Quantitative analysis of crude oils by fluorescence lifetime and steady state measurements using 380-nm excitation

Fluorescence emission and lifetime spectroscopy using 380-nm excitation have been successfully applied to the determination of API gravity and aromatic concentration of crude petroleum oils. Twelve crude oils from the North Sea with American Petroleum Institute (API) gravities of between 20 and 50 were analyzed; emission spectra were obtained with a standard fluorometer and the lifetime data with a pulsed ultraviolet light-emitting diode (UV-LED) coupled to a fluorescence lifetime system assembled in-house. A strong correlation between API gravity and the ratio of the fluorescence intensities recorded at 650 nm and 450 nm yielded a partial least-squares (PLS) regression model with an accuracy of ±4.6 °API; using the fluorescence intensity ratio at 650 nm and 550 nm gave an accuracy of ±5.8 °API. The average fluorescence lifetimes (τ̄) of the oils ranged from 1.9 to 18.6 ns, with τ̄ first increasing and then decreasing as the emission wavelength increased. These effects are due to the interplay between energy transfer and the quenching processes, which are governed by oil composition. A strong linear correlation was found between the ratio of the fluorescence lifetime measured at 650 nm and 550 nm and the aromatic concentration. A PLS regression model using this ratio had an accuracy of ±4% for aromatic concentration. Combining the fluorescence intensity with the lifetime measurements at 550 nm and 650 nm provides a rapid, quantitative, non-contact, nondestructive method of oil characterization.