Reformulated 17O correction of mass spectrometric stable isotope measurements in carbon dioxide and a critical appraisal of historic ‘absolute’ carbon and oxygen isotope ratios

Abstract

Mass-spectrometric stable isotope measurements of CO 2 use molecular ion currents at mass-to-charge ratios m/ z 44, 45 and 46 to derive the elemental isotope ratios n( 13C)/ n( 12C) and n( 18O)/ n( 16O), abbreviated 13C/ 12C and 18O/ 16O, relative to a reference. The ion currents have to be corrected for the contribution of 17O-bearing isotopologues, the so-called ‘ 17O correction’. The magnitude of this correction depends on the calibrated isotope ratios of the reference. Isotope ratio calibrations are difficult and are therefore a matter of debate. Here, I provide a comprehensive evaluation of the existing 13C/ 12C ( 13 R), 17O/ 16O ( 17 R) and 18O/ 16O ( 18 R) calibrations of the reference material Vienna Standard Mean Ocean Water (VSMOW) and CO 2 generated from the reference material Vienna Pee Dee Belemnite (VPDB) by reaction with 100% H 3PO 4 at 25 °C (VPDB-CO 2). I find 17 R VSMOW / 10 - 6 = 382.7 - 2.1 + 1.7 , 18 R VSMOW/10 −6 = 2005.20 ± 0.45, 13 R VPDB- CO 2 / 10 - 6 = 11124 ± 45, 17 R VPDB- CO 2 / 10 - 6 = 391.1 - 2.1 + 1.7 and 18 R VPDB- CO 2 / 10 - 6 = 2088.37 ± 0.90 . I also rephrase the calculation scheme for the 17O correction completely in terms of relative isotope ratio differences ( δ values). This reveals that only ratios of isotope ratios (namely, 17 R/ 13 R and 13 R 17 R/ 18 R) are required for the 17O correction. These can be, and have been, measured on conventional stable isotope mass spectrometers. I then show that the remaining error for these ratios of isotope ratios can lead to significant uncertainty in the derived relative 13C/ 12C difference, but not for 18O/ 16O. Even though inter-laboratory differences can be corrected for by a common ‘ratio assumption set’ and/or normalisation, the ultimate accuracy of the 17O correction is hereby limited. Errors of similar magnitude can be introduced by the assumed mass-dependent relationship between 17O/ 16O and 18O/ 16O isotope ratios. For highest accuracy in the 13C/ 12C ratio, independent triple oxygen isotope measurements are required. Finally, I propose an experiment that allows direct measurement of 13 R 17 R/ 18 R.