Time-resolved fluorescence of oils and oil distillates in artificial seawater at low excitation wavelengths: Assessing the use of lifetimes to decouple oil and dissolved organic matter (DOM) fluorophores in natural waters.

It is challenging to decouple typical protein-like chromophoric dissolved organic matter (CDOM) fluorophores from oil-related fluorophores in natural waters using standard steady-state fluorescence techniques. In the present work, time-resolved florescence spectroscopy was explored as a means of differentiating between these two types of fluorophores. Fluorescence lifetimes of oil products were measured as a function of excitation and emission wavelength in artificial seawater. A triexponential model gave τ = ~1-10 ns, τ = ~3-30 ns, and τ = ~0.2-2 ns. Time-resolved fluorescence amplitudes were dominated by τ (τ 55-65 %; τ 25-29 %; τ 11-16 %) and contributions to steady-state fluorescence were dominated by τ and τ. Lifetimes increased with decreasing λ. Fluorescence was quenched by halide ions, but ion concentrations in natural waters are insufficient for quenching to significantly change lifetimes. Heavier, denser oils had red-shifted emission and lower lifetimes. Results suggest that time-resolved spectroscopy can decouple oil and protein-like CDOM fluorophores at λ < 300 nm.