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

Mass-spectrometric stable isotope measurements of CO2 use molecular ion currents at mass-to-charge ratios m/z 44, 45 and 46 to derive the elemental isotope ratios n(C-13)/n((12) C) and n(O-18)/n(O-16), abbreviated C-13/C-12 and O-18/O-16, relative to a reference. The ion currents have to be corrected for the contribution of O-17-bearing isotopologues, the so-called 'O-17 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 (R-13), O-17/O-16 (R-17) and O-18/O-16 (R-18) calibrations of the reference material Vienna Standard Mean Ocean Water (VSMOW) and CO) generated from the reference material Vienna Pee Dee Belemnite (VPDB) by reaction with 100% H3PO4 at 25 degrees C (VPDB-CO2). I find R-17(VSMOW)/10(-6) = 382.7(-2.1)(+1.7), R-18(VSMOW)/10(-6) = 2005.20 +/- 0.45, R-13(VPDB-CO2)/10(-6) = 11124 +/- 45, R-17(VPDB-CO2)/10(-6) = 391.1(-1.2)(+1.7) and R-18(VPDB-CO2)/10(-6) = 2088.37 +/- 0.90. I also rephrase the calculation scheme for the O-17 correction completely in terms of relative isotope ratio differences (delta values). This reveals that only ratios of isotope ratios (namely, R-17/R-13 and (RR)-R-13-R-17/R-18) are required for the O-17 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 C-13/C-12 difference, but not for O-18/O-16. Even though inter-laboratory differences can be corrected for by a common 'ratio assumption set and/or normalisation, the ultimate accuracy of the O-17 correction is hereby limited. Errors of similar magnitude can be introduced by the assumed mass-dependent relationship between O-17/O-16 and O-18/O-16 isotope ratios. For highest accuracy in the C-13/C-12 ratio, independent triple oxygen isotope measurements are required. Finally, I propose an experiment that allows direct measurement of (RR)-R-13-R-17/R-18. (c) 2007 Elsevier Ltd. All rights reserved.