Isotopic and molecular analyses of hydrocarbons and monocarboxylic acids of the Murchison meteorite

Abstract

The monocarboxylic acids and hydrocarbons of the Murchison meteorite (CM2) were isolated for isotopic analysis. The nonvolatile hydrocarbons were analyzed as crude methanol and benzene-methanol extracts and also after separation by silica gel chromatography into predominantly aliphatic, aromatic, and polar hydrocarbon fractions. The volatile hydrocarbons were obtained after progressive decomposition of the meteorite matrix by freeze-thaw, hot water, and acid treatment. Molecular analyses of the aromatic hydrocarbons showed them to comprise a complex suite of compounds in which pyrene, fluoranthene, phenanthrene, and acenaphthene were the most abundant components, a result similar to earlier analyses. The polar hydrocarbons also comprise a very complex mixture in which aromatic ketones, nitrogen, and sulfur heterocycles were identified. Both δ^(13)C and δD values were obtained for all preparations. The monocarboxylic acids, aliphatic, aromatic, and polar hydrocarbons, and the indigenous volatile hydrocarbons were found to be D-rich with δD values ranging from about +100 to +1000. The δ^(13)C values ranged overall from −13 to +17. The deuterium enrichment observed in these compounds is suggestive of a relationship to interstellar organic compounds. In two separate analyses, the δD values of the nonvolatile hydrocarbons were observed to increase in the following order: aliphatic < aromatic < polar. This finding is consistent with an early solar system or parent body conversion of aromatic to aliphatic compounds as well as the earlier suggestion of pyrolytic formation of aromatic from aliphatic compounds.

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