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

Abstract

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.