Pre-Existing Carbon Structure and Its Effect on Site-Specific Carbon Isotopes in Small Organic Molecules

Abstract:

The ability to measure site-specific isotopes in organic molecules allows for better understanding of the mechanisms of their biosynthetic and/or catagenic formation and destruction. Here we examine for site-specific isotopic composition of propane from natural and synthetic sources using novel instruments and techniques gas source mass spectrometry ¹, and discuss the possible relationship of our findings to recent independent evidence from NMR measurements for the isotopic structures of long-chain alkanes².

A recent NMR study² demonstrates that n-alkanes can be divided into three groups according to their site-specific carbon isotope structure: long (C₁₆+) even carbon number, long (C₁₇+) odd carbon number , and short (C₁₁-C₁₅). We modeled the isotopic site-specific composition of propane derived from these three distinct groups. If propane is cleaved from such long-chain hydrocarbons without fractionation, the long odd-numbered and the shorter alkanes would produce propane with an average terminal position 6-7‰ lighter than the center position, while the long even-numbered chain compounds would produce propane with a terminal position averaging around 7‰ heavier than the center. If, instead the fractionation associated with cleaving propane from such parent molecules is ~10‰ (as seems likely), then these average terminal — center differences should be decreased by ~5 ‰ (i.e., to -11-12 and +1-2 ‰, respectively). We will compare these predictions with our previous demonstrations of the changes in bulk and site specific compositions in propane due to isotope exchange equilibria, diffusion and conventional models of kerogen ‘cracking’, and will use these models as a framework for interpreting the observed site-specific isotopic compositions of propane from diverse natural gas deposits.

1. Piasecki, A. et al. Site-Specific Carbon Isotope Measurement of Organics by Gas Source Mass Spectrometry. Mineralogical Magazine 77, (2013).

2. Gilbert, A., Yamada, K. & Yoshida, N. Organic Geochemistry. Organic Geochemistry 62, 56–61 (2013).