Assessing the maturity of crude petroleum oils using total synchronous fluorescence scan spectra

There have been many applications of fluorescence methods for the analysis of crude petroleum oils down through the years. However, none of these studies has yielded a robust qualitative or quantitative method for quantifying the chemical composition, or assessing the maturity of crude oils. Simple fluorescence parameters such as lifetime, intensity, and intensity ratios do not correlate well with chemical composition particularly for medium weight crude oils [A. G. Ryder, T. J. Glynn, and M. Feely (2003). Proc. SPIE-Int. Soc. Opt. Eng. 4876, 1188-1195.]. A better approach may be to use the Total Synchronous Fluorescence Scan (TSFS) method to fully interrogate the complex chemical composition of the oils [D. Patra and A. K. Mishra (2002). Anal. Bioanal. Chem. 373, 304-309.]. We present TSFS spectra from 18 crude petroleum oils of varying composition, sourced from around the world. The TSFS plots of these oils are very complex, with the contours being spread over the full 250 700 nm wavelength range (lambda(ex)) and 40-200 nm wavelength interval (Deltalambda) sampled. The 3-D contour maps tend to two contour concentrations one at lambda(em) < 300 nm, Δλ = 120-200 nm, and a second near λ(ex) ∼ 380-400 nm, Δλ = 40-60 nm. The first feature represents fluorescence emission originating mainly from energy transfer processes with the second, longer wavelength feature originating from fluorescence emission generated by a higher proportion of direct excitation as opposed to emission resulting from energy transfer. The topography of the 3D contour plots is therefore influenced by the balance between energy transfer and direct fluorescence emission, which is governed by the chemical composition of the crude oils. We discuss how the gross chemical composition affects TSFS spectra and how TSFS can be used to assess oil maturity with a view to developing quantitative methods.