Chromatography-vacuum low temperature method of methane enrichment and isotopic fractionation in gas samples

Abstract

Methane (CH4) clumped isotope analysis plays a crucial role in the fields of climate change, energy exploration, and planetary research. The purity of CH4 in samples directly affects the precision and accuracy of high-resolution mass spectrometry in clumped isotope analysis. Addressing the challenge associated with enriching and purifying CH4 components in gas samples, this study optimized conditions such as carrier gas line speed and sample injection volume based on the principles of gas chromatography (GC) component separation, with real-time monitoring of component peak shapes. Additionally, the recovery rate was quantified using an external standard method and purity was verified through GC component analysis to ensure the effectiveness of the purification process. By optimizing the chromatography-vacuum low-temperature enrichment preparation method, the optimal carrier gas line speed for the IBEX system was determined to be 12 mL/min, with a CH4 injection volume less than 12 mL. This facilitated visualization of GC peak shapes, thus ensured that the CH4 peak was essentially separated from the adjacent N2 interference peak, achieving high-purity enrichment of the CH4 single component. When the CH4 content in gas samples was less than 70% and the air content was high, secondary purification was required to improve CH4 purity. The causes of CH4 isotopic fractionation during purification using adsorbents like 5Å molecular sieves were discussed, and extending the CH4 collection time was proposed to eliminate the interference from the 5Å molecular sieve. Currently, this method requires approximately 90 min for a single purification process, with CH4 recovery and purity ranging from 90.1% to 95.7% and 97.3% to 98.9%, respectively. The differences in isotopic composition (δ13CVPDB and δDVSMOW, Δ13CH3D, and Δ12CH2D2) are all less than the analytical error of the mass spectrometer, making them almost negligible.

Keywords

• ch4
• gas chromatography
• purity
• recovery rate
• isotopic fractionation