Liquid-phase microextraction–gas chromatography–mass spec(3)

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.