δ13C and δ2H measurement of methane from ecological and geological sources by gas chromatography/combustion/pyrolysis isotope-ratio mass spectrometry.

RATIONALE

The carbon and hydrogen isotopes of methane are useful in differentiating biological (e.g. wetlands, ruminants, biomass burning) and geological methane sources (e.g. fossil fuels, gas hydrates), as well as quantifying pathways of methanotrophism. Continuous-flow isotopic measurements of methane present a set of analytical challenges, including sample size restrictions and separation of CH4 from atmosphere, hydrocarbons, and CO2 .

METHODS

Small-scale modifications were made to a commercial trace-gas preconcentration and sampling unit (Thermo Scientific PreCon-GasBench) for improved isotopic analysis of methane (δ(13)C/δ(2)H) across a range of gas concentrations.

RESULTS

The long-term reproducibility of δ(13)C-CH4 values is less than ±0.2‰ (1σ). The limit-of-quantitation of δ(13)C-CH4 values is less than 0.8 nmol, conveniently measurable within standard gas sampling vials. A reproducibility of better than ±4‰ (1σ) is regularly achieved for δ(2) H values from sample sizes greater than 2 nmol. The range of measurement, for both δ(13)C and δ(2)H values, is easily extended from ambient concentration (~1.7 ppm-v) for preconcentrated samples to percent methane concentrations under subsampling.

CONCLUSIONS

The automated measurement of δ(13)C-CH4 and δ(2)H-CH4 values, from ambient to percentage concentrations, is possible with minimal modifications to a commercial preconcentration/gas chromatography inlet. Sample matrix interferences (CO2 , Cn Hy , air) are eliminated and simultaneous isotopic measurements of methane and CO2 and/or C1 -C4 light hydrocarbons are possible, while still retaining functionality for isotopic measurements of other gas species (e.g. CO2, N2, O2).