Origin and behavior of atmospheric methane inferred from carbon isotopic analyses(I) : Preparation of air sample for carbon isotopic analyses

DOI IR IR (HANDLE) Open Access

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  • 大気中メタンの炭素同位体比測定とその挙動解析(I): 微量メタンの濃縮・分離とその炭素同位体比測定

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Carbon isotopic ratios give us useful information on the origin and behavior of atmospheric methane which contributes to the greenhouse effect. The concentration of methane in the atmosphere is too low to measure its carbon isotopic ratios without enrichment. We assemble a preparation line for the analyses of these ratios. This line enriches methane in air sample more than 1m^3 by adsorption under low temperature, and separates it from the other gas components with gas chromatography. The enriched methane is finally turned to carbon dioxide through the combustion tube in this line. The efficiency of methane collection with this line is examined using air sample and standard methane diluted with nitrogen, and it is more than 80% for the former sample and more than 90% for the latter one. The carbon isotopic ratios (^<14>C/^<13>C) in the latter two samples are 0.00512±0.00091 and 0.01184±0.00072. These ratios are almost the background level (0.00587±0.00004), and that shows the contamination with the atmospheric air of high ^<14>C/^<13>C ratio in this line is little and negligible.

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