Stable isotope analysis of atmospheric CO using a Gasbench II-Cold Trap-IRMS setting.

Rationale: The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO ) is a useful technique for the investigation and identification of the sources and sinks of the most abundant greenhouse gases by far. For this reason, we are presenting a measuring system here that enables a wide range of users to carry out stable isotope analysis of atmospheric CO using off-the-bench hardware and software.

Methods: The fully automated system uses cryogenic and gas chromatographic separation to analyse CO from 12-mL whole air samples and consists of an autosampler, a Gasbench II (GB), a downstream cryo trap and a continuous flow gas interface feeding into a sector field mass spectrometer (GC Pal/GB/Cold Trap/ConFlo IV/DeltaV Plus). The evaluation of the system performance was based on the analysis of samples prepared from eight CO sources (four CO reference gases and four artificial air tanks).

Results: The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO source) in the determination of the carbon and oxygen isotope ratio was 0.04‰ and 0.09‰ (n = 24). Furthermore, we were able to show that the measurement data also allowed for the quantification of the CO mole fraction, with a precision of 1.2 μmol mol in the analysis range of 400-500 μmol mol .

Conclusions: Our protocol provides a detailed description of the measurement set-up and the analysis procedure, how raw data should be evaluated and gives recommendations for sample preparation and sampling to enable a fully automated whole air sample analysis. The quantification limit of CO mole fractions and measurement precision for carbon and oxygen isotope ratios of CO should meet the requirements of a wide range of users.