Temperature Dependence of Dissolved Organic Matter Fluorescence.

The temperature dependence of organic matter fluorescence apparent quantum yields (Φ) 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 ( E) for radiationless decay processes of the excited singlet state. E was calculated from temperature dependent Φ 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 Φ values, with E ranging from 5.4 to 8.4 kJ mol at an excitation wavelength of 350 nm. Conversely, soil humic acid isolates exhibited little or no temperature dependence in Φ. E varied with excitation wavelength in most cases, typically exhibiting a decrease between 350 and 500 nm. The narrow range of E values observed for these samples when compared to literature E values for model fluorophores (∼5-30 kJ mol) 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.