Infrared spectroscopy quantification of functional carbon groups in kerogens and coals: A calibration procedure

The determination of the abundances of the CH_x, C = O and aromatic groups in chondritic Insoluble Organic Matter (IOM) and coals by Infrared (IR) spectroscopy is a challenging issue due to insufficient knowledge on the absorption cross-sections and their sensitivity to the molecular environment. Here, we report a calibration approach based on a ¹³C synthetic model material whose composition was unambiguously determined by Direct-Pulse/Magic Angle Spinning Nuclear Magnetic Resonance (DP/MAS NMR). Ratios of the cross-sections of the CH_x, C = O and aromatic groups have been determined, and the method has been applied to IOM samples extracted from four chondrites as Orgueil (CI), Murchison (CM), Tagish Lake (C2-ungrouped) and EET 92042 (CR2), and to a series of coals. The estimate of the aliphatic to aromatic carbon ratio (n_CHx/n_Aro) in IOM samples from Orgueil, Murchison and Tagish Lake chondrites is in good agreement with Single-Pulse/NMR estimates earlier published, and is lower by a factor of 1.3 in the case of the CR chondrite EET 92042 (but the error bars overlap). In contrast, the aliphatic to carbonyl ratio (n_CHx/n_C=O) is overestimated for the four chondrites. These discrepancies are likely due to the control of the absorption cross-section of the C = O and C = C bonds by the local molecular environment. Regarding coals, the use of published NMR analyses has brought to light that the integrated cross-section ratio A_CHx/A_Aro varies with the vitrinite reflectance over an order of magnitude. Here as well, the local oxygen speciation plays a critical control in A_Aro, which decreases with increasing the vitrinite reflectance. We provide an analytical law that links A_CHx/A_Aro and vitrinite reflectance, which will allow the determination of n_CHx/n_Aro for any coal sample, provided its vitrinite reflectance is known.