PREPARATION AND HANDLING OF METHANE FOR RADIOCARBON ANALYSIS AT COLOGNEAMS

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.