Evaluating Raman spectra of carbonaceous matter by automated, iterative curve-fitting

•An operator independent, automated curve-fitting method is presented.•Two parameters are derived from the method that correlates with vitrinite reflectance.•The parameters are valid for multiple excitation wavelengths.

Raman spectroscopy of carbonaceous material (RSCM) estimates the maximum metamorphic temperature of coal, graphite and CM enclosed in metapelites. However, the evaluation of CM Raman spectra is often biased by subjectivity, which reduces the overall comparability of RSCM results. To decrease subjectivity, an automated evaluation software is presented. It is based on the iterative and simultaneous modeling of signal and baseline functions. Two new RSCM parameters are introduced, termed Raman Area Ratio (RAR) and Scaled Total Area (STA). These parameters are tested on a sample suite of organic rich shales and slates of the Teuschnitz Syncline (Thuringia, Germany) of well constrained very low to low metamorphic grade, defined by illite crystallinty and vitrinite reflectance. A correlation of both RSCM parameters with vitrinite reflectance verifies that the RSCM parameters reflect the metamorphic conditions. This correlation prevails at different excitation wavelengths (488 nm, 633 nm, 785 nm) used for the Raman measurements.