In-Line Combustion System for the Measurement of δ13C–CH4 in Gas Reference Materials Using Optical Isotope Ratio Spectroscopy

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High Resolution Image Download MS PowerPoint Slide 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.

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