Isotopic analyses of nitrogenous compounds from the Murchison meteorite: Ammonia, amines, amino acids, and polar hydrocarbons

Abstract

The combined volatile bases (ammonia, aliphatic amines, and possibly other bases), ammonia, amino acids, and polar hydrocarbons were prepared from the Murchison meteorite for isotopic analyses. The volatile bases were obtained by cryogenic transfer after acid-hydrolysis of a hot-water extract and analyzed by combined gas chromatography-mass spectrometry of pentafluoropropionyl derivatives. The aliphatic amines present in this preparation comprise a mixture that includes both primary and secondary isomers through C_5 at a total concentration of ≥ 100 nmoles g^(−1). As commonly observed for meteoritic organic compounds, almost all isomers through C_5 are present, and the concentrations within homologous series decrease with increasing chain length. Ammonia was chromatographically separated from the other volatile bases and found at a concentration of 1.1–1.3 μmol g^(−1) meteorite. The ammonia analyzed includes contributions from ammonium salts and the hydrolysis of extractable organic compounds, e.g., carboxamides. Stable isotope analyses showed the volatile bases to be substantially enriched in the heavier isotopes, relative to comparable terrestrial compounds (δD ≤ + 1221%.; δ^(13)C = + 22%.; δ(15)N = + 93%.). Ammonia, per se, was found to have a somewhat lower δ^(15)N value (+69%.) than the total volatile bases; consequently, a higher δ^(15)N (>93%.) can be inferred for the other bases, which include the amines. Solvent-extractable polar hydrocarbons obtained separately were found to be enriched in ^(15)N (δ^(15)N = + 104%.). Total amino acids, prepared from a hydrolyzed hot-water extract by cation exchange chromatography, gave a δ^(15)N of + 94%., a value in good agreement with that obtained previously. Nitrogen isotopic data are also given for amino acid fractions separated chromatographically. The δ^(15)N values of the Murchison soluble organic compounds analyzed to date fall within a rather narrow range (δ^(15)N = + 94 ± 8%.), an observation consistent with their formation, or formation of their precursors, by interstellar chemistry.

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