Optical and Photochemical Properties of Suwannee River Natural Organic Matter Fractions Generated by Preparative Free Flow Electrophoresis

Abstract Fractionation of dissolved organic matter (DOM) has been used for decades to simplify the challenging task of bulk sample characterization and to gain insight into the structural features governing DOM reactivity. Here, we used preparative free-flow electrophoresis (PFFE), which separates molecules based on their charge-to-size ratio, to separate Suwannee River natural organic matter (SRNOM) into fractions with varying electrophoretic mobility. Optical and photochemical data of the fractions were paired with extensive analytical characterization. The least electrophoretically mobile fraction had the lowest number-average molecular weight (Mn), aromaticity, and specific ultraviolet absorbance (SUVA254), while showing the largest quantum yields of fluorescence as well as singlet oxygen and hydroxyl radical formation. Structure–activity relationships for the fractions, which exhibited substantial differences in mobility, aromaticity, and Mn did not follow the conventional trends observed in size-based DOM fractionation studies (e.g., SUVA254 ∝ aromaticity). These findings highlight the role of charge and structural differences in driving optical and photochemical properties apart from bulk aromaticity and Mn. The distribution of absorbance, the sum of vanillyl and syringyl phenols, and DIOHBA (3,5-dihydroxybenzoic acid) indicates that the most electrophoretically mobile fractions with the highest absorbance are dominated by DIOHBA, a phenol monomer derived from tannins and flavonoids, rather than lignin. DIOHBA was inversely related to the fluorescence index and β/α. Overall, these results provide insight into SRNOM photophysics and highlight the utility of PFFE as a tool for resolving DOM structure–reactivity relationships.