Compound-Specific Radiocarbon Analysis of Atmospheric Methane: A New Preconcentration and Purification Setup

Abstract

Methane contributes substantially to global warming as the second most important anthropogenic greenhouse gas. Radiocarbon (14C) measurements of atmospheric methane can be used as a source apportionment tool, as they allow distinction between thermogenic and biogenic methane sources. However, these measurements remain scarce due to labor-intensive methods required. A new setup for the preparation of atmospheric methane samples for radiocarbon analysis is presented. The system combines a methane preconcentration line with a preparative gas chromatography technique to isolate pure methane samples for a compound-specific radiocarbon analysis. In order to minimize sample preparation time, we designed a simplified preconcentration line for the extraction of methane from 50 L atmospheric air, which corresponds to 50 µg C as required for a reliable 14C analysis of methane-derived CO2 gas measurement with accelerator mass spectrometry (AMS). The system guarantees a quantitative extraction of methane from atmospheric air samples for 14C analysis, with a good repeatability and a low processing blank. The setup was originally designed for the measurement of samples with low methane concentrations, but it can also be adapted to apportion sources from environmental compartments with high methane levels such as freshwaters or wetlands.

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…As with the stable isotope ratios, a more immediate benefit of Δ 14 CH 4 could come at the regional scale; however, in regions where nuclear sources exist, careful consideration needs to be paid to properly assess and quantify this interfering component (Graven et al, ; Townsend‐Small et al, ). Coupling regional transport models to measurements of Δ 14 CH 4 would allow quantification of the fossil component of CH 4 ; however, the sampling methods to make this possible are only starting to be developed (Espic et al, ). In addition, background stations must implement routine and long‐term measurements to accurately quantify the regional Δ 14 CH 4 background.…”

14C Research at the Laboratory for the Analysis of Radiocarbon with AMS (LARA), University of Bern

“…of ambient air and purified by several traps and a preparative gas chromatography (GC). [34] This process removes CO 2 and other carbon-containing gases quantitatively, which can be verified for each sample by reinjection of the purified CH 4 into the GC. Air components are also removed completely except for krypton that co-elutes with CH 4 from the GC.…”

“…2). [34] With this procedure, we have determined 14 CH 4 at various sites in Switzerland, including the Beromünster tower.…”

A sealed‐tube method for offline δ13C analysis of CO2 via a Gas Bench II continuous‐flow isotope ratio mass spectrometer

“…For example, recent developments in both atmospheric and aqueous methane 14 C dating can be used to generate methane δ 13 C data from the same sample without the need for collecting or extracting additional samples. 9,10 Another example is 14 C studies of marine dissolved organic carbon (DOC), which often require contemporaneous δ 13 C values for environmental interpretation. [11][12][13] Third, many difficult sample types requiring offline preparation have low sample throughput (e.g.…”

“…These studies often benefit from contemporaneous measurements of both 14 C and δ 13 C data for geochemical interpretation. For example, recent developments in both atmospheric and aqueous methane 14 C dating can be used to generate methane δ 13 C data from the same sample without the need for collecting or extracting additional samples 9,10 . Another example is 14 C studies of marine dissolved organic carbon (DOC), which often require contemporaneous δ 13 C values for environmental interpretation 11–13 …”