OpticalPropertiesofHumicSubstancesandCDOMRe

OpticalPropertiesofHumicSubstancesandCDOM:RelationtoStructureERINS.BOYLE,†NICOLASGUERRIERO,†ANTHONYTHIALLET,†ROSSANADELVECCHIO,*,‡ANDNEILV.BLOUGH*,†DepartmentofChemistryandBiochemistry,UniversityofMaryland,CollegePark,Maryland20742,andEarthSystemScienceInterdisciplinaryCenter,UniversityofMaryland,CollegePark,Maryland20742ReceivedNovember20,2008.RevisedmanuscriptreceivedJanuary30,2009.AcceptedFebruary3,2009.Thespectraldependenciesofabsorptionandfluorescenceemission(emissionmaxima(λmax),quantumyields(φ),andmeanlifetimes(τm))wereacquiredforacommerciallignin,SuwanneeRiverhumic(SRHA)andfulvic(SRFA)acids,andaseriessolidphaseextracts(C18)fromtheMiddleAtlanticBight(MABextracts).Theseparameterswerecomparedwiththerelativeaveragesizeandtotalligninphenolcontent(TLP).TLPwasstronglycorrelatedwithabsorptionat280and355nmfortheMABextracts,SRHA,andSRFA.Thespectraldependenceofλmax,φ,andτmwasverysimilarforallsamples,suggestingacommonphotophysicalandthusstructuralbasis.Astrongdecreaseofφandτmwithincreasingaveragesizeindicatesthatintramolecularinteractionsmustbeimportant.Whencombinedwithpreviouswork,theresultsleadustoconcludethattheopticalpropertiescommonlyassociatedwithterrestrialhumicsubstancesandchromophoricdissolvedorganicmatterariseprimarilyfromanensembleofpartiallyoxidizedligninsderivedfromvascularplantsources.Theyfurtherprovideadditionalsupportforanelectronicinteractionmodelinwhichintramo-lecularenergytransfer,excited-stateelectrontransfer,aswellaschargetransferlikelyplayimportantrolesinproducingtheobservedopticalandphotochemicalpropertiesofthesematerials.IntroductionAlthoughtheopticalpropertiesofhumicsubstances(HS)andchromophoricdissolvedorganicmatter(CDOM)havebeeninvestigatedfordecades(1-7),thestructuralbasisofthesepropertiesremainsunclear(3-8).Thesematerialsexhibitratherunusualabsorptionandemissioncharacter-istics:absorptiondecreaseswithincreasingwavelengthinanapproximatelyexponentialfashion,butextendswellintothevisibleandinsomecasesthenear-infrared;emissionmaxima(λmax)shiftcontinuouslytotheredwithincreasingexcitationwavelength(λexg375nm)whilethefluorescencequantumyields(φ)monotonicallydecrease.Theseresultsprovideexceedinglystrongevidencethattheabsorptionandemissionspectramustarisefromaverylargenumberofabsorbingandemittingspeciesorstates(8,9).However,previousresultsindicatethatthespectracannotresultsolelyfromasimplelinearsumofthespectraofnumerousindependentlyabsorbingandemittingchromophores,butinsteadthatintramolecularelectronicinteractionsbetweenchromophoresmustalsobeplayinganimportantrole(8).Basedonthisstudy(8),aswellasothers(10-22),theopticalpropertiesofhumicsubstanceswereproposedtoresultinpartfromintramolecularcharge-transferinteractionsbe-tweenhydroxy-aromaticdonorsandquinoid(orother)acceptorsformedthroughthepartialoxidationofligninandpossiblyotherpolymerichydroxy-aromaticprecursors(8).TogainfurtherinsightintothestructuralbasisofHSopticalproperties,thespectraldependenceofthefluores-cenceemissionmaxima,quantumyields,andlifetimes(τ)ofacommerciallignin,SuwanneeRiverfulvic(SRFA)andhumic(SRHA)acids,andaseriesofsamplesobtainedfromdiversenaturalwatersthroughsolidphaseextraction(C18)werecompared.Thedataobtainedfromthesemeasurementswerefurthercomparedwiththerelativeaveragemolecularsizeandtotalligninphenolcontent.Theresultsprovideadditionalsupportforanelectronicinteractionmodelinwhichintramolecularenergytransfer,excited-stateelectrontransfer,aswellaschargetransferlikelyplayimportantrolesinproducingtheobservedopticalandphotochemicalpropertiesofHS,CDOM,andextracted(partiallyoxidized)lignins.MaterialsandMethodsReagents.WaterwasobtainedfromaMilliporeMilli-QPluspurificationsystem.SRFAandSRHAwereobtainedfromtheInternationalHumicSubstancesSociety.Na2HPO4andNaH2PO4usedforbuffersolutionwereobtainedfromJ.T.Baker.Alkali-extractedcarboxylatedlignin(LAC)wasac-quiredfromAldrich(lotno.19714DS).AsreportedbyAldrich,theweightaverage(Mw)andnumberaverage(Mn)molecularweightofLACis∼175000and∼16000,respectively.ExtractedSamples.Naturalwaters(CDOM)andextracts(C18)wereobtainedfromsamples(20L)collectedonboardtheresearchvesselR/VCapeHenlopenortheR/VHughSharpintheMiddleAtlanticBight(MAB)alongatransectfromtheDelawareRivertothewesternboundaryoftheGulfStreamduringtwosummercruises(September2005andAugust2006)(Table1):midDelawareBay,2m,1_MDB_2m_(06)or6_MDB_2m_(05);continentalshelf,2m,2_Shelf_2m_(06)or7_Shelf_2m_(05);continentalshelf,35or43m,3_Shelf_35m_(06)or8_Shelf_43m_(05);GulfStream,2m,4_GS_2m_(06);andGulfStream,1000m,5_GS_1000m_(06).Watersampleswerefirstfiltered(0.2µmGellmanfilter),acidifiedtopH2andpumpedthroughthesolventphaseextractioncartridge(C18extractioncolumn,UnitedChemicalTechnologies,Inc.)attheflowrateof50mLmin-1.Thecartridgeswerepretreatedwith100mLofhighpuritymethanolfollowedby50mLofacidified(pH2)Milli-Qwaterpriortoextraction.Afterextractioneachcartridgewasrinsedwith1Lofacidified(pH2)Milli-Qwatertoremovesaltsandstoredintherefrigerator(4°C)untilfurtherprocessing.TheextractedmaterialwaselutedfromtheC18cartridgeswithhighpuritymethanolandevaporated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