Performance of Cryogenic Adsorbents for Use in Methane Bulk and Clumped Isotope Analysis

Cryogenic trapping of methane is essential for bulk and clumped isotope analyses, requiring adsorbent materials that enable efficient recovery and preserve isotopic signatures. This study evaluates the performance - capacity, isotopic fractionation, and ease of use - of silica gels, zeolite molecular sieves, and activated carbon under various trapping and desorption conditions. A focus is set on the preservation of methane clumped isotope signatures.

A well-characterized methane reference gas (40 mL) was cryofocused at 77 K in containers filled with silica gels, zeolite molecular sieves (5A and 13X), or activated carbon alongside non-loaded containers. After loading, the containers were warmed in a water bath (21°C-95°C) for various dwell times. The bulk (𝛿D-CH₄ and 𝛿¹³C-CH₄) and clumped (∆¹³CH₃D and ∆¹²CH₂D₂) isotopic composition of the desorbed methane were measured against the untreated reference gas using novel quantum cascade laser absorption spectroscopy (QCLAS).

The best results were achieved with coarse-grained (1-3 mm) silica gels heated to 50°C for at least 5 min or at 21°C-22°C for a minimum of 120 min. Elevated desorption temperatures (80°C-95°C) compromised clumped isotope signatures. Although effective for gas trapping, zeolite molecular sieves, and activated carbon introduced significant bulk and clumped isotopic shifts due to catalytic effects and chromatographic isotopologue separation. Methane cryofocused without adsorbents retained its bulk and clumped isotopic composition without significant fractionation.

Among the tested adsorbents, silica gels demonstrated superior performance, preserving δ¹³C-CH₄, δD-CH₄, ∆¹³CH₃D, and ∆¹²CH₂D₂ values close to or within performance targets while offering high adsorption capacity, reproducibility, and ease of regeneration. Adsorbent-free cryotrapping is a viable alternative for sufficiently large methane volumes, where vapor pressure isotope effects (VPIEs) become negligible. However, cryogenic adsorbents remain indispensable for ensuring isotopic accuracy for small sample volumes and high-precision applications.