气体中甲烷单组分的色谱—真空低温富集方法及其同位素分馏效应

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

刘清梅 李嘉成 蒋文敏 熊永强

LIU Qingmei LI Jiacheng JIANG Wenmin XIONG Yongqiang

1. 中国科学院 广州地球化学研究所, 有机地球化学国家重点实验室, 广州 510640; 2. 中国科学院 深地科学卓越创新中心, 广州 510640; 3. 中国科学院大学, 北京 100049

1. State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China; 2. CAS Center for Excellence in Deep Earth Science, Guangzhou, Guangdong 510640, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China

甲烷(CH4)团簇同位素分析在气候变化、能源勘探和行星生命等领域中发挥了重要作用。样品中CH4的纯度直接了影响高分辨质谱团簇同位素分析的精度和准确性。针对气样中CH4组分的富集纯化难题，根据气相色谱(GC)组分分离原理，实时监测组分峰形，进一步优化了载气线速、进样量等条件。同时，通过外标法量化回收率，GC组分分析验证纯度，保证纯化的有效性。通过优化色谱—真空低温富集制备方法，确定了IBEX系统载气最佳线速为12 mL/min，CH4进样量需小于12 mL等实验条件，可视化GC峰形确保CH4峰与相邻N2干扰峰基本分离，实现了CH4单组分的高纯富集。当气样中CH4含量小于70%而空气含量较高时，需要进行二次纯化以提高CH4纯度。讨论了5分子筛等吸附剂在纯化过程中可能引起CH4同位素分馏的原因，并通过适当延长CH4收集时间来消除5分子筛干扰。目前，该方法单次纯化过程约90 min，CH4的回收率和纯度分别为90.1%～95.7%和97.3%～98.9%，对同位素组成(δ13CVPDB和δDVSMOW、Δ13CH3D和Δ12CH2D2)的差异均小于质谱仪的分析误差，几乎可以忽略不计。

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.

国家自然科学基金项目“深层油气中甲烷团簇同位素地球化学研究”(42073065)资助。

https://doi.org/10.11781/sysydz202403621