublichealth[1,2].Long-termelevateddietarylevelsofbromideexertchangesonthestatusofthethyroidgland,witheventualsubstitutionofiodidebybro-mideduringbiosynthesisofthyroidhormones[3,4].Brominatedandiodinatedcompoundsareproducedduringthedisinfectionofwaterbychlorineinthepresenceofbromideandiodide[5,6].Toxicologicalstudiessuggestthatbrominatedandiod-inatedcompoundsmaybemorecarcinogenicthanchlorinatedanalogues[7].Ozone,usedasanalternativedisinfectanttochlo-rine,generatesbromateasaby-productinwaterscontainingthe

Correspondingauthor.Tel.:+917612678103;fax:+917612403252.E-mailaddress:arichna@sancharnet.in(K.K.Verma).

precursorbromide.Bromatehasbeenclassi edasagroup2Bprobablehumancarcinogen[8].In1993,theWHOsetalimitof25 gl 1forbromateindrinkingwater[9].Despitethehealthrisks,thisguidelinewasbasedonthelimitationsofthemeasure-menttechniquesforbromate.TheUSEnvironmentalProtectionAgency(EPA)setamaximumcontaminantlevelof10 gl 1bromateunderStage1Disinfectants/DisinfectionByproductsRulein1998[10].TheEUreducedtheirregulatoryvaluefrom50to10 gl 1bromateindrinkingwater[11]andtheWHOalsosetaguidelineof10 gl 1[12].Acancerriskof1in105hasbeenassessedat0.5 gl 1bromatebasedoningestionofdrinkingwater[13].

Ionchromatographyhasbeentraditionallyusedfordeter-miningbromate,otheroxyhalidesandhalidesusingacarbonateeluentandsuppressedconductivitydetection[14].Ahydroxideeluenthasbeenfoundtohavelowersup-pressedbackgroundconductivity[15].Preconcentrationbefore

0021-9673/$–seefrontmatter©2007PublishedbyElsevierB.V.doi:10.1016/j.chroma.2007.03.027

146K.Reddy-Nooneetal./J.Chromatogr.A1148(2007)145–151

ionchromatography[16],andpostcolumnreactionwitho-dianisidine[17,18],chlorpromazine[19],reducedfuchsine[20]oriodide-heptamolybdate[21]andphotometricdetectionhavealsobeenproposedtolowerthelimitofdetectionofbromateandtoavoidmatrixinterference.Chlorideconcentrationdrasticallyin uencesthebromatepeakshapeinionchromatographyduetothechloridecompetitiononcolumnactivesite[22,23].Chlo-ridealsosuppressesbromateresponseinmassspectrometricdetection[23].

Forcapillaryelectrophoresis,theseparationcapacityofthesystemrequiresoptimizationsinceelectromigrationdispersionofhighlyconcentratedchloridepeakimpairstheresolutionofhalides[24].Highlevelsofchloridealsoaffecttheionchro-matographyofiodide[25,26]andbromide[27],andnitrateelutesclosetobromide.Detectionsuchasbyconductiv-ity,amperometryorpotentiometryhaveinadequatesensitivity[28].Postcolumnreactionandspectrophotometry[29]orcon-versionintoamoredetectableorganicderivativearetwowaysusedtoovercomethisproblemforiodide[30–33]andbromide[34,35].Inourexperience,manyofthesederivati-zationreagentshaveinnatelimitations,e.g.,acetanilide[34]andethylacetate[35]arenotapplicabletoiodide,N,N-dimethylanilinehasvariablestoichiometrywhenbromideisdetermined[32,33],phenylboronicacid–mercuricnitrate[36]isunsuitablewhenlargeexcessofchlorideispresent,andreactionwith2,6-dimethylphenolisslow[30,31].Thesecon-siderationsareimportantwhenbothbromideandiodide,orbromateandiodate,aretobedetermined.Wehavefoundinthisworkthat2,6-dimethylanilinecanbeusedintherapid,sensitiveandsimultaneousdeterminationofbromate/bromideandiodate/iodide.

Theinherentadvantagesinderivatizationofiodideandbro-midetocorrespondinghalo-organicsandtheirGCledustoapplythistechniquetothesensitivedeterminationofbro-mateandiodateaftertheirreductiontohalides.However,themajorprobleminthisapproachisowingtoanybromideandiodidethatmayalreadybepresentinthesample,anditmakesnecessaryapre-separationofbromideandiodidebeforereductionofoxyhalides.Differencesinthesolubilityproduct(Ksp)ofsilversaltsofhalidesandoxyhalideswereexploitedtosolvethisproblem.Reactionwithsparinglysol-ublesilverchloridehasbeenusedtogivemoreinsoluble(lowerKsp)silverbromideandiodide,whichwerethuspre-cipitated,whileoxyhalidesremainedunaffectedinsolutionsincetheirsilversaltsaremoresoluble(higherKsp)thansilverchloride.

Itwasalsonecessarythatthederivativesshouldbeamenabletocleanupandenrichmentbyoneofsamplepreparationmeth-ods,forexampleliquid-phasemicroextraction(LPME)asusedinthiswork.4-Halo-2,6-dimethylanilinerespondedfavorablywelltomicroextractioninanumberoforganicsolvents.Thus,acombinationofanionexchangewithsilverchloride,derivatiza-tiontohaloanilines,LPMEofthederivativesandtheirGC–MShasresultedintoasensitivemethodforthe gl 1 1determi-nationofbromatewithlimitofdetectionatngllevel.Itwasalsopossibletousethemethodforiodate,bromideandiodide.

2.Experimental

2.1.EquipmentandGCconditions

TheGC–MSinstrumentationusedconsistedofAgilentG1800BGCDsystem(HP5890seriesIIwithaquadrupolemassdetector)withaHP-5(5%phenylsubstitutedmethylpolysilox-ane)30m×0.25mm; lmthickness0.25 m,capillarycolumn.Helium(99.999%)ascarriergasata owrateof1mlmin 1wasused.Theinjectortemperaturewasmaintainedat250 Candallinjectionsweremadeinsplitlessmode.TheGCoventemper-ature washeldat90 Cfor3min,programmedto220 Cat20Cmin 1andheldforfurther2min.TheGC–MStransferlinewasmaintainedat300 C,electronionizationat70eVandthemassspectrumscannedfromm/z45–450.ChromatographicdatawereacquiredusingaHPChemstationsoftwareG1074BversionA.01.00.TheGC–FIDanalysiswasperformedusingHewlett-Packard5890seriesIIgaschromatograph(AgilentTechnologiesInc.,PaloAlto,CA).Theseparationwereper-formedonaHP-5(5%phenylsubstitutedmethylpolysiloxane),30m×0.32mm;0.25 m lmthickness,capillarycolumn.Nitrogen (99.999%)1wasusedascarriergasata owrateof3mlmin.Extractionvials,4ml,withPTFEsiliconeseptumandscrewcapwithahole(Supelco,Bellefonte,PA,USA),8mm×1.5mmmagneticstirbars,amagneticstirrerandanin-housemadesyringestandwereused.LiChrolutENsolid-phaseextractioncartridges(2.8ml)containing200mgofthesorbentwereobtainedfromMerck,Darmstadt,Germany.Detectionatfullscanmodefromm/z45–450wascarriedouttoidentifythepeaks.Selectedionmonitoring(SIM)wasdoneforionsm/z199,120forbromate/bromide,m/z247,120foriodate/iodide,andm/z329,250fortheinternalstandard.2.2.Reagentsandstandards

AllaqueoussolutionswerepreparedinHPLCgradewater(Millipore-India,Mumbai,India).Sodium2-iodosobenzoatereagentwasmadebystirring400mgofthefreeacid(Sigma,St.Louis,MO,USA)withaslightmolarexcessofsodiumhydrox-ide(7.6mlof0.2Msodiumhydroxide)anddilutingto100mlwithwaterinastandard ask.Itwas lteredthrougha0.45 mmembrane merciallyavailable30%solutionofhydrogenperoxide(RanKem,NewDelhi,India)wasused.Asolutionof100 lof2,6-dimethylaniline(Aldrich,Mil-waukee,WI,USA)in50mlmethanolwasusedashalogenscavenger.Thephosphatebuffercontained4geachofNaH2PO4andNa2HPO4(Qualigens,Mumbai,India)in100mlofwater,andwasadjustedtopH6.4.Ascorbicacid(Qualigens,Mumbai,India)solutionwasmadebydissolving50mgofthereagentin100mlof1Maceticacid.GCgradecyclohexane,hexane,isooc-taneandtoluenewereobtainedfromMerck,Mumbai,India.Theinternalstandard2,4,6-tribromoaniline(Aldrich,Milwaukee,WI,USA),100mg,wasdissolvedin100mlofmethanol.Ana-lyticalreagentgradealkalioxyhalidesandhalides(Qualigens,Mumbai,India)wereovendriedandtheirseparatesolutions,1000mgl 1each,wereprepared.Lessconcentratedsolutions

K.Reddy-Nooneetal./J.Chromatogr.A1148(2007)145–151147

weremadebysequentialdilutionwithdeionizedwaterandstoredinacoolplace.

2.3.Determinationofiodideandbromide

2.3.1.MethodinvolvingSDME

A0.5–2mlaliquotofthecombinedstandardsolutioncontain-ing0.05 g–25mgofiodide/bromidewasmixedina4mlvialwith250 leachofphosphatebufferandof2,6-dimethylaniline,and500 lof2-iodosobenzoate.Thesolutionwasmixedwellandkeptfor10minatambienttemperature(26 C)foriodina-tion.Thereafter,250 lof1Msulfuricacidwasaddedandthesolutionmixedwellandkeptforabout1minforbromination.Thesolutionwasmixedwith500 lof1Msodiumhydroxide,20 loftheinternalstandardanddilutedto4mlwithdeionizedwater.Theneedleofa5 lHamiltonsyringe, lledwith2 loftoluene,waspenetratedthroughtheseptumofthevialuntilthetipprotruded1cmbelowthemeniscusofthesolution.Theplungerwasdepressedtocausethesolventtoformadropsus-pendedatthetip.After15min(stirrate300rpm),thedropwasdrawnbackintothesyringeandimmediatelytransferredintotheGCinjectionportforanalysis.

2.3.2.MethodinvolvingLPME

A0.5–2mlaliquotofthecombinedstandardsolutioncon-taining0.05 g–25mgofiodide/bromidewasmixedina5mlstandard askhavingagroundglassstopperwith250 leachofphosphatebufferandof2,6-dimethylaniline,and500 lof2-iodosobenzoate.Thesolutionwasmixedwellandkeptfor10minatambienttemperature(26 C)foriodination.There-after,250 lof1Msulfuricacidwasaddedandthesolutionmixedwellandkeptforabout1minforbromination.Thesolu-tionwasmixedwith500 lof1Msodiumhydroxide,20 loftheinternalstandardanddilutedto5mlwithdeionizedwater.A50- lportionoftoluenewasaddedtothe askandshakenvigorouslyforabout1min.The askwaskeptundisturbedforthetoluenedropletstocoalesceandsettleasaseparatelayeronthetopoftheaqueousphase.A2 lportionoforganicphasewascarefullywithdrawnbya5 lHamiltonsyringeandinjectedintotheGC.

2.4.Determinationofiodideandbromideintablesaltandseawater

Forthedeterminationoftotaliodide/bromide,a25gpor-tionofhomogenizedtablesaltwasdissolvedinabout80mlofwater,mixedwith500 lofascorbicacid,keptfor1–2minand nallydilutedto100mlinastandard ask.A10mlaliquotofseawaterwasmixedwith500 lofascorbicacidand1mlof0.5%EDTA(disodiumsalt),keptfor1–2minanddilutedto25mlwithdeionizedwaterinastandard ask.TwomillilitersportionofeachsamplesolutionwaspassedthroughLiChrolutENcartridgethathadpreviouslybeenactivatedwith2mlofmethanolandequilibratedwith2mlofHPLCgradewater.Theeluatewasmixedwith500 lofhydrogenperox-ideandusedforiodide/bromidedeterminationasdescribedabove.

Forfreeiodidedetermination,a20mlaliquotofseawaterwasmixedwith1mlof0.5%EDTA(disodiumsalt)anddilutedto25mlinastandard ask.A2mlaliquotofthissolutionwaspassedthroughLiChrolutENcartridgeandtheeluatewassubjectedtoanalysisasdescribedabove.2.5.Determinationofbromateandiodate

A0.5–2mlaliquotofbromateandiodate(thetotalvolumeadjustedto2mlwithdeionizedwater,ifnecessary)wasmixedina5mlcentrifugetubewithabout50mgofpowderedsilverchloride,stirredforabout2minandcentrifugedat3000rpm.A500 lportionoftheclearsupernatantsolutionwasmixedina4mlvial(forSDME)ora5mlstandard ask(forLPME)with500 lofascorbicacid,swirledfor1minforcompletereduc-tionofbromate/iodate.Itwasthenmixedwith250 leachofphosphatebufferandof2,6-dimethylaniline,500 lofhydro-genperoxideand500 lof2-iodosobenzoate.Thesolutionwasmixedwellandkeptfor10minforiodination.Thereafter,250 lof1Msulfuricacidwasaddedandthesolutionwasagainmixedwellandkeptforabout1minforbromination.Thesolutionwasmixedwith500 lof1Msodiumhydroxideand20 loftheinternalstandard,dilutedtovolume(4mlor5ml,respectively)withwater,andsubjectedtoextractionandGCasdescribedabove.

3.Resultsanddiscussion3.1.TheChemistry

Thedeterminationofiodideandbromideisbasedonasequenceofprecolumnreactionsinvolvingtheiroxidationwith2-iodosobenzoate,andelectrophilicsubstitutionreactionof2,6-dimethylanilinetoyield4-halo-2,6-dimethylaniline.Oxidationoftwohalideionsshouldnotoccursimultaneouslynorbro-midebeoxidized rstasinthesesituationsiodide/iodinecanundergoasecondaryreactionwithbrominetoformiodatewhichisincapableofformingiodo-phenolderivative.Thestan-dardreductionpotential(E )of2-iodosobenzoateat25 Cwasfoundtobe1.21VatpH1,1.08VatpH2,0.53VatpH4and0.48VatpH7[37].TheE foriodineandbrominehavebeenreportedas0.53and1.06V,respectively[38].Thus,inthe rststepofoxidation,carriedoutatpH6.4,onlyiodidewasoxidizedtoiodinewhichonreactionwith2,6-dimehylanilineformed4-iodo-2,6-dimethylaniline.Inthesecondstep,whentheoxidationwascarriedoutatpH2–3,brominewasproducedwhichformed4-bromo-2,6-dimethylaniline(Fig.1).Anotherimportantconsiderationwasthatofiodination/brominationof2,6-dimethylaniline.IodinationwasoptimumatpH6.4(com-pletingwithin10min)whileitwastooslowoverthepHrange1–4(over20minrequiredforcompletion).BrominationonthecontrarywasveryrapidoverthepHrange1–3(complet-ingwithin1min).Asthereductionpotentialofchlorineis1.39V,chloridedidnotparticipateintheredoxreactionwith2-iodosobenzoate.Itwasnecessarytoreduceiodateandbro-matetothecorrespondinghalideswithascorbicacidsinceoxyhalides,assuch,donotundergoelectrophilichalogenation.

148K.Reddy-Nooneetal./J.Chromatogr.A1148(2007)

145–151

Fig.1.Thederivatizationreactionsusedforiodideandbromide.

2,6-Dimethylanilinewasconsideredasahalogenscavengerofchoicesince,contrarytootherreagents[32–34],itrespondedforbothiodinationandbrominationreaction,andthatonlyoneisomerofeachhaloanilinewasformed.ThehaloanilineswereamenableforSDME/LPMEduringpreconcentrationandcleanupstepofanalysis.

Determinationofbromateandiodateisbasedontheirreductiontohalidesbyascorbicacid.Beforereduction,anyfreebromideandiodidepresentinthesamplewasquantitativelyremovedbyanionexchangewithAgCl,areactionthatisbasedondifferencesinsolubilityproduct(Ksp)ofsilversalts,beingAgCl,1.8×10 10,AgBr,5.0×10 13,AgI,8.3×10 17,AgBrO3,5.5×10 5,andAgIO3,3.1×10 8[39].BothAgBrandAgIhaveKsplowerthanthatofAgClandthusbromideandiodideareprecipitatedastheirsilversaltsontreatmentwithAgCl.BromateandiodateremaininsolutionastheirsilversaltshaveKsphigherthanthatofAgCl.InsolutionsofhalidestreatedwithAgCl,atequilibriumAgL=Ag+the[Ag++L ]=√,1.8×10 10=1.34×10 5(AgL=AgCl);[Br ]=√ 13=7.07×10 7(AgL=AgBr);and[I ]=√5.0×108.3×10 17=9.0×10 9(AgL=AgI)(allvaluescal-culatedwithoutinvolvingcouplingofequilbria).Thus,[Ag+][Br ]=9.47×10 12and[Ag+][I ]=1.21×10 13,bothvaluesarehigherthantheKspofAgBrandAgI,respectively,andunderthisconditionbromideandiodidearecompletelyprecipitatedupontreatmentwithAgCl.Whenasolutionof10mgl 1eachofbromideandiodidewastreatedwithAgClandthesupernatantsolutionwassubjectedtohalidesdeter-minationbytheproposedmethod,nopeakforhalo-organiccompoundswasobtained(Fig.2).At[Ag+]=1.34×10 5(fromAgCl),precipitationofAgBrO3required[BrO3 ]=K 5spAgBrO3/[Ag+]=5.5×10 5/1.34×10=4.10moll 1 ;

andprecipitationofAgIO3required[IO3]=K+spAgIO3/[Ag]=3.1×10 8/1.34×10 5=2.31×10 3moll 1.BothconditionsareunrealisticwithpracticalsamplesandthusthereisnopossibilityofAgBrO3orAgIO3precipitationontreatmentwithAgCl.Thiswascon rmedbyexperimentsinwhichaliquotsofstandardscontaining10mgl 1eachofbromateandiodateweretreatedwithAgCl,andoxyhalidesweredeterminedinthesupernatantsolutionbythepresentmethod;therecoveryofbromateandiodatewas

99.6–100.5%.

Fig.2.GC–MSof10mgl 1eachofbromideandiodideafterderivatizationto4-bromo-2,6-dimethylanilineand4-iodo-2,6-dimethylaniline(uppertrace),andaftertreatmentwithAgClandtheirdeterminationinthesupernatantsolution(lowertrace).LPMEin50 loftoluene.IS,theinternalstandard2,4,6-tribromoaniline.Inset,magni edbaselineoflowertrace.

3.2.Extractionofhaloanilines

SDME[40,41]usestypically1–3 loforganicsolventandcombinessamplingandpreconcentrationinthesamestep,andsincethewholevolumeofextractedanalyte(s)istransferredtothechromatograph,thesensitivityattainedissimilartosolid-phasemicroextraction(SPME).Asalwaysafreshdropofsolventisusedforextractionthereisnochanceofanalytecarry-overinSDME,whichisacommonlimitationofSPME.ThereareseveralparameterswhichcontroltheoptimumperformanceofSDMEandincludethenatureofsolvent,dropsizeofsolvent,extractiontime,stirring,ionicstrengthofsolution,etc.Theseparameterswereseparatelyevaluatedtooptimizetheextraction.4-Iodo-2,6-dimethylaniline,4-bromo-2,6-dimethylanilineandtheinternalstandard2,4,6-tribromoanilineexistmostlyinunprotonatedforminthe nalsolutionofpH7.5–8.SDMEofhaloanilinesformedinthedeterminationof1mgl 1eachofiodideandbromidewascarriedoutwith1 lofhexane,cyclohexane,isooctaneandtoluene,theoptimumextractionwasobtainedwithtolueneina15minequilibrationtime(Fig.3).Theextractionalsoincreasedwithincreasingdropvolumeinrange1–4 l;a2 ldropoftoluenewasusedinsubsequentexperimentssinceitgavesuf cientsensitivityandwasstableduringextraction.Theextractionreachedoptimumvaluein15min(rangestudied1–25min).Highsaltconcentrationaffectstheextractionef ciencyandforthisitisnecessarytodilutethesamples.Thisbehaviourhasbeenreportedearlier[42,43].Saltinwaterperhapschangedthenatureof

Nernst

Fig.3.Effectofsolvent,1 ldrop,ontheSDMEof1mgl 1ofiodideandbromideafterderivatizationto4-halo-2,6-dimethylaniline.Equilibrationtime,15min.

K.Reddy-Nooneetal./J.Chromatogr.A1148(2007)145–151

149

Fig.4.SDME–SIMforthedeterminationoftotalbromideandiodide(0.5mgl 1each)(lowertrace),andbromate(0.1mgl 1)andiodate(0.08mgl 1)aftertreatmentofstandardwithAgCl(uppertrace)inastandardmixtureofhalidesandoxyhalides.Halideswerederivatizedto4-bromo-2,6-dimethylaniline(retentiontime8.13min)and4-iodo-2,6-dimethylaniline(retentiontime8.91min)andSDME.Theinternalstandard2,4,6-tribromoaniline(retentiontime9.51min).Inset,massspectraofpeakat8.13minandat8.91min.

diffusion lmaroundthedropmakingitsuccessivelymorepolarwithincreasingionicstrengththatreducedtherateofdiffusionoftargetorganicmolecules.

LPMEwithorganicsolventinthestandard askandwith-drawalof lportionofextractsettledinthe askabovethemeniscusmarkforanalysisbyGChaspreviouslybeenusedbyusasaconvenientmethodofsamplepreparation[32,44].ToluenewasusedasextractionsolventsinceitgaveoptimumextractionincomparisontoothersolventsaswereusedinSDME.Withconventional5mlstandard ask,downto50 lofsolventcouldbeusedconveniently.Contrarytothelattermethod,andsimilartoclassicalsolventextraction,analytemasstransfertoorganicphaseisfavoredbyhighsaltconcentrationinthismethod.3.3.Validation

Thechromatogramobtainedforiodideandbromideintablesalt,afterderivatizationandSDME,isshowninFig.4(lowertrace);theexcess2,6-dimethylanilinefromreactionwaselutedwithin5min(notshowninthechromatogram).Insetsarethemassspectracorrespondingtohalo-derivativepeaks.Theelec-tronionization(70eV)massspectrumcontainedmolecularionasthebasepeakalongwithdiagnosticfragmentions.Thechro-matographicpeaksforderivatizediodideandbromidewereidenti edbycomparisonofretentiontimeandmassspectraoftheauthenticsubstances.Thequantitativenatureofderivatiza-tionreactionwascon rmedbyGCwhenequalmolarmassesofauthentic4-iodo-and4-bromo-2,6-dimethylaniline,andofiodideandbromide,afterderivatization,ontheirseparatechro-matographyproducedpeakareas,relativetothatoftheinternalstandard,within3–5.5%.Theaverageconversionwas97.4%(RSD4%).Laboratorysamplesoftablesaltwerepreparedbyaddingknownamountsofiodideandbromidetosodiumchloridethatwasfoundtobefreefromanyiodideandbro-mide,andmixingthoroughly.Agreementbetweenthestandardadditionstotablesaltandseawater,andthemassesofiodideandbromidefoundbythepresentmethodservedtovalidate

thenewmethod.Theaveragerecoveryofspikeswas99.3%(range97.2–102.3%)withRSDof3.7%(range2.7–5.1%)foriodide,and99.6%(range97.4–103.2%)withRSDof2.5%(range1.6–4.2%)forbromide.TheresultsobtainedaregiveninTable1.Theaveragerecoveryofspikediodide/iodateandbro-mide/bromatetoseawaterwas99.7%(range96.8–105.7%)and99.2%(range96.7–105.2%),respectively.TheresultsobtainedaregiveninTable2.Chromatogramsobtainedintheanalysisofiodide/iodateandbromide/bromateintheirstandardsolutioninvolvingtheuseofAgClforseparationofhalidesisgiveninFig.4.Therecoveryofiodate(at0.08mgl 1level)andbromate(at0.1mgl 1level)was98.6%and97.4%,respectively.3.4.Calibrationgraphanddetectionlimits

Rectilinearcalibrationgraphs(atsixteenconcentrationlev-els;on2mlsamplesize)wereobtainedfor0.05 g–25mgl 1ofbromate/bromideandiodate/iodidebyderivatizationandGC–MS;thecorrelationcoef cient(r2)andlimitofdetection(LOD,S/N=3)were0.9986and20ngl 1ofbromate,15ngl 1ofiodate,20ngl 1ofbromideand10ngl 1ofiodide(byLPMEin50 loftoluene).ThecorrespondingvaluesusingSDMEwere0.9996and65,50,70and45ngl 1.Inprecisionstud-ies,sixaliquotsofsampleswereseparatelyderivatizedandoneinjectioneachwasmade.TheRSDinthedeterminationof0.02,0.1and10mgl 1ofiodideandbromidestandardsbythepresentmethodwas3.1%,3.0%and2.8%,respectivelyforiodide,and4.2%,3.0%and2.6%,respectivelyforbromide(byLPMEin50 loftoluene).TheRSDinthedeterminationof0.5 gl 1ofbromateandiodatewasrespectively,1.2%and1.8%byLPMEin50 loftoluene,and7.5%and6.7%bySDMEin2 loftoluene.Thus,LPMEwasmoresensitiveandprecisemethodthanSDME.LPME–GC–FIDcouldalsobeusedforthedetermi-nationof2 g–25mgl 1ofbromate/bromideandiodate/iodide;theaveragecorrelationcoef cient(r2)andLODwere0.9995and100ngl 1ofbromate,75ngl 1ofiodate,90ngl 1ofbro-mideand60ngl 1ofiodide.GC–FIDforthedeterminationof5 gl 1ofbromateisgiveninFig.5.

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Table1

DeterminationofiodideandbromideinthechloridematrixafterSDMEofderivatives(allresultsaretheaverageof vedeterminations)

Iodide(mg/100g)

LaboratorymadetablesaltsamplesSampleno.1cSampleno.2dSampleno.3eCommercialtablesaltsamplesRawsalt(India)

Sampleno.1(Spain)Sampleno.2(Spain)Sampleno.3(Spain)Sampleno.4(Tata)

Sampleno.5(NatureFresh)Sampleno.6(Star555)

Sampleno.7(CaptainCook)Sampleno.8(NirmaShudh)Sampleno.9(Knorr)Othersamples

Potassiumchloride(ThomasBaker)ConcentratedHCl(Merck)

abcdef

Recovery(%)ofspikea2.53.54.298.299.0101.697.598.497.2101.3102.098.198.297.5102.3

RSD(%)

Bromide(mg/100g)3.12.73.58.5830.1317.1828.8818.6613.2027.8030.6528.0824.7116.230.41

Recovery(%)ofspikeb

RSD(%)

1.964.869.870.490.045.075.862.622.183.522.602.832.900.44<LODf

5.1510.6015.363.52.73.54.03.63.93.13.74.23.73.85.1

99.0101.5102.198.597.498.997.998.298.6103.2101.498.5

1.82.52.91.61.82.02.23.62.81.92.44.2

2mgofiodide/100goftablesaltwasspiked.5mgofbromide/100goftablesaltwasspiked.

Thesamplecontained2mgofiodideand5mgofbromidein100gofsodiumchloride.Thesamplecontained5mgofiodideand10mgofbromidein100gofsodiumchloride.Thesamplecontained10mgofiodideand15mgofbromidein100gofsodiumchloride.Belowthelimitofdetection.

3.5.Analysisofrealsamples

Thepresentmethodhasbeenappliedtoseawaterandtablesaltsamplestodetermineiodide/iodateandbro-mide/bromate.Typicalchromatogramswereobtainedforanalysisofiodide/iodateandbromideinseawaterinvolv-ingsamplepreparationbySDME.Bromatewasnotfoundinseawater;possiblyitwasbelowtheLODinthesam-pleanalyzedbythepresentmethod.Similarobservationhasbeenmadebyusingarecentlyreportedmethodbasedonion

chromatography-inductivelycoupledplasma-massspectrome-try[45].3.6.Interferences

Theinterferenceofanumberofdifferentionshasbeenstud-iedbyspikingthestandardswithknownquantitiesofforeignsubstancesandanalyzingitbythepresentmethod.Anyorganicmatterthatalsoundergoesiodinationorbromination,suchasanilineandphenol,interferedseverelyandshouldberemoved,

Table2

Determinationofbromate/bromideandiodate/iodideinseawaterafterLPMEofderivatives(allresultsaretheaverageof5determinations)Sample

Iodide( g/l)Found

Seawaterno.1

25.6

Spiked10.020.030.025.050.075.025.050.075.0

Recoverya(%)97.8102.596.998.8105.799.0103.597.8101.9

Iodate( g/l)Found31.4

Spiked10.025.035.025.075.0100.025.050.075.0

Recoveryb(%)102.198.4104.396.897.9103.698.7103.698.6

Bromide(mg/l)Found60.8

Spiked15.025.035.025.050.075.030.060.075.0

Recoveryc(%)97.598.6103.6102.598.598.699.5105.198.5

Bromate( g/l)Found<LODe

Spiked5.020.030.010.025.075.025.050.075.0

Recoveryd(%)97.9104.296.7103.697.596.9104.699.2105.2

Seawaterno.215.236.555.4<LOD

Seawaterno.311.838.659.9<LOD

abcde

TheRSDwasintherange2.5–4.8%.TheRSDwasintherange3.6–5.4%.TheRSDwasintherange1.6–3.0%.TheRSDwasintherange4.2–6.5%.Belowthelimitofdetection.

K.Reddy-Nooneetal./J.Chromatogr.A1148(2007)145–151

151

Fig.5.GC–FIDforthedeterminationof5 gl 1ofbromate.TheGCcol-umnwasHP-5(5%phenylsubstitutedmethylpolysiloxane)30m×0.32mm; lmthickness0.25 m,capillarycolumn;oventemperaturewasheldat90 Cfor3min,programmedto220 Cat20 Cmin 1andheldforfurther2min;carriergasnitrogen,3mlmin 1;hydrogen,25–30mlmin 1,andair,250–300mlmin 1.LPMEin50 loftoluene,injectionvolume,2 l,splitless.

e.g.bysolid-phaseextractionwithLiChrolutENcartridgebeforederivatization.Reducingionsdeplete2-iodosobenzoateleadingtoincompletederivatizationreaction.Thissituationwasavoidedbyemployingalargeramountsof2-iodosobenzoatereagent,orbymixingthesampleswith500 lof20%hydrogenperoxidebeforeaddingotherreagents.Sul de,thiosulfate,sul- te,thiocyanate,nitrite,iron(II)andmanganese(II)producednotmorethan3–5%errorwhenpresentuptoa3-fold(m/m)excessoverbromate/bromideandiodate/rgeexcess,atleast1000-fold,ofchloride,bromide,nitrate,phosphate,perchlorate,sulfate,hydrogencarbonate,ammonium,calcium,magnesium,zinc,cadmiumandcobaltcouldbeavoided,theerrorwasnotmorethan2–3%.

Besidesremovingbromideandiodide,prereactionwithAgClhasalsobeeneffectiveinremovingcertainotherinterferingionsowingtolowerKspoftheirsilversaltsthanthatofAgCl;examplesinclude(Kspofrespectivesilversaltinparentheses)cyanide(2.2×10 16),thiocyanate(1.0×10 12)andsul de(1.0×10 50).4.Conclusions

TheproposedmethodinvolvinguseofAgClforremovalofbromideandiodide,anddeterminationofbromateandiodatebytheirreductionandconversion,respectivelyinto4-bromo-and4-iodo-2,6-dimethylanilineisasimpleandconvenientprotocolofanalysis,anditavoidsmanyinconveniencesofionchromatog-raphy.The4-bromo-and4-iodo-2,6-dimethylanilinearegoodcandidatesforliquid-phasemicroextractionandanalyzingtheextractsbyGC–MSproducedsensitivedetectionatngl 1levelofanalyteions.LPMEwith50 lofsolventhasbeenfoundtobemorerobustandtogivebetterextractioninshorterperiodthanSDME.Acknowledgement

A.J.thankstheDepartmentofScience&Technology,NewDelhi,fortheawardofaresearchgrantunderWomenScientistScheme(SR/WOS-A/CS-02/2004).References

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