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

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 17O-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 17O-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 λ of the three-isotope fractionation line and the ordinate axis intercept 103 ln(1 + k) on a 103 ln(1 + δ17O) versus 103 ln(1 + δ18O) plot. At the heart of the approach is the relationship between 17R, which is the 17O/16O ratio of the sample CO2, and other assigned or empirically determined parameters needed for the δ13C evaluation:218Rref17R(1+k)17Rref1λ-3(17R)2+217R45R-46R=0With 45R and 46R as the respective 45/44 and 46/44 ion abundance ratios of the sample, as obtained by measurement of δ45(CO2) and δ46(CO2) values reported relative to a reference material (usually VPDB-CO2), and 17Rref and 18Rref being the respective 17O/16O and 18O/16O ratios in the same reference material, 17R can readily be obtained by numerical methods, for given λ 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 δ13C 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 17O and 18O.