Atmospheric Measurement Techniques A combustion setup to precisely reference δ 13 C and δ 2 H isotope ratios of pure CH 4 to produce isotope reference gases of δ 13 CCH 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 δ13CCH4 or δH-CH4. 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 δ13CCH4 values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH 4 gases from fossil and biogenic sources and determined the δ13C andδ2H values of the produced CO 2 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 ‰ forδ2H in the fossil and biogenic CH 4, 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 atmosphericδC-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 CH 4 gases with ultrapure N2/O2 (CH4 ≤ 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 δ13C within the uncertainties of our detection limit (we cannot conclude this for δ2H because our system is currently not prepared for δH-CH4 measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting δH-CH4 or other gas species.