Size exclusion chromatography (SEC) in combination with optical measurements (e.g., absorbance and fluorescence) has become a popular form of analysis to characterize dissolved organic matter (DOM). In this study, SEC was coupled with online absorbance, fluorescence, and total organic carbon (TOC) measurements to derive the apparent fluorescence quantum yield (Φ_f) as a function of molecular weight for DOM. This methodology allows for various components of DOM to be analyzed as a function of their apparent molecular weight (AMW). Individual instrument specific SEC-fluorescence detector correction factors were developed by comparing data to an off-line fluorescence detector. Salicylic acid was used as a well-characterized fluorescence standard to confirm the accuracy of calculated Φ_f values. The method was then applied to various sample sets including natural water samples from Boulder Creek and a DOM isolate (Suwannee River Fulvic Acid, SRFA) before and after ozonation to demonstrate its ability to measure the Φ_f of unknown DOM samples and to track changes across biogeochemical gradients.

Results demonstrate that although the fluorescence and absorbance intensities are highest for compounds in the high AMW range (>1000 Da), the Φ_f increases with decreasing AMW (ranging from <0.5-3.0%). Therefore, highly fluorescent DOM components (in terms of fluorescence efficiency) are composed of compounds in the medium to low AMW ranges (<1000 Da). Additionally, ozonation of SRFA resulted in an increased Φ_f at high AMW and a decreased Φ_f in the medium to low AMW range.

This method will provide a way to identify and isolate the highly fluorescent size fractions of DOM for further analysis, and be a tool used to differentiate between sources of organic matter (e.g. the mixing of the effluent organic matter in a treated wastewater effluent with the natural organic matter present in the receiving surface waters).

Urban Wünsch

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