Direct radiocarbon ( 14 C) analysis of methane using positive ion mass spectrometry

Abstract The reduction of anthropogenic methane emissions is a priority due to its potent global warming potential. Radiocarbon ( 14 C) can distinguish between methane from natural biogenic and fossil fuel sources, however, the analysis of methane 14 C by conventional accelerator mass spectrometry (AMS) techniques is demanding. At SUERC, a prototype positive ion mass spectrometer (PIMS) is set up to directly analyze 14 C in methane with minimal sample preparation. Methane gas was mixed with a stoichiometric amount of oxygen in an open split and admitted directly into the source. A series of modern, blank and unknown methane samples were clearly distinguishable by their 14 C/ 13 C raw ratios. The collision cell gas flowrate was then increased to lower the limit of detection. We obtained a corrected 14 C/ 13 C raw ratio of less than 2 × 10 –13 for blank fossil methane which corresponds to a radiocarbon age greater than 50 kyr. Modern biogenic methane had a measured 14 C/ 13 C raw ratio approaching 1 × 10 –10 which is consistent with the nominal value of contemporary atmospheric methane. These first results indicate that PIMS has the potential to be a valuable new analytical technique for screening the 14 C content of biogas and in climate research studies.