A general algorithm for the O-17 abundance correction to C-13/C-12 determinations from CO2 isotopologue measurements, including CO2 characterised by 'mass-independent' oxygen isotope distributions

Miller, M. F.; Röckmann, T. and Wright, Ian (2007). A general algorithm for the O-17 abundance correction to C-13/C-12 determinations from CO2 isotopologue measurements, including CO2 characterised by 'mass-independent' oxygen isotope distributions. Geochimica Et Cosmochimica Acta, 71(13) pp. 3145–3161.

DOI: https://doi.org/10.1016/j.gca.2007.03.007

Abstract

It is widely recognised that a significant limitation to the ultimate precision of carbon stable isotope ratio measurements, as obtained from dual-inlet mass spectrometric measurements Of CO2 isotopologue ion abundances at m/z 44, 45, and 46, is the correction for interference from O-17-bearing molecular ions. Two long-established, alternative procedures for determining the magnitude of this correction are in widespread use (although only one has IAEA approval); their differences lead to small but potentially significant discrepancies in the magnitude of the resulting correction. Furthermore, neither approach was designed to accommodate oxygen three-isotope distributions which do not conform to terrestrial mass-dependent behaviour. Stratospheric CO2, for example, contains a strongly 'mass-independent' oxygen isotope composition. A new strategy for determining the O-17-bearing ion correction is presented, for application where the oxygen three-isotope characteristics of the analyte CO2 are accurately known (or assigned) in terms of the slope lambda of the three-isotope fractionation line and the ordinate axis intercept 10(3) In(1 + k) on a 10(3) In(1 +delta O-17) versus 10(3) ln(1 + delta O-18) plot. At the heart of the approach is the relationship between R-17, which is the O-17/O-16 ratio of the sample CO2, and other assigned or empirically determined parameters needed for the delta C-13 evaluation: 2 R-18(ref)[R-17/(1+k)R-17(ref)](1/lambda) -3(R-17)(2) + 2 (RR)-R-17-R-45 -R-46 = 0 With R-45 and R-46 as the respective 45/44 and 46/44 ion abundance ratios of the sample, as obtained by measurement of delta(45)(CO2) and delta(46)(CO2) values reported relative to a reference material (usually VPDB-CO2), and R-17(ref) and R-18(ref) being the respective O-17/O-16 and O-18/O-16 ratios in the same reference material, R-17 can readily be obtained by numerical methods, for given lambda and k values. The correction procedure involves no approximations, in principle, and is equally applicable to CO2 of terrestrial, mass-dependent oxygen isotopic composition, as well as to more 'exotic' sources. Besides isotopic characterisation of stratospheric CO2, potential applications include high precision delta C-13 measurements of CO2 derived from the oxidation of tropospheric CO (also characterised by significantly 'mass-independent' oxygen isotopic composition); high precision isotopic monitoring of atmospheric CO2; the metrology of carbonate isotopic reference materials; and the isotopic characterisation of CO2 which has been equilibrated with waters artificially 'labelled' with known enrichments of O-17 and O-18. (c) 2007 Elsevier Ltd. All rights reserved.