CH4 is the second most important anthropogenic greenhouse gas and originates from different sources. The use of radiocarbon (14C) analysis of CH4 opens up the possibility to differentiate geological and agricultural origin. At the CologneAMS facility, the demand for 14C analysis of CH4 required the development of a sample handling routine and a vacuum system that converts CH4 to CO2 for direct injection of CO2 into the AMS. We evaluated the processing of CH4 using several series of gas mixtures of 14C-free and modern standards as well as biogas with sample sizes ranging from 10 to 50 µg C. The results revealed a CH4 to CO2 conversion efficiency of 94–97% and blank values comparable to blank values achieved with our routinely used vacuum system for processing CO2 samples. The tests with a near modern CH4:CO2 biogas mixture gave reproducible results with a near modern 14C content of 0.967–1.000 F14C, after applying the background correction.

Increasing demands for small-scale radiocarbon (14C) analyses required the installation of a “SO-110 B” type ion source (HVE Europa B.V.) at our 6 MV Tandetron AMS (HVE) dedicated for the direct injection of CO2 using either the gas injection system (GIS) from Ionplus AG or a EuroVector EA 3000 elemental analyzer (EA). We tested both systems with multiple series of 14C-free and modern standards (2.5–50 µg C) combusted in quartz ampoules or EA containers and were able to quantify exogenous C introduced. In EA-GIS-AMS analysis exogenous C is mainly derived from the EA sample containers. Blank values for 50 µg C combusted in solvent-cleaned tin (Sn) vessels were 0.0127 ± 0.0012 F14C (boats) and 0.0090 ± 0.0010 F14C (capsules), while they were much higher for thermally cleaned silver (Ag) capsules. The processing of gas samples for GIS-AMS yields similar blank values corresponding to 0.30 ± 0.08 µg exogenous C with 0.93 ± 0.23 F14C consisting of 0.28 µg C modern and 0.02 µg C fossil C. The combustion of larger amounts of blank material (1 mg C) in a single quartz tube split into aliquots gives lower blanks (0.0064 ± 0.0008 F14C; 50 µg C). Thus, 14C analysis of small, gaseous samples is now possible at CologneAMS.