Cryogen-free fully automated preconcentration unit to enable Δ13CH3D and Δ12CH2D2 analysis

Clumped isotope analysis is a powerful tool to constrain methane's origin in various geological, biogeochemical, and environmental settings. The extremely low abundance of ¹²CH₂D₂ and ¹³CH₃D isotopologues poses a challenge for the existing analytical systems. Mole fraction enhancement and cleaning of the environmental samples are necessary to fully exploit the potential of modern analyzers, requiring 50 to 1000 μmol of pure CH₄. To enable high-precision Δ¹²CH₂D₂ and Δ¹³CH₃D analysis with dual-QCL (8.6 μm and 9.3 μm) absorption spectroscopy in a multipass cell (400 m), we have developed a new automated cryogen-free unit for methane Cleaning and Extraction – CleanEx.

The unit can process up to 18 liters of sample air at a high flow rate of 900 ml min⁻¹. Methane (T_BP = −161 °C) is separated from major air components on a high capacity trap filled with HayeSep D (Trap 1, −176 °C). Sequential desorption and transfer to a second trap (Trap 2, −181°C) ensure complete O₂ and Ar removal. Substances with a higher boiling point, i.e., CO₂, N₂O, H₂O, C_nH_m, remain on Trap 1 to be removed at a later conditioning phase. CleanEx demonstrates equal performance for gas mixtures with initial methane mole fraction ranging from 2 ppm up to 2%. In contrast to the previously developed single trap TREX system (Eyer et al., 2016), atmospheric O₂ and Ar are effectively separated by cryo-focusing of CH₄ on the second trap. Ongoing work is focused on the separation of atmospheric gases with boiling points close to methane, e.g., Kr (T_BP = −153 °C).

We present the instrument design, performance, and details of its operation. Fractionation effects, methane recovery efficiency, and implications for high-precision δ¹³C-CH₄, δD-CH₄, Δ¹³CH₃D, and Δ¹²CH₂D₂ analyses are being discussed.

Acknowledgments. This work was supported by the Swiss National Science Foundation (SNSF) under R'Equip project QCL4CLUMPS (no. 206021_183294) and the project STELLAR (grant no. 19ENV05; Stable isotope metrology to enable climate action and regulation) which has received funding from the EMPIR programme co-financed  by  the  Participating  States  and  from  the  European  Union's  Horizon  2020  research  and innovation programme.

 

References

Eyer, S., Tuzson, B., Popa, M. E., van der Veen, C., Röckmann, T., Rothe, M., Brand, W. A., Fisher, R., Lowry, D., Nisbet, E. G., Brennwald, M. S., Harris, E., Zellweger, C., Emmenegger, L., Fischer, H., and Mohn, J.: Real-time analysis of δ¹³C- and δD-CH₄ in ambient air with laser spectroscopy: method development and first intercomparison results, Atmos. Meas. Tech., 9, 263–280, https://doi.org/10.5194/amt-9-263-2016, 2016.