Temperature Dependence of Dissolved Organic Matter Fluorescence

The temperature dependence of organic matter fluorescence apparent quantum yields (Φf) was measured for a diverse set of organic matter isolates (i.e., marine aquatic, microbial aquatic, terrestrial aquatic, and soil) in aqueous solution and for whole water samples to determine apparent activation energies (Ea) for radiationless decay processes of the excited singlet state. Ea was calculated from temperature dependent Φf data obtained by steady-state methods using a simplified photophysical model and the Arrhenius equation. All aquatic-derived isolates, all whole water samples, and one soil-derived fulvic acid isolate exhibited temperature dependent Φf values, with Ea ranging from 5.4 to 8.4 kJ mol–1 at an excitation wavelength of 350 nm. Conversely, soil humic acid isolates exhibited little or no temperature dependence in Φf. Ea varied with excitation wavelength in most cases, typically exhibiting a decrease between 350 and 500 nm. 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 similar photophysical mechanism for singlet excited states nonradiative inactivation across organic matter isolates of diverse source and character. In addition, this approach to temperature dependent fluorescence analysis provides a fundamental, physical basis, in contrast to existing empirical relationships, for correcting online fluorescence sensors for temperature effects.