Quantum Yields of Natural Organic Matter and Organic Compounds: Implications for the Fluorescence-based Interpretation of Organic Matter Composition

Absorbance and fluorescence spectroscopy are efficient tools for tracing the supply, turnover and fate of dissolved organic matter (DOM). The colored and fluorescent fractions of DOM (CDOM and FDOM, respectively) are linked by the apparent fluorescence quantum yield (AQY), which reflects the likelihood that chromophores emit fluorescence after absorbing light. Compared to the number of studies investigating CDOM and FDOM, few studies have systematically investigated AQY spectra for DOM, and compared them to quantum yields of organic compounds. Quantum yields of investigated organic compounds varied between 0.0008 and 0.36, whereas the average AQY for DOM samples at 350 nm was 0.0086 ± 0.0038. Moreover, marine and estuarine waters exhibited contrasting spectral dependencies of AQY values. We found that the presence of non-fluorescent chromophores greatly suppressed calculated AQYs and therefore hypothesize that AQYs can be used as an indicator of changes in the relative sizes of CDOM and FDOM pools. Additionally, the fluorescence spectra of potential DOM constituents were determined and compared to modelled spectra in the OpenFluor database. The feasibility of potential matches was subsequently evaluated using molar fluorescence and absorbance. Five organic compounds were potential matches with modelled spectra from 16 studies; however, the ability to confirm matches was limited due to multiple compounds exhibiting very similar spectra. This reiterates the fact that spectral similarity alone is insufficient evidence of the presence of particular compounds and additional evidence is required.