Quantitative analysis of powdered organic matter by transmission infrared microspectroscopy using a diamond-window compression cell

Abstract

An artificial coalification series of Mahakam coal (type III organic matter) was analysed by transmission FT-i.r. microspectroscopy using a diamond anvil compression cell. Micro-infrared spectra were compared with spectra recorded in global mode using the standard KBr pelletization technique. Results indicated that infrared microspectroscopy provides higher-quality spectra (baseline and signal/noise ratio) as well as more information (accurate OH and v aromatic CH integration) than the bulk approach. The interference fringes on the micro-infrared spectra were used for sample thickness determination. Direct application of the Lambert-Beer law then allowed the absorbances of major infrared bands to be used to reveal the chemical changes taking place during maturation. Aliphatic CH species were shown to decrease roughly linearly whereas aromatic CH increased exponentially with increasing pyrolysis temperature. The end member of the artificial coalification series exhibited a fourfold lower aliphatic content and fifteenfold higher aromatic content than the original sample. Although oxygenated functions (OH, CO and CO) were substantially removed during the maturation, the contrasted behaviour of individual CO bands (1710, 1735 and 1650 cm−1) strongly suggested a rearrangement among the CO functions during artificial coalification. Direct absorbances of selected infrared bands were plotted against widely used geochemical parameters (HC and OC atomic ratios and Rock-Eval hydrogen index). The good correlations obtained indicate that transmission infrared microspectroscopy using a diamond anvil compression cell is a promising technique for statistically deducing the HC, OC and HI values of kerogen.