Raman spectroscopy is an interesting tool to assess the thermal maturity of solid organic matter. For carbonaceous material with moderate to high maturities, several studies have found good correlations between Raman spectral parameters and thermal maturity, expressed as vitrinite reflectance (VR) or bitumen reflectance (BR). However, at low maturities a large part of the Raman peaks is lost under an intense background radiation, caused by fluorescence. This fluorescence problem mainly occurs at 0.4-1.0% VR (the oil window), and makes it difficult to recognize the original spectrum. In this study, Raman parameters that have been put forward in literature were tested on a low maturity, solid bitumen sample of approximately 0.61% BR. The investigated parameters include the peak’s full width at half maximum FWHM, peak position W, peak area A, area ratio AD/AG and intensity ratio ID/IG. Fluorescence in this sample is very high and covers Raman peaks. It was found that during consecutive measurements at a single location (i.e. irradiation with the Raman laser), fluorescence decreases with time and Raman peaks appear. This is in line with Quirico et al. (2005), who observed the same effect at coal measurements. An interesting observation is the behaviour of Raman parameters during ongoing irradiation. The full widths at half maximum do not change at all for every investigated peak in our spectra. Also peak positions remain the same. The peak areas do change with irradiation, and show a decrease with decreasing fluorescence. Comparison of areas under individual peaks and total spectrum area however suggests that A and fluorescence decrease at equal speeds. This is the case for most important Raman peaks at 1370 (D-band), 1600 (G-band) and 3200 cm-1, with correlation coefficients of 0.66, 0.97 and 0.92 respectively. Lastly, the area ratio AD/AG and intensity ratio ID/IG(approximation) show no trend with fluorescence, indicating that the shape of the spectrum remains the same with irradiation. This is a promising result, because it suggests that fluorescence can be controlled without changing spectral parameters. Although not all peak parameters in this study (FWHM and AD/AG) correspond to parameters from literature regarding maturity, the behaviour of the Raman peak parameters in combination with the decreasing fluorescence is an exciting outcome. If further research proves that the original parameters are not altered by irradiation, this will provide an answer to the problem of fluorescence at low maturity samples.
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RBINS Staff Publications 2016
