Semi-automated separation of methane from ambient air for analysis of Δ13CH3D and Δ12CH2D2

Bulk isotope ratios (δ¹³C-CH₄ and δD-CH₄) are used as tracers to help determine the contribution of methane (CH₄) sources and sinks to the atmospheric burden. The multiply substituted (clumped) isotopologues are now potentially available as additional tracers to improve these distinctions in the global understanding of the CH₄ budget. Measurement of Δ¹³CH₃D and Δ¹²CH₂D₂, however, is more challenging than measurements of bulk isotopes and requires more advanced instrumentation. Our ongoing project, POLYGRAM (www.polygram.ac.uk), is developing the sampling strategy, sample preparation, mass spectrometry, and modelling work to begin monitoring these new isotopologue ratios.

Use of a custom-built automated preconcentrator is a key step in our approach during sample preparation, as HR-IRMS requires ultra-pure CH₄ samples to measure the multiply substituted isotopologues. For ambient air studies we obtain 150 ml samples (1 bar) containing around 1% amount fraction of CH₄, which can then be easily transported and further purified for final analysis. Importantly this separation technique is automated, free of liquid cryogen, and requires minimal manual intervention. We have also developed the system to be flexible and allow for preparation from any natural sources containing less than 1% CH₄. We will present the validation of the method as well as a discussion to support the motivation for future development of monitoring Δ¹³CH₃D and Δ¹²CH₂D₂ in the global atmosphere.