Continuous-flow IRMS technique for determining the ¹⁷O excess of CO₂ using complete oxygen isotope exchange with cerium oxide

This paper presents an analytical system for analysis of all single substituted isotopologues (12C16O17O, 12C16O18O, 13C16O16O) in nanomolar quantities of CO2 extracted from atmospheric air samples. CO2 is separated from bulk air by gas chromatography and CO2 isotope ratio measurements (ion masses 45/44 and 46/44) are performed using isotope ratio mass spectrometry (IRMS). The 17O excess (Δ17O) is derived from isotope measurements on two different CO2 aliquots: unmodified CO2 and CO2 after complete oxygen isotope exchange with cerium oxide (CeO2) at 700 °C. Thus, a single measurement of the 17O excess requires two injections of 1 mL of air with a CO2 mole fraction of 390 μmol mol−1 at 293 K and 1 bar pressure (corresponding to 16 nmol CO2 each). The required sample air size (including flushing) is 2.7 mL of air. A single analysis (one pair of injections) takes 15 min. The analytical system is fully automated for unattended measurements over several days. The standard deviation of the 17O excess analysis is 1.7‰. Repeated analyses of an air sample reduce the measurement uncertainty, as expected for the statistical standard error. Thus, the uncertainty for a group of ten measurements is 0.58‰ for Δ17O in 2.5 h analysis. 270 repeat analyses of one air sample decrease the standard error to 0.20‰. The instrument performance was demonstrated by measuring CO2 on stratospheric air samples obtained during the EU project RECONCILE with the high-altitude aircraft Geophysica. The precision for RECONCILE data is 0.03‰ (1σ) for δ13C, 0.07‰ (1σ) for δ18O and 0.55‰ (1σ) for δ17O for sample of 10 measurements. The samples measured with our analytical technique agree with available data for stratospheric CO2.