Reformulated17 O correction of mass spectrometric stable isotope measurements in carbon dioxide and a critical appraisal of historic absolute carbon and oxygen isotope ratios
Geochimica et Cosmochimica Acta 72.5: 1312-1334
ISSN/ISBN: 0016-7037 DOI: 10.1016/j.gca.2007.12.011
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(13 C)/n(12 C) and n(18 O)/n(16 O), abbreviated13 C/12 C and18 O/16 O, relative to a reference. The ion currents have to be corrected for the contribution of17 O-bearing isotopologues, the so-called '17 O 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 existing13 C/12 C (13 R),17 O/16 O (17 R) and18 O/16 O (18 R) calibrations of the reference material Vienna Standard Mean Ocean Water (VSMOW) and CO2 generated from the reference material Vienna Pee Dee Belemnite (VPDB) by reaction with 100% H3 PO4 at 25 degrees C (VPDB-CO2 ). I find17 R(VSMOW) /10(-6) =382.7(-2.1)(+1.7) ,18 R(VSMOW) /10(-6) =2005.20+ or -0.45,13 R(VPDB-CO2) /10(-6) =11124+ or -45,17 R(VPDB-CO2) /10(-6) =391.1(-2.1)(+1.7) and18 R(VPDB-CO2) /10(-6) =2088.37+ or -0.90. I also rephrase the calculation scheme for the17 O correction completely in terms of relative isotope ratio differences (delta values). This reveals that only ratios of isotope ratios (namely,17 R/13 R and13 R17 R/18 R) are required for the17 O 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 relative13 C/12 C difference, but not for18 O/16 O. Even though inter-laboratory differences can be corrected for by a common 'ratio assumption set' and/or normalisation, the ultimate accuracy of the17 O correction is hereby limited. Errors of similar magnitude can be introduced by the assumed mass-dependent relationship between17 O/16 O and18 O/16 O isotope ratios. For highest accuracy in the13 C/12 C ratio, independent triple oxygen isotope measurements are required. Finally, I propose an experiment that allows direct measurement of13 R17 R/18 R.