Structure-Dependent Reactivity of Low Molecular Weight Fulvic Acid Molecules during Ozonation

摘要

Size-exclusion chromatography coupled to quadrupole time-of-flight mass spectrometry(SEC-Q-TOF-MS)was used to study changes in the molecular composition of a Suwannee River fulvic acid isolate by ozonation.The composition of all three SEC fractions showed strong changes and a relative increase of the low molecularweight anions.Further mass spectrometric investigations focused on the low molecular weight fulvic acid molecules,where a preferential removal of fulvic acid molecules with a low oxidation state(low O/C ratio)and a high degree of unsaturation(low H/C ratio)was observed.Besides their elemental composition,also the structure of the fulvic acid molecules influenced their reactivity toward ozone.The data suggest that molecules with a more extended carbon skeleton and less carboxylate substituents showed higher reactivity whereas some highly unsaturated molecules did not show measurable removal up to a specific ozone dose of 2.5 mg/mg of DOC due to sterical shielding of the reactive structures.Newly formed molecules were determined by SEC-Q-TOF-MS,which were characterized by a very high number of carboxylate groups(high 0/C ratio)and a highly saturated carbon skeleton(high H/C ratio).These investigations explain on a molecular level many observations previously made with whole mixtures orfractions of natural organic matter.