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

• Published in: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
• Main Authors: PHAN, Van T.H., QUIRICO, Eric, BECK, Pierre, LE BRECH, Yann, JOVANOVIC, Lora, LE GUILLOU, Corentin, BERNARD, Sylvain, BONAL, Lydie, CARRASCO, Nathalie, GAUTIER, Thomas, RAYA, Jesus
• Format: Other/Unknown Material
• Language: English
• Published: Elsevier 2021
• Subjects: Chimie/Chimie organique; Tagish; Tagish Lake; Murchison
• Online Access: https://oskar-bordeaux.fr/handle/20.500.12278/40161; https://doi.org/10.1016/j.saa.2021.119853

The determination of the abundances of the CH x/sub>, 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 13 C 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 (CR), 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 AAro, 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.