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

Abstract

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.