The Trace Gas/Isoprime instrument analyses the 13C/12C stable isotope ratio in methane using continuous flow gas chromatography-isotope ratio mass spectrometry. The instrument is located in the greenhouse gas laboratory at Royal Holloway University of London. Air samples that have been collected in a flasks or bags are analysed. The instrumentation has been described in Fisher et al., 2006, Rapid Communication in Mass Spectrometry, doi: 10.1002/rcm.2300.