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