T01_P07

A new setup for CH₄ analysis at CologneAMS

Melchert J¹, Rethemeyer J¹, Gierga M¹, Gwozdz M²,

¹University Of Cologne - Institute for Geology and Mineralogy, Cologne, Germany, ²University Of Cologne - Institute for Nuclear Physics, Cologne, Germany

The radiocarbon analysis of CH₄ required the development of a new sample handling routine and the establishment of a new vacuum system that converts CH₄ to CO₂ for direct measurement with the gas injection system of the AMS at the CologneAMS facility. First tests with multiple series of ₁₄C-free and modern standards, as well with a biogas mixture with sample sizes ranging from 20 to 50 μg C resulted in a CH₄ to CO₂ conversion efficiency of 94 – 97%. Processed standards were further evaluated for contamination with extraneous carbon. With this new set up blank values achieved 0.006 ± 0.003 F¹⁴C, which is comparable to blank values achieved with our routinely used CO2 vacuum system. With the processed standard series, we were able to quantify a low contribution of 0.26 ± 0.13 μg modern and 0.33 ± 0.12 μg dead exogenous carbon, respectively, for the new system. Both sources of contamination resulted in 0.58 ± 0.18 μg of extraneous C, introduced during sample handling and pre-treatment, with a corresponding F14C of 0.447 ± 0.245. First tests with a near modern CH₄:CO₂ biogas mixture delivered reproducible results with a ¹⁴C content of 0.978 – 1.010 F¹⁴C, after applying the correction for extraneously introduced carbon.