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

[摘要] This paper presents an analytical system for analysis of all singlesubstituted isotopologues (¹²C¹⁶O¹⁷O,¹²C¹⁶O¹⁸O, ¹³C¹⁶O¹⁶O) in nanomolar quantitiesof CO₂ extracted from stratospheric air samples. CO₂ isseparated from bulk air by gas chromatography and CO₂ isotope ratiomeasurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratiomass spectrometry (IRMS). The ¹⁷O excess (Δ¹⁷O) isderived from isotope measurements on two different CO₂ aliquots:unmodified CO₂ and CO₂ after complete oxygen isotope exchange withcerium oxide (CeO₂) at 700 °C. Thus, a single measurement ofΔ¹⁷O requires two injections of 1 mL of air with a CO₂mole fraction of 390 μmol mol⁻¹ 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 ofinjections) takes 15 minutes. The analytical system is fully automated forunattended measurements over several days. The standard deviation of the¹⁷O excess analysis is 1.7‰. Multiplemeasurements on an air sample reduce the measurement uncertainty, asexpected for the statistical standard error. Thus, the uncertainty for agroup of 10 measurements is 0.58‰ for Δ¹⁷O in 2.5 h of analysis. 100 repeat analyses of one air sampledecrease the standard error to 0.20‰. The instrumentperformance was demonstrated by measuring CO₂ on stratospheric airsamples obtained during the EU project RECONCILE with the high-altitudeaircraft Geophysica. The precision for RECONCILE data is 0.03‰ (1σ) for δ¹³C, 0.07‰ (1σ) for δ¹⁸O and 0.55‰ (1σ) for δ¹⁷O for a sample of 10measurements. This is sufficient to examine stratospheric enrichments, whichat altitude 33 km go up to 12‰ for δ¹⁷Oand up to 8‰ for δ¹⁸O with respect totropospheric CO₂ : δ¹⁷O ~21‰ Vienna Standard Mean Ocean Water (VSMOW), δ¹⁸O ~41‰ VSMOW (Lämmerzahl et al., 2002). The samplesmeasured with our analytical technique agree with available data forstratospheric CO₂.

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