In-Line Combustion System for the Measurement of δC-CH in Gas Reference Materials Using Optical Isotope Ratio Spectroscopy.

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