C-H and C-C clumping in ethane by high-resolution mass spectrometry

Abstract:

Ethane (C₂H₆) is an important natural compound, and its geochemistry can be studied through ¹³C-¹³C, ¹³C-D and/or D-D clumping. Such measurements are potentially important both as a stepping stone towards the study of more complex organic molecules and, in its own regard, to understand processes controlling the generation, migration and destruction of natural gas. Isotopic clumping on C-C and C-H bonds could be influenced by thermodynamics, chemical kinetics, diffusion or gas mixing. Previous work showed that ¹³C-D clumping in methane generally reflects equilibrium and provides a measure of formation temperature (Stolper et al 2014a), whereas ¹³C-¹³C clumping in ethane is likely most controlled by chemical-kinetic processes and/or inheritance from the isotopic structure of source organic compounds (Clog et al 2014). ¹³C-D clumping in ethane has the potential to provide a thermometer for its synthesis, as it does for methane. However, the difference in C-H bond dissociation energy for these two compounds may suggest a lower ‘blocking temperature’ for this phenomenon in ethane (the blocking temperature for methane is ≥~250 C in geological conditions). We present analytical techniques to measure both ¹³C-¹³C and ¹³C-D clumping in ethane, using a novel two-instrument technique, including both the Thermo 253-Ultra and the Thermo DFS. In this method, the Ultra is used to measure the relative abundances of combinations nearly isobaric isotopologues: (¹³C¹²CH₆ + ¹²C₂DH₅)/¹²C₂H₆ and (¹³C₂H₆ + ¹²C¹³CDH₅)/¹²C₂H₆, free of other isobaric interferences like O₂. The DFS, a very high resolution single-collector instrument, is then used to measure the ratios of isotopologues of ethane at a single cardinal mass: ¹²C₂DH₅/¹³C¹²CH₆, and ¹²C¹³CDH₅/¹³C₂H₆, with precisions of ~1 permil. Those 4 measurements allow us to calculate the bulk isotopic composition (D and ¹³C) as well as the abundance of ¹³C₂H₆ and ¹³C¹²CDH₅. We also present progress on the development of software tools to use the data measured with the DFS efficiently.