Andrea Watzinger

Rationale The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO 2 ) 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 2 using off‐the‐bench hardware and software. Methods The fully automated system uses cryogenic and gas chromatographic separation to analyse CO 2 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 2 sources (four CO 2 reference gases and four artificial air tanks). Results The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO 2 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 2 mole fraction, with a precision of 1.2 μmol mol −1 in the analysis range of 400–500 μmol mol −1 . 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 2 mole fractions and measurement precision for carbon and oxygen isotope ratios of CO 2 should meet the requirements of a wide range of users.