Continuous-flow IRMS technique for determining the .sup.17O excess of CO.sub.2 using complete oxygen isotope exchange with cerium oxide

• Published in: Atmospheric measurement techniques 2015-02, Vol.8 (2), p.811
• Main Authors: Mrozek, D. J, van der Veen, C, Kliphuis, M, Kaiser, J, Wiegel, A. A, Röckmann, T
• Format: Article
• Language: English
• Subjects: Analysis; Cerium; Mass spectrometry; Methods; Rare earth metal compounds
• ISSN: 1867-1381; 1867-8548

This paper presents an analytical system for analysis of all single substituted isotopologues (.sup.12 C.sup.16 O.sup.17 O, .sup.12 C.sup.16 O.sup.18 O, .sup.13 C.sup.16 O.sup.16 O) in nanomolar quantities of CO.sub.2 extracted from stratospheric air samples. CO.sub.2 is separated from bulk air by gas chromatography and CO.sub.2 isotope ratio measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio mass spectrometry (IRMS). The .sup.17 O excess (Δ.sup.17 O) is derived from isotope measurements on two different CO.sub.2 aliquots: unmodified CO.sub.2 and CO.sub.2 after complete oxygen isotope exchange with cerium oxide (CeO.sub.2) at 700 °C. Thus, a single measurement of Δ.sup.17 O requires two injections of 1 mL of air with a CO.sub.2 mole fraction of 390 μmol mol.sup.-1 at 293 K and 1 bar pressure (corresponding to 16 nmol CO.sub.2 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 .sup.17 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 Δ .sup.17 O 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 CO.sub.2 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 [delta].sup.13 C, 0.07‰ (1σ) for [delta].sup.18 O and 0.55‰ (1σ) for δ.sup.17 O for a sample of 10 measurements. This is sufficient to examine stratospheric enrichments, which at altitude 33 km go up to 12‰ for δ.sup.17 O and up to 8‰ for [delta].sup.18 O with respect to tropospheric CO.sub.2 : δ.sup.17 O ~ 21‰ Vienna Standard Mean Ocean Water (VSMOW), [delta].sup.18 O ~ 41‰ VSMOW (Lämmerzahl et al., 2002). The samples measured with our analytical technique agree with available data for stratospheric CO.sub.2.