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δ 13 C and δ 2 H measurement of methane from ecological and geological sources by gas chromatography/combustion/pyrolysis isotope‐ratio mass spectrometry
Author(s) -
Yarnes Chris
Publication year - 2013
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.6549
Subject(s) - chemistry , methane , isotope ratio mass spectrometry , gas chromatography , analytical chemistry (journal) , isotopes of carbon , environmental chemistry , mass spectrometry , stable isotope ratio , detection limit , isotope , chromatography , total organic carbon , physics , organic chemistry , quantum mechanics
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 CH 4 from atmosphere, hydrocarbons, and CO 2 . 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‐CH 4 values is less than ±0.2‰ (1σ). The limit‐of‐quantitation of δ 13 C‐CH 4 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‐CH 4 and δ 2 H‐CH 4 values, from ambient to percentage concentrations, is possible with minimal modifications to a commercial preconcentration/gas chromatography inlet. Sample matrix interferences (CO 2 , C n H y , air) are eliminated and simultaneous isotopic measurements of methane and CO 2 and/or C 1 ‐C 4 light hydrocarbons are possible, while still retaining functionality for isotopic measurements of other gas species (e.g. CO 2 , N 2 , O 2 ). Copyright © 2013 John Wiley & Sons, Ltd.