Measuring doubly 13C-substituted ethane by mass spectrometry

Ethane (C₂H₆) is present in non-negligible amounts in most natural gas reservoirs and is used to produce ethylene for petrochemical industries. It is one of the by-products of lipid metabolism and is the arguably simplest molecule that can manifest multiple ¹³C substitutions. There are several plausible controls on the relative abundances of ¹³C₂H₆ in natural gases: thermodynamically controlled homogeneous isotope exchange reactions analogous to those behind carbonate clumped isotope thermometry; inheritance from larger biomolecules that under thermal degradation to produce natural gas; mixing of natural gases that differ markedly in bulk isotopic composition; or combinations of these and/or other, less expected fractionations. There is little basis for predicting which of these will dominate in natural samples. Here, we focus on an analytical techniques that will provide the avenue for exploring these phenomena. The method is based on high-resolution gas source isotope ratio mass spectrometry, using the Thermo 253-Ultra (a new prototype mass spectrometer). This instrument achieves the mass resolution (M/Δ M) up to 27,000, permitting separation of the isobaric interferences of potential contaminants and isotopologues of an analtye or its fragments which share a cardinal mass. We present techniques to analyze several isotopologues of molecular and fragment ions of C₂H₆. The critical isobaric separations for our purposes include: discrimination of ¹³C₂H₆ from ¹³C¹²CDH₅ at mass 32 and separation of the ¹³CH₃ fragment from ¹²CH₄ at mass 16, both requiring at least a mass resolution of 20000 to make an adequate measurement. Other obvious interferences are either cleanly separated (e.g., O₂, O) or accounted for by peak-stripping (CH₃OH on mass 32 and NH₂ on mass 16). We focus on a set of measurements which constrain: the doubly-substituted isotopologue, ¹³C₂H₆, and the ¹³CH₃/¹²CH₃ ratio of the methyl fragment, which constrains the bulk δ ¹³C. Similar methods can be used to measure the D/H ratio, among other species. The precision on the δ ¹³C is better than 0.25 permil (1 s.e.) on the CH₃ fragment. Calculating δ ¹³C and δ D simultaneously on the intact isotopologues on masses 30 to 32 yields precisions of respectfully 0.2 and 4 permil (1 s.e.). Ratios of mass 32 to mass 30 species are measured to better than +/-0.7 per mil (1 s.e.). The corresponding precision on Δ ¹³C₂H₆ (defined as 1000 * ((¹³C₂H₆/C₂H₆)_measured/(¹³C₂H₆/C₂H₆)_stochastic)-1)) is +/-0.85 per mil (1 s.e.). All precision reflects counting statistics and can be improved with longer counting times. Accuracy determinations are underway.