Developing a preconcentration and purification setup for 14C measurements of atmospheric methane

Methane contributes substantially to global warming as the second most important anthropogenic greenhouse gas. Its sources are diverse and remain poorly quantified and not well understood [1]. The radiocarbon (14C) content of these emissions is of growing interest since it can be used as a tool for a methane source apportionment [2]. Indeed, contemporary methane (e.g. agriculture, biomass burning) contains present-day 14C levels, whereas fossil methane (e.g. fossil fuels, geologic sources) is 14C-free. However, this task is challenging given the very large amounts of methane required and its very low concentration in the atmosphere. Methane is usually separated from other trace gases in a stepwise process: first CO2 is cryogenically removed, then CO is oxidized to CO2 and also cryogenically removed, before methane can finally be oxidized and isolated as CO2. For this simplified process, cross contamination of CO2 from one fraction to the next remains an issue difficult to overcome [3].

Our research aims at enabling the extraction of pure methane from atmospheric air and performing 14C measurements with the accelerator mass spectrometer MICADAS in our laboratory [4]. We have developed a methane preconcentration and purification setup (MPPS). Here, we combine a methane preconcentration line with a preparative gas chromatography technique [5] to obtain pure methane samples from the atmosphere.

The MPPS accepts flasks or aluminum bags filled with 50-100 liters of dry atmospheric air, where the whole sample is pumped through a glass and stainless steel line. First, CO2 is cryogenically removed in a custom-made Russian Doll trap [6]. Then, two successive preconcentration stages [7] enable the removal of most of bulk air before the sample enters the GC column, where methane is chromatographically separated from remaining bulk air and other trace gases and collected in an individual trap. The purity of the methane is verified before it is transferred to a copper oxide oven, where it is converted into CO2 and finally recovered in a sealed glass ampoule, ready for a 14C-AMS gas measurement. In this work, we present details of the setup, the evaluation of the methane isolation procedure and first 14C results of methane extracted from ambient air.

[1] Dlugokencky et al., Philos. Trans. R. Soc. London Ser. A, 369, 2058 (2011)
[2] Lassey et al., Atmospheric Chemistry and Physics, 7, 2141 (2007)
[3] Pack et al., Organic Geochemistry, 78, 89 (2015)
[4] Szidat et al., Radiocarbon, 56, 561 (2014)
[5] Özek and Demirci, Methods in Molecular Biology, 864, 275 (2012)
[6] Brenninkmeijer and Röckmann, Analytical Chemistry, 68, 3050 (1996)
[7] Bräunlich, Study of atmospheric carbon monoxide and methane using isotopic analysis, PhD thesis, University of Heidelberg (2000)