Development and verification of preconcentrator to measure clumped isotopologues (Δ13CH3D and Δ12CH2D2) of methane from the atmosphere and sources

Bulk isotopic signatures (δ13C-CH4 and δD-CH4) are widely used for determination of methane source types and relative contributions. For example, these measurements are implemented as additional tracers in top-down studies. However, for some sources, for example certain fossil fuels sources in Europe and waste sector, the bulk isotopic signatures are overlapping, thus some methane sources remain indistinguishable1–4. The multiply substituted (clumped) isotopes can be used as additional tracers to better distinguish methane sources, and potentially, better understand methane sinks. Measurement of methane clumped isotopes, Δ13CH3D and Δ12CH2D2 is more challenging than measurements of bulk isotopes and requires more advanced instruments 3,5–8. Currently, a NERC project called POLYGRAM aims to develop the sample preparation (automated preconcentration) and measurement infrastructure to measure atmospheric air samples using High Resolution - Isotope Ratio Mass Spectrometer (HR-IRMS), to determine clumped isotopes from air samples collected at the world-recognised global monitoring sites at Cape Point, South Africa and station Zeppelin, Svalbard. Moreover, the project also aims to determine the clumped isotopes ratios of methane sources, like wetlands, agriculture or coal mines, as currently clumped isotopes database is constrained. Use of custom-built preconcentrator is a key step in the measurement chain, as HR-IRMS requires ultra-pure methane samples to measure clump isotopes. For ambient air studies, our aim is to obtain, at ambient pressure, a 150 ml sample containing at least 1% of methane from hundreds of litres of ambient air, where CH4 mole fraction is less than 2 ppm. To achieve it, we aim to concentrate methane in our sample by up to 62500 times. Additionally, we develop our preconcentrator to prepare samples containing at least 1% of methane from gas samples containing <1% CH4, like air in coal mines, landfill emissions, etc. During the conference, we will be focused on overcoming technical and scientifical challenges and made progress in developing CH4 preconcentrator. We will present the results of validation exercises to ensure repeatability and lack of fractionation effects, both for ambient air and methane source samples. References: Turner, A. J., Frankenberg, C. & Kort, E. A. Interpreting contemporary trends in atmospheric methane. Proc. Natl. Acad. Sci. 116, 2805–2813 (2019). Saunois, M. et al. The Global Methane Budget 2000–2017. Earth Syst. Sci. Data 12, 1561–1623 (2020). Chung, E. & Arnold, T. Potential of Clumped Isotopes in Constraining the Global Atmospheric Methane Budget. Glob. Biogeochem. Cycles 35, (2021). Menoud, M. et al. New contributions of measurements in Europe to the global inventory of the stable isotopic composition of methane. Earth Syst. Sci. Data 14, 4365–4386 (2022). Douglas, P. M. J. et al. Methane clumped isotopes: Progress and potential for a new isotopic tracer. Org. Geochem. 113, 262–282 (2017). Haghnegahdar, M. A., Schauble, E. A. & Young, E. D. A model for 12CH2D2 and 13CH3D as complementary tracers for the budget of atmospheric CH4. Glob. Biogeochem. Cycles 31, 1387–1407 (2017). Sivan, M. & Röckmann, T. Extraction, purification, and clumped isotope analysis of methane (Δ13CDH3 and Δ12CD2H2) from sources and the atmosphere. (2023). Haghnegahdar, M. A. et al. Tracing sources of atmospheric methane using clumped isotopes. 120, (2023).