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

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 C₂ molecules using a conventional isotope ratio mass spectrometer. Target molecules include ethane, ethene and ethanol.

¹³ C-¹³ C isotope species in C₂ molecules were measured as C₂ F₆ using a conventional isotope ratio mass spectrometer. Ethane and ethene are directly fluorinated to C₂ F₆ . Ethanol is measured after dehydration to ethene and subsequent fluorination of the latter. The method enables the measurement of the Δ¹³ C¹³ C values normalized against a reference working standard.

The reproducibility of the whole protocol, including chemical modification steps and measurement of C₂ F₆ isotopologues, is better than ±0.14‰ for all the compounds. Ethane from natural gas samples and biologically derived ethanol show a narrow range of Δ¹³ C¹³ C values, varying from 0.72‰ to 0.90‰. In contrast, synthetic ethanol as well as putative abiotic ethane show Δ¹³ C¹³ C values significantly different from this range with values of 1.14‰ and 0.25‰, respectively.

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.