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Abstract:
RATIONALE: The isotopic composition of greenhouse gases helps to constrain global budgets and to study sink and
source processes. We present a new system for high-precision stable isotope measurements of carbon, hydrogen and
oxygen in atmospheric methane and carbon dioxide. The design is intended for analyzing flask air samples from existing
sampling programs without the need for extra sample air for methane analysis.
METHODS: CO2 and CH4 isotopes are measured simultaneously using two isotope ratio mass spectrometers, one for the
analysis of δ13Candδ18Ovalues and the second one for δ2Hvalues. The inlet carousel delivers air from 16 sample positions
(glass flasks 1-5 L and high-pressure cylinders). Three 10-port valves take aliquots fromthe sample stream.CH4 from100-mL
air aliquots is preconcentrated in 0.8-mL sample loops using a new cryo-trap system. A precisely calibrated working
reference air is used in parallel with the sample according to the Principle of Identical Treatment.
RESULTS: It takes about 36 hours for a fully calibrated analysis of a complete carousel including extractions of four
working reference and one quality control reference air. Long-term precision values, as obtained from the quality control
reference gas since 2012, account for 0.04‰(δ13C values of CO2), 0.07‰(δ18Ovalues of CO2), 0.11‰(δ13C values ofCH4)
and 1.0‰(δ2H values of CH4). Within a single day, the system exhibits a typical methane δ13C standard deviation (1σ) of
0.06‰for 10 repeated measurements.
CONCLUSIONS: The system has been in routine operation at the MPI-BGC since 2012. Consistency of the data and
compatibility with results from other laboratories at a high precision level are of utmost importance. A high sample
throughput and reliability of operation are important achievements of the presented systemto cope with the large number
of air samples to be analyzed.
