CleanEx: A Versatile Automated Methane Preconcentration Device for High-Precision Analysis of ¹³CH₄, ¹²CH₃D, and...

The relative abundance of methane isotopologues offers key insights into the global methane (CH₄) cycle. Advances in laser spectroscopy enable routine high-precision measurements even for rare deuterated methane isotopologues, ¹²CH₃D and ¹³CH₃D, provided there are sufficiently high methane amount fractions and reproducible measurement conditions, which can be achieved by CH₄ adsorption-desorption techniques. We present a new cryogen-free automated preconcentration device─CleanEx─designed for quantitative extraction of CH₄ from large volumes of sample gas and for cleaning by stepwise temperature-controlled desorption to separate interferant gases. We show that CleanEx has the capability to preconcentrate methane by almost 2000-fold from ∼18 L of air. The performance is demonstrated in a range of methane amount fractions between 2 ppm (μmol mol^-1), which corresponds to the present-day ambient air, up to 1000 ppm, representative for close to source or process conditions. Advantages over existing devices are a significantly larger primary adsorption trap and a secondary cryo-focusing step, which ensures separation of methane from major atmospheric compounds, i.e., O₂, Ar, and CO₂. We have demonstrated quantitative extraction of methane, with no significant isotopic fractionation and high repeatability of 0.2‰, 0.6‰, and 0.8‰ (n = 42) for the studied isotopologue ratios, ¹³CH₄/¹²CH₄, ¹²CH₃D/¹²CH₄, and ¹³CH₃D/¹²CH₄, during cryogenic adsorption-desorption on HayeSep D material. The developed device in combination with a suitable laser spectrometer offers a robust and autonomous method for precise continuous monitoring of δ¹³C-CH₄ and δD-CH₄ in ambient air and optionally Δ¹³CH₃D in process-derived methane.