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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.
