The temperature-dependence of organic matter fluorescence quantum yields (Φf) was measured for a diverse set of isolates (microbial and terrestrial aquatic, soil, and lignite-derived) in aqueous solution to determine activation energies (Ea) for radiationless decay processes of the excited singlet state. Ea was calculated from the temperature-dependent Φf data measured between 10-40 °C using a simplified photophysical model and the Arrhenius equation. All aquatic-derived isolates and one soil-derived fulvic acid isolate exhibited temperature-dependent Φf values consistent with a single radiationless decay process dominating over this temperature range, with Ea ranging from 5.9 to 8.6 kJ mol-1 at an excitation wavelength of 370 nm. Conversely, soil and lignite-derived humic acid isolates exhibited little or no temperature-dependence in Φf. There was a significant relationship between Φf and the rate of radiationless decay derived from the photophysical model, implying a structural basis underpinning the measured Ea values. The narrow range of Ea values observed for these samples when compared to literature Ea values for model fluorophores (~ 5-30 kJ mol-1) points to a consistent group of fluorophores across organic matter isolates of diverse character. Furthermore, spectral shape was unchanged as a function of temperature, indicating that the distribution of emitting states was unchanged. In addition, this fundamental approach to temperature-dependent fluorescence analysis complements existing empirical relationships for the correction of online fluorescence sensors for temperature effects.

Temperature dependence of the fluorescence of dissolved organic matter: Implications for DOM photophysics

255th National Meeting (2018)

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