Abstract
Measurements of the stable carbon isotope ratio in methane (δ(13)C-CH(4)) are used in determining the source of CH(4) emissions on local, regional, and global scales. To achieve the required level of data comparability for atmospheric monitoring networks, accurate gas reference materials of δ(13)C-CH(4) in air are required with high levels of reproducibility. We describe a method to determine the δ(13)C-CH(4) of CH(4) in synthetic air reference materials reported against the Vienna Pee Dee Belemnite (VPDB) scale. The measurement principle converts CH(4) into carbon dioxide (CO(2)) via direct combustion using a platinum catalyst. Subsequently, the CO(2) resulting from CH(4) combustion was analyzed for δ(13)C-CO(2) using OIRS (optical isotope ratio spectroscopy) against CO(2) in synthetic air reference materials traceable to the δ(13)C(VPDB) scale. The δ(13)C-CH(4) of four nominally 410 μmol mol(-1) CH(4) in synthetic air reference materials, prepared in pairs from two CH(4) sources with distinct δ(13)C-CH(4) were certified with an average δ(13)C-CH(4) of -39.07‰ and -51.91‰. Measurement reproducibility within 0.17‰ was demonstrated between measurements of the two reference materials from each CH(4) source. Agreement to traceable measurements of the pure CH(4) was achieved within reported measurement reproducibility. Combined expanded uncertainties (k= 2) between 0.4 and 1.5‰ have been demonstrated. The sensitives of the conversion system to flow rate have been assessed and found to have a negligible impact on the certification of δ(13)C-CH(4). Thus, we demonstrate that a combustion system coupled to OIRS measurement of CO(2) can provide a means of useful measurement of δ(13)C-CH(4) traceable to VPDB.