Extraction, purification, and clumped isotope analysis of methane (δ¹³CDH₃and δ¹²CD₂H₂) from sources and the atmosphere

Measurements of the clumped isotope anomalies (13CDH3 and 12CD2H2) of methane have shown potential for constraining methane sources and sinks. At Utrecht University, we use the Thermo Scientific Ultra high-resolution isotope-ratio mass spectrometer to measure the clumped isotopic composition of methane emitted from various sources and directly from the atmosphere. We have developed an extraction system with three sections for extracting and purifying methane from high (>1%), medium (0.1% to 1%), and low-concentration (<0.1%) samples, including atmospheric air (g1/42ppmCombining double low line0.0002%). Depending on the methane concentration, a quantity of sample gas is processed that delivers 3±1mL of pure methane, which is the quantity typically needed for one clumped isotope measurement. For atmospheric air with a methane mole fraction of 2ppm, we currently process up to 1100L of air. The analysis is performed on pure methane, using a dual-inlet setup. The complete measurement time for all isotope signatures is about 20h for one sample. The mean internal precision values of sample measurements are 0.3±0.1‰ for 13CDH3 and 2.4±0.8‰ for 12CD2H2. The long-term reproducibility, obtained from repeated measurements of a constant target gas, over almost 3 years, is around 0.15‰ for 13CDH3 and 1.2‰ for 12CD2H2. The measured clumping anomalies are calibrated via the 13CDH3 and 12CD2H2 values of the reference CH4 used for the dual-inlet measurements. These were determined through isotope equilibration experiments at temperatures between 50 and 450°C. We describe in detail the optimized sampling, extraction, purification, and measurement technique followed in our laboratory to measure the clumping anomalies of methane precisely and accurately. This paper highlights the extraction and one of the first global measurements of the clumping anomalies of atmospheric methane.