A fluorination method for measuring the ¹³C‐¹³C isotopologue of C₂ molecules

Abstract

Rationale: Doubly substituted isotope species ("clumped" isotopes) can provide insights into the biogeochemical history of a molecule, including its temperature of formation and/or its (bio)synthetic pathway. Here, we propose a new fluorination method for the measurement of 13 C-13 C species in C 2 molecules using a conventional isotope ratio mass spectrometer. Target molecules include ethane, ethene and ethanol.Methods: 13 C-13 C isotope species in C 2 molecules were measured as C 2 F 6 using a conventional isotope ratio mass spectrometer. Ethane and ethene are directly fluorinated to C 2 F 6 . Ethanol is measured after dehydration to ethene and subsequent fluorination of the latter. The method enables the measurement of the Δ 13 C 13 C values normalized against a reference working standard. Results:The reproducibility of the whole protocol, including chemical modification steps and measurement of C 2 F 6 isotopologues, is better than ±0.14‰ for all the compounds. Ethane from natural gas samples and biologically derived ethanol show a narrow range of Δ 13 C 13 C values, varying from 0.72‰ to 0.90‰. In contrast, synthetic ethanol as well as putative abiotic ethane show Δ 13 C 13 C values significantly different from this range with values of 1.14‰ and 0.25‰, respectively. Conclusions:The method presented here provides alternative means of measuring 13 C-13 C species to that using high-resolution mass spectrometry. Preliminary data from natural and synthetic molecules re-emphasizes the potential of 13 C clumped isotope species as a (bio)marker.

Low 13C-13C abundances in abiotic ethane

“…The data presented here will be obtained solely by the method presented in ref. 19 that gives reproducible Δ 13 C 13 C values with no scale compression 20 (see Methods).…”

“…Symbols are the same as shown in the legend of panel b . Grey circles denote bio-ethanol from three plants: the C3-type, the C4-type, and the Crassulacean Acid Metabolism (CAM) 19 . Light blue triangles represent ethene (C 2 H 4 ) produced by propane pyrolysis (see Methods).…”

“…We have developed a method to determine the relative abundances of the three isotopologues in ethane ( 12 CH 3 12 CH 3 , 12 CH 3 13 CH 3 , and 13 CH 3 13 CH 3 ) reported as ∆ 13 C 13 C values 19 , 20 (see Methods). Here, we show that this tool can be used to distinguish between abiotic and biotic hydrocarbons.…”

Hydrocarbon Cycling in the Tokamachi Mud Volcano (Japan): Insights from Isotopologue and Metataxonomic Analyses

“…Further studies should include culture experiments with different substrates and electron acceptors as well as the analysis of the ion concentrations in the samples (especially sulfate, nitrate and iron species) to determine their role in hydrocarbon production and consumption. Finally, the study should be pursued including newly developed isotopologue measurements such as the 13 C− 13 C isotopologue of ethane [ 17 , 18 , 47 ] and the 13 CH 3 D and CH 2 D 2 isotopologues of methane [ 14 , 15 , 34 , 48 ]. These new tracers are currently used to understand the sources and sinks of hydrocarbons.…”

“…Isotopologues are molecules that differ in the number and/or position of isotope substitution [ 11 , 12 ]. These include multiply substituted isotope species such as 13 C H 3 D and CH 2 D 2 for methane [ 13 , 14 , 15 , 16 ], 13 C H 3 - 13 C H 3 for ethane [ 17 , 18 ], as well as position-specific isotopologues such as 13 C H 3 -CH 2 -CH 3 and CH 3 - 13 C H 2 CH 3 of propane [ 19 , 20 , 21 , 22 ]. The latter approach has proven particularly useful for the detection of hydrocarbon anaerobic oxidation in natural gas basins [ 23 ].…”