Temperature Dependence of Dissolved Organic Matter Fluorescence

The temperature dependence of organic matter fluorescence apparent quantum yields (Phi(f)) 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-a) for radiationless decay processes of the excited singlet state. E-a was calculated from temperature dependent Phi(f) 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 Phi(f) values, with E-a 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 Phi(f). E-a varied with excitation wavelength in most cases, typically exhibiting a decrease between 350 and 500 nm. The narrow range of E-a values observed for these samples when compared to literature E-a values for model fluorophores (similar to 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.