Chemical and Carbon Isotopic Alteration of Organic Matter during Stepped Combustion

Stepped combustion was examined for its applicability to the resolution of isotopically distinct carbon components of complex organic matter such as kerogen. A kerogen (isolated from Green River shale), a standard oil (NBS-22), and two single-component, pure, organic compounds (sucrose and pentadecane) were subjected to a modified procedure of stepped combustion. The modified procedure was designed to test for completeness of reaction at an arbitrarily selected relatively low combustion temperature (225 °C) in repeated periods of heating. In sequential periods at 225 °C, the rate of carbon release as CO₂ and CO generally diminished from several percent C per hour to near-zero percent C per hour, over a combined combustion time of up to 38 h. Nearly half of the carbon in all four materials remained as charred, oxidation-resistant residues that were readily combusted at 500°C. The carbon fractions released at 225 and 500°C, respectively, were Isotopically relatable to the chemical alteration of the organic substances that occurred during the analytical process, rather than to isotopically distinct components in the original materials.