A new method to determine the 17 O isotopic abundance in CO 2 using oxygen isotope exchange with a solid oxide

• Published in: Rapid communications in mass spectrometry 2001-12, Vol.15 (24), p.2426-2437
• Main Authors: Assonov, S.S., Brenninkmeijer, C. A. M.
• Format: Article
• Language: English
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.529

This paper discusses a simple method to determine 17 O isotope excess or deficiency (‘mass‐independent isotopic composition’) in CO 2 gas. When applying conventional mass spectrometry of CO 2 ( m/z 44, 45 and 46) to determine the 17 O/ 16 O ratio, the 13 C/ 12 C ratio has to be established separately. This can be achieved by analysing an aliquot of sample CO 2 before and after subjecting it to oxygen isotope exchange with a pool of oxygen with ‘normal’ 17 O/ 16 O ratio, i.e. with Δ 17 O≡δ 17 O−0.516 × δ 18 O = 0. Cerium oxide has been shown to be practically well suited for the exchange of CO 2 oxygen; the reagent is safe and does not produce any contamination. The CO 2 ‐CeO 2 exchange reaction has 99.8 ± 0.7% recovery yield. At 650 °C this reaction reaches equilibrium in 30 min and, as tested, results in complete oxygen replacement. Δ 17 O determinations depend on accuracy of CO 2 delta measurements: the repeatability of ±0.015‰ (1σ) in δ 45 R and δ 46 R deter‐mination relative to the working reference results in an error of Δ 17 O as small as ±0.33‰. Such a precision is sufficient for Δ 17 O determination in stratospheric CO 2 . The calculated Δ 17 O value systematically depends on absolute 17 R and 13 R ratios in isotopic reference materials, which are presently not yet known with certainty (the 17 R value is most important), and may be inadequate for 17 O‐correction with a = 0.516. Within the present uncertainty, Δ 17 O determined in 17 O‐enriched CO 2 agrees with the value directly measured in the enriched O 2 from which this CO 2 was produced. Besides Δ 17 O determination, investigated CO 2 ‐CeO 2 equilibration may have several other implications. Fast, complete isotopic exchange of CO 2 by reaction with CeO 2 may also be employed to get reproducible 17 O‐correction and, hence, to better monitor small δ 13 C shifts and to isotopically equilibrate mixtures of CO 2 gases. Copyright © 2001 John Wiley & Sons, Ltd.