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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 13CCH4 or 2H-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 13CCH4 values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH4 gases from fossil and biogenic sources and determined the 13C and 2H 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 13C and −170.1 ‰ and −317.4‰ for 2H in the fossil and biogenic CH4, respectively. We used both CH4 types as parental gases from which we mixed two filial CH4 gases. Their 13C was determined to be −42.21‰and −47.25‰representing glacial and present atmospheric 13C-CH4. The 2H 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 13C within the uncertainties of our detection limit (we cannot conclude this for 2H because our system is currently not prepared for 2H-CH4 measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting 2H-CH4 or other gas species.