Raman spectroscopy of dispersed vitrinite — Methodical aspects and correlation with reflectance

•Raman spectroscopy of dispersed vitrinites can be used as a maturation index (STA-RSCM).•Final polishing (0.05 μm) changes the shape of Raman spectra of dispersed vitrinites.•Without final polish, vitrinites of < 7% Rmax can be measured by Raman spectroscopy.•Low wavelength lasers (405–488 nm) allow us to analyze low mature vitrinites.

Raman spectroscopy of dispersed vitrinites needs polished surfaces to safely identify organic particles in whole rock samples. Carbonaceous matter (CM) is very sensitive to the polishing process which leads to erroneous Raman spectra of these materials. It is unknown to which extent Raman spectra of low ordered CM are influenced by the polishing process. Therefore, vitrinites of high volatile bituminous coal, through anthracite to semi-graphite rank to graphitic CM were analyzed by multi-wavelength Raman spectroscopy and measured for minimum and maximum reflectance. A step-wise preparation and measuring protocol allows us to access at which point the Raman spectra are altered by sample preparation. The effect of polishing on the Raman spectra of vitrinites is not significant up to maximum reflectance values of ~ 7%. The multi-wavelength approach shows that short-wavelength lasers (488 nm and less) invoke far less fluorescence background than long-wavelength lasers, which permits us to measure very low mature vitrinites. Wavelength-dependent frequency shifts in the vitrinite Raman spectra reconstruct a two stage evolution of aromatic compounds during organic metamorphism. The first stage shows a wavelength independent downshift of the defect-band (D-band) from ~ 1370 to 1330 cm− 1, which is assumed to be related to the progressive growth of linear aromatic structures. The second stage is characterized by diverging D-band positions, which is caused by the growth of compact polycyclic aromatic hydrocarbons. Consequently, a strong linear correlation between the reflectance values and the derived Raman spectroscopic parameters was found. Because the comparison was conducted on the single grain level, it is shown that the scaled total area (STA) Raman parameter relates to the true maximum vitrinite reflectance. Thus, a linear and robust maturity index, which covers a maximum vitrinite reflectance range of ca. 1 to 7%, is established.