A combustion setup to precisely reference δ 13C and δ 2H isotope ratios of pure CH 4 to produce isotope reference gases of δ 13C-CH 4 in synthetic air
Isotope records of atmospheric CH 4 can be used to infer changes in the biogeochemistry of CH 4. 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 δ 13C-CH 4 or δ 2H-CH 4. We present a method to produce isotope reference gases for CH 4 in synthetic air that are precisely anchored to the VPDB and VSMOW scales and have δ 13C-CH 4 values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH 4 gases from fossil and biogenic sources and determined the δ 13C and δ 2H values of the produced CO 2 and H 2O 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 δ 13C and −170.1‰ and −317.4‰ for δ 2H in the fossil and biogenic CH 4, respectively. We used both CH 4 types as parental gases from which we mixed two filial CH 4 gases. Their δ 13C was determined to be −42.21‰ and −47.25‰ representing glacial and present atmospheric δ 13C-CH 4. The δ 2H isotope ratios of the filial CH 4 gases were found to be −193.1‰ and −237.1‰, respectively. Next, we mixed aliquots of the filial CH 4 gases with ultrapure N 2/O 2 (CH 4 ≤ 2 ppb) producing two isotope reference gases of synthetic air with CH 4 mixing ratios near atmospheric values. We show that our method is reproducible and does not introduce isotopic fractionation for δ 13C within the uncertainties of our detection limit (we cannot conclude this for δ 2H because our system is currently not prepared for δ 2H-CH 4 measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting δ 2H-CH 4 or other gas species.