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

This paper presents an analytical system for analysis of all single substituted isotopologues (COO, COO, COO) in nanomolar quantities of CO2 extracted from stratospheric 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 O excess (1O) 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 1O requires two injections of 1 mL of air with a CO2 mole fraction of 390 μmol mol at 293 K and 1 bar pressure (corresponding to 16 nmol CO2 each). The required sample size (including flushing) is 2.7 mL of air. A single analysis (one pair of injections) takes 15 minutes. The analytical system is fully automated for unattended measurements over several days. The standard deviation of the O excess analysis is 1.7 ‰. Multiple measurements on an air sample reduce the measurement uncertainty, as expected for the statistical standard error. Thus, the uncertainty for a group of 10 measurements is 0.58 ‰ for 1O in 2.5 h of analysis. 100 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 δC, 0.07 ‰ (1σ) for δO and 0.55 ‰ (1σ) for δO for a sample of 10 measurements. This is sufficient to examine stratospheric enrichments, which at altitude 33 km go up to 12 ‰ for δO and up to 8 ‰ for δO with respect to tropospheric CO2 : δ O ≈ 21 ‰ Vienna Standard Mean Ocean Water (VSMOW), δO ≈ 41 ‰ VSMOW (Lämmerzahl et al., 2002). The samples measured with our analytical technique agree with available data for stratospheric CO2.