A combustion setup to precisely reference δ13C and δ2Hisotope ratios of pure CH4 to produce isotope reference gases ofδ13C-CH4 in synthetic air

[摘要] Isotope records of atmospheric CH₄ can be used to infer changes inthe biogeochemistry of CH₄. One factor currently limiting thequantitative interpretation of such changes are uncertainties in the isotopemeasurements stemming from the lack of a unique isotope reference gas,certified for δ¹³C-CH₄ or δ²H-CH₄. We present a method toproduce isotope reference gases for CH₄ in synthetic air that areprecisely anchored to the VPDB and VSMOW scales and haveδ¹³C-CH₄ values typical for the modern and glacialatmosphere. We quantitatively combusted two pure CH₄ gases fromfossil and biogenic sources and determined the δ¹³C andδ²H values of the produced CO₂ and H₂O relativeto the VPDB and VSMOW scales within a very small analytical uncertainty of0.04‰ 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 fossiland biogenic CH₄, respectively. We used both CH₄ types asparental gases from which we mixed two filial CH₄ gases. Theirδ¹³C was determined to be −42.21‰ and−47.25‰ representing glacial and present atmosphericδ¹³C-CH₄. The δ²H isotope ratios of thefilial CH₄ gases were found to be −193.1‰ and−237.1‰, respectively. Next, we mixed aliquots of the filialCH₄ gases with ultrapure N₂/O₂ (CH₄ ≤2 ppb) producing two isotope reference gases of synthetic air withCH₄ mixing ratios near atmospheric values. We show that our method isreproducible and does not introduce isotopic fractionation forδ¹³C within the uncertainties of our detection limit (wecannot conclude this for δ²H because our system is currently notprepared for δ²H-CH₄ measurements in air samples). Thegeneral principle of our method can be applied to produce synthetic isotopereference gases targeting δ²H-CH₄ or other gas species.

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