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

Measurements of the stable carbon isotope ratio in methane (δ¹³C-CH₄) are used in determining the source of CH₄ emissions on local, regional, and global scales. To achieve the required level of data comparability for atmospheric monitoring networks, accurate gas reference materials of δ¹³C-CH₄ in air are required with high levels of reproducibility. We describe a method to determine the δ¹³C-CH₄ of CH₄ in synthetic air reference materials reported against the Vienna Pee Dee Belemnite (VPDB) scale. The measurement principle converts CH₄ into carbon dioxide (CO₂) via direct combustion using a platinum catalyst. Subsequently, the CO₂ resulting from CH₄ combustion was analyzed for δ¹³C-CO₂ using OIRS (optical isotope ratio spectroscopy) against CO₂ in synthetic air reference materials traceable to the δ¹³C_VPDB scale. The δ¹³C-CH₄ of four nominally 410 μmol mol^-1 CH₄ in synthetic air reference materials, prepared in pairs from two CH₄ sources with distinct δ¹³C-CH₄ were certified with an average δ¹³C-CH₄ of -39.07‰ and -51.91‰. Measurement reproducibility within 0.17‰ was demonstrated between measurements of the two reference materials from each CH₄ source. Agreement to traceable measurements of the pure CH₄ was achieved within reported measurement reproducibility. Combined expanded uncertainties (<i>k</i>= 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 δ¹³C-CH₄. Thus, we demonstrate that a combustion system coupled to OIRS measurement of CO₂ can provide a means of useful measurement of δ¹³C-CH₄ traceable to VPDB.