Stable isotope analysis of atmospheric CO2 using a Gasbench II – Cold Trap – IRMS setting

The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO₂) is a useful technique for the investigation and identification of the sources and sinks of the most abundant greenhouse gas 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 CO₂ using off the bench hard- and software.

The fully automated system uses cryogenic and gas chromatographic separation to analyse CO₂ from 12 mL whole air samples and consists of an autosampler, a Gasbench II, a downstream cryo trap and a continuous flow gas interface feeding into a sector field mass spectrometer (GC Pal/Gasbench II/Cold Trap/Conflo IV/DeltaV Plus). The evaluation of the system performance was based on the analysis of samples prepared from eight CO₂ sources (four CO₂ reference gases and four artificial air tanks).

The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO₂ 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 CO₂ mole fraction, with a precision of 1.2 µmol mol^-1 in the analysis range of 400 to 500 µmol mol^-1.

The method to be presented was summarized and published in the form of a protocol (DOI: 10.1002/rcm.9647) providing a detailed description of the measurement setup 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. We look forward to further discussion with interested users to elaborate on potential improvements/extensions/application options.