A general algorithm for the 17O abundance correction to 13C/ 12C determinations from CO 2 isotopologue measurements, including CO 2 characterised by ‘mass-independent’ oxygen isotope distributions

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 CO 2 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 CO 2, 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 CO 2 are accurately known (or assigned) in terms of the slope λ of the three-isotope fractionation line and the ordinate axis intercept 10 3 ln(1 + k) on a 10 3 ln(1 + δ 17O) versus 10 3 ln(1 + δ 18O) plot. At the heart of the approach is the relationship between 17R, which is the 17O/ 16O ratio of the sample CO 2, and other assigned or empirically determined parameters needed for the δ 13C evaluation: 2 18 R ref 17 R ( 1 + k ) 17 R ref 1 λ - 3 ( 17 R ) 2 + 2 17 R 45 R - 46 R = 0 With 45R and 46R as the respective 45/44 and 46/44 ion abundance ratios of the sample, as obtained by measurement of δ 45(CO 2) and δ 46(CO 2) values reported relative to a reference material (usually VPDB-CO 2), and 17R ref and 18R ref 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 CO 2 of terrestrial, mass-dependent oxygen isotopic composition, as well as to more ‘exotic’ sources. Besides isotopic characterisation of stratospheric CO 2, potential applications include high precision δ 13C measurements of CO 2 derived from the oxidation of tropospheric CO (also characterised by significantly ‘mass-independent’ oxygen isotopic composition); high precision isotopic monitoring of atmospheric CO 2; the metrology of carbonate isotopic reference materials; and the isotopic characterisation of CO 2 which has been equilibrated with waters artificially ‘labelled’ with known enrichments of 17O and 18O.