A combustion setup to precisely reference δ13C and δ2H isotope ratios of pure CH4 to produce isotope reference gases of δ13C-CH4 in synthetic air
Abstract. Isotope records of atmospheric CH4 can be used to infer changes in the biogeochemistry of CH4. One factor currently limiting the quantitative interpretation of such changes are uncertainties in the isotope measurements stemming from the lack of a unique isotope reference gas, certified for δ13C-CH4 or δ2H-CH4. We present a method to produce isotope reference gases for CH4 in synthetic air that are precisely anchored to the VPDB and VSMOW scales and have δ13C-CH4 values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH4 gases from fossil and biogenic sources and determined the δ13C and δ2H values of the produced CO2 and H2O relative to the VPDB and VSMOW scales within a very small analytical uncertainty of 0.04‰ and 0.7‰, respectively. We found isotope ratios of −39.56‰ and −56.37‰ for δ13C and −170.1‰ and −317.4‰ for δ2H in the fossil and biogenic CH4, respectively. We used both CH4 types as parental gases from which we mixed two filial CH4 gases. Their δ13C was determined to be −42.21‰ and −47.25‰ representing glacial and present atmospheric δ13C-CH4. The δ2H isotope ratios of the filial CH4 gases were found to be −193.1‰ and −237.1‰, respectively. Next, we mixed aliquots of the filial CH4 gases with ultrapure N2/O2 (CH4 ≤ 2 ppb) producing two isotope reference gases of synthetic air with CH4 mixing ratios near atmospheric values. We show that our method is reproducible and does not introduce isotopic fractionation for δ13C within the uncertainties of our detection limit (we cannot conclude this for δ2H because our system is currently not prepared for δ2H-CH4 measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting δ2H-CH4 or other gas species.
