A new automated method for high-throughput carbon and hydrogen isotope analysis of gaseous and dissolved methane at atmospheric concentrations.

Rationale: The dual isotope ratio analysis, carbon (δ ¹³ C value) and hydrogen (δ ² H value), of methane (CH ₄ ) is a valuable tracer tool within a range of areas of scientific investigation, not least wetland ecology, microbiology, CH ₄ source identification and the tracing of geological leakages of thermogenic CH ₄ in groundwater. Traditional methods of collecting, purification, separating and analysing CH ₄ for δ ¹³ C and δ ² H determination are, however, very time consuming, involving offline manual extractions.
Methods: Here we describe a new gas chromatography, pyrolysis/combustion, isotope ratio mass spectrometry (IRMS) system for the automated analysis of either dissolved or gaseous CH ₄ down to ambient atmospheric concentrations (2.0 ppm). Sample introduction is via a traditional XYZ autosampler, allowing either helium (He) purging of gas or sparging of water from a range of suitable, airtight bottles.
Results: The system routinely achieves precision of <0.3‰ for δ ¹³ C values and <3.0‰ for δ ² H values, based on long-term replicate analysis of an in-house CH ₄ /He mix standard (BGS-1), corrected to two externally calibrated reference gases at near atmospheric concentrations of methane. Depending upon CH ₄ concentration and therefore bottle size, the system runs between 21 (140-mL bottle) and 200 samples (12-mL exetainer) in an unattended run overnight.
Conclusions: This represents the first commercially available IRMS system for dual δ ¹³ C and δ ² H analysis of methane at atmospheric concentrations and a step forward for the routine (and high-volume) analysis of CH ₄ in environmental studies.
(© 2021 United Kingdom Research and Innovation, as represented by the British Geological Survey. Rapid Communications in Mass Spectrometry published by John Wiley & Sons Ltd.)