Further Cryogenic Separation and Mass Spectrometry Developments: Towards Ambient Air Methane Clumped Isotopes Measurements

The multiply-substituted isotopologues of methane are of significant interest due to their increased ability to distinguish between methane formation and destruction processes, in comparison to singly-substituted isotopologues, as shown previously by Thiagarajan et al.(2022) and Sivan et al.(2022). Methane isotopologues, unlike the isotopologues of carbon dioxide, do not easily transition towards thermodynamic equilibrium in the atmosphere and therefore, ambient air methane isotopologues, offer constraints on the atmospheric methane sources and sinks (Chung and Arnold, 2021). These processes potentially offer new insight into the causes of variation in methane concentrations in the last two decades.

We present developments in the separation of the components of air, using a helium-cooled cryostat. Working on both pre-concentrated air mixtures and laboratory created gas mixtures, we extract methane from the contaminants and other atmospheric gases using the cryostat, applicable to a minimum methane concentration of ~1% (Stolper et al., 2015), then we analyse using a TFS Ultra HR-IRMS. We demonstrate that our cryostat separations successfully extract methane and krypton from laboratory gas mixtures containing the components of atmospheric air, without causing methane fractionation.

We also present further developments in measuring and calibrating the isotopologues of methane by high-resolution mass spectrometry. We successfully created thermodynamically equilibrated samples of methane in the 250-500^oC range using a nickel catalyst and are working on the 1-250^oC range using a γ-Al₂O₃ catalyst (Eldridge et al., 2019). It is essential to have an extensive calibration curve to best constrain the effects of scale compression on the calculated deltas and therefore reduce sources of further error, hence the extension of this calibration range.

Further work will add additional reference and sample points to the absolute reference frame created by the equilibrated samples, optimise the cryogenic/gas chromatographic purification methods for more complex gas mixtures, and optimise the IRMS workflow to reduce the necessary air sample sizes.

 

References:

• Chung, E. and Arnold, T., 2021. Potential of clumped isotopes in constraining the global atmospheric methane budget. Global Biogeochemical Cycles, 35(10), p.e2020GB006883.
• Eldridge, D.L., Korol, R., Lloyd, M.K., Turner, A.C., Webb, M.A., Miller III, T.F. and Stolper, D.A., 2019. Comparison of Experimental vs Theoretical Abundances of 13CH3D and 12CH2D2 for Isotopically Equilibrated Systems from 1 to 500 C. ACS Earth and Space Chemistry, 3(12), pp.2747-2764.
• Liu, Q., Li, J., Jiang, W., Li, Y., Lin, M., Liu, W., Shuai, Y., Zhang, H., Peng, P. and Xiong, Y., 2024. Application of an absolute reference frame for methane clumped-isotope analyses. Chemical Geology, p.121922.
• Sivan, M., Röckmann, T., Slomp, C.P., van der Veen, C. and Popa, M.E., 2022, May. Isotopic characterization of methane: insights from clumped isotope (13CDH3 and CD2H2) measurements. In EGU General Assembly Conference Abstracts (pp. EGU22-4029).
• Stolper, D.A., Martini, A.M., Clog, M., Douglas, P.M., Shusta, S.S., Valentine, D.L., Sessions, A.L. and Eiler, J.M., 2015. Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues. Geochimica et Cosmochimica Acta, 161, pp.219-247.
• Thiagarajan, N., Pedersen, J.H., Brunstad, H., Rinna, J., Lepland, A. and Eiler, J., 2022. Clumped isotope constraints on the origins of reservoir methane from the Barents Sea. Petroleum Geoscience, 28(2), pp.petgeo2021-037.