A rapid method for preparing low volume CH4 and CO2 gas samples for 14C AMS analysis

14C measurements of CH4 in environmental samples (e.g. soil gas, lake water, gas hydrates) can advance understanding of C cycling in terrestrial and marine systems. The measurements are particularly useful for detecting the release of old C from climate sensitive environments such as peatlands and hydrate fields. However, because 14C CH4 measurements tend to be complex and time consuming, they are uncommon. Here, we describe a novel vacuum line system for the preparation of CH4 and CO2 from environmental samples for 14C analysis using accelerator mass spectrometry (AMS). The vacuum line is a flow-through system that allows rapid preparation of samples (1 h for CH4 and CO2, 30 min for CH4 alone), complete separation of CH4 and CO2 and is an easy addition to multipurpose CO2 vacuum lines already in use. We evaluated the line using CH4 and CO2 standards with different 14C content. For CH4 and CO2, respectively, the total line blank was 0.4 ± 0.2 and 1.4 ± 0.6 μg C, the 14C background 51.1 ± 1.2 and 48.4 ± 1.5 kyr and the precision (based on pooled standard deviation) 0.9‰ and 1.3‰. The line was designed for sample volumes of ca. 180 ml containing 0.5-1% CH4 and CO2, but can be adjusted to handle lower concentration and larger volume samples. This rapid and convenient method for the preparation of CH4 and CO2 in environmental samples for 14C AMS analysis should provide more opportunities for the use of 14C CH4 measurements in C cycle studies.