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 indistinguishable^1–4.

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.

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