YanKorshin2013ChapterQuantitationOfInteractions

QuantitationofInteractionsofSuwanneeRiverFulvicAcidwithProtonsBasedonNumericalDeconvolutionofDifferentialAbsorbanceandFluorescenceSpectraMingquanYanandGregoryV.KorshinAbstractInthischapter,weappliedanalternativeapproachthatcombinedthegenerationofdifferentialspectrawiththeirdeconvolutionthatalloweddetermin-ingtheemergenceandsignificanceoffeaturesseeninthespectroscopicdataobtainedatvaryingprotonactivities.ThebehaviorofSuwanneeRiverfulvicacid(SRFA)wasascertainedusingdifferentialabsorbancespectraandfluores-cenceexcitation
emissionmatrixes.ThepH-differentialabsorbancespectrawereprocessedvianumericdeconvolutiontodeterminethecontributionsofdiscretefeatures.DeconvolutionshowedthepresenceofsixGaussianbandsthatwereoperationallyreferredtoasA5,A4,A3,A2,A1,andA0withmaximalocatedintheorderofincreasingenergy(ordecreasingwavelengths)at2.25eV(551nm),3.240.02eV(383nm),3.990.01eV(311nm),4.520.03eV(275nm),5.110.06eV(243nm),and5.970.03eV(208nm).ThepH-differentialexcitation
emissionmatrixesofSRFAwereprocessedbyparallelfactoranalysisintosixdiscretefluorophorescomponentsreferredtoasF5,F4,F3,F2,F1,andF0withmaximalex/lemlocatedat(220,330)nm/430nm,(250,340)nm/450nm,(240,340)nm/460nm,(220,310)nm/380nm,(270,400)nm/490nm,and(280,340)nm/540nm.ExaminationofrelationshipsbetweenthepropertiesofthesixGaussianbandsandthesixfluorophoresandtheirdependencesvs.pH,aswellasthecomparisonoftheSRFAdataandthoseforselectedmodelcompounds,allowedmakingconclusionsconcerningtheengage-mentofpotentiallydiscretefunctionalities(e.g.,salicylic,polyhydroxyphenolic)andalsoofinteractionscharacteristicofhumicspeciesonlyandindicativeoftheirconformations.M.Yan(*)DepartmentofEnvironmentalEngineering,PekingUniversity,Beijing100871,Chinae-mail:[e-mail]andEnvironmentalEngineering,UniversityofWashington,Box352700Seattle,WA98195-2700,USAe-mail:[e-mail].(eds.),FunctionsofNaturalOrganicMatterinChangingEnvironment,DOI10.1007/978-94-007-5634-2_42,#ZhejiangUniversityPressandSpringerScience+BusinessMediaDordrecht2013233KeywordsAbsorbance•Dissolvedorganicmatter•Fluorescence•Numericaldeconvolution•pHIntroductionDissolvedorganicmatter(DOM)playsadominantroleindefiningtheenviron-mentalfate,bioavailability,andtoxicityofmetalsandothercontaminants.Becauseabsorbanceandfluorescencespectroscopyprovideimportantinformationabouthumicspecies,thesemethodshavebeenusedtoexaminecation–humicinteractionsinsitu,mostlyviafluorescencequenching.Changesoftheabsorbanceofhumicspeciescausedbytheircomplexationreactionshavebeenexaminedmuchless,mostlybecauseDOMchromophoresproduceoverlappingbandsresultinginfea-turelessabsorbancespectrathatarenotreadilyamenabletodeconvolutiontodeterminethecontributionsofdiscretefunctionalgroups(Korshinetal.,1997;Yanetal.,2012).MaterialsandMethodsSamples.SuwanneeRiverfulvicacid(SRFA)(1S101F)wasobtainedfromtheInternationalHumicSubstancesSociety(IHSS).SRFAwaspreparedata5.0mgL-1finalTOCconcentrationinMilli-Qwater.Titration.Acid–baseconditionsoftheSRFAsolutionwerecontrolledbytheadditionofHClO4orNaOHatconcentrationsof1.0,0.1,or0.01MduringtitrationsfrompH3.0to11.0.AftereachadditionofHClO4orNaOH,thesolutionswereallowedtoequilibratefor30minpriortotheremovalof5-mLand1-mLaliquotsforabsorbancespectraandfluorescencespectraanalysis,respectively.AbsorbancemeasurementswererecordedusingaPerkin-ElmerLambda18UV/Visspectro-photometer(witha5-cmcell)atwavelengthsfrom200to600nm.Theexcitatio-n–emissionmatrixfluorescencespectroscopymeasurementswererecordedusingaPerkin-ElmerLS50Bluminescencespectrometer(witha1-cmcell).ResultsandDiscussionToexaminethestructureofpH-differentialspectrainmoredetail,theywereprocessedtodeterminethepresenceofdistinctbandsthatcontributetotheiroverallshape.Similarlytotheconceptpresentedinpriorresearch(Korshinetal.,1997),suchbandswereassumedtohaveaGaussianshapewhenrepresentedvs.photonenergy(measuredineV).ThedifferentialspectrawereassumedtocompriseseveralGaussianbands,eachcharacterizedbythelocationofitsmaximum(E0i),width234M.YanandG.V.Korshin(Wi),andintensityatE¼E0i(A0i).Accordingly,thedifferentialspectraobtainedatvaryingpHsconcentrationsweremodeledasDAðEÞ¼XiDA0iexp
E
E0iWi2!ResultsoftheapplicationofthisconcepttomodelingofthepH-differentialspectraofSRFAareshowninFig.(1).Theyindicatethatthisapproachallowedforaveryclosefittingbetweentheobservedandmodeledspectra.ThepositionsofthesebandsdidnotundergosignificantchangesasthepHincreased,buttheirintensityand,insomecases,theirwidthdidchange.ThemaximaofthebandsoperationallyreferredtoasA5,A4,A3,A2,A1,andA0werelocated,intheorderofincreasingenergy(ordecreasingwavelengths)at2.25eV(551nm),3.240.02eV(383nm),3.990.01eV(311nm),4.520.03eV(275nm),5.110.06eV(243nm),and5.970.03eV(208nm)(Fig.1).AllthefluorescenceEEMsofSRFAatvariouspHcouldbesuccessfullydecomposedbyparallelfactoranalysis(PARAFAC)intoasix-componentmodel(StedmonandBro,2008).EachPARAFACcomponentdescribesthecontributionfromonefluorophore,andthemixtureEEMissplitintocontributionsfromindividualfluorophores.ThePARAFACemissionloadingsthenprovid