Reformulated (super 17) O correction of mass spectrometric stable isotope measurements in carbon dioxide and a critical appraisal of historic absolute carbon and oxygen isotope ratios
Kaiser Jan
Geochimica et Cosmochimica Acta 72(5): 1312-1334
2008
ISSN/ISBN: 0016-7037 Accession: 023484445
Mass-spectrometric stable isotope measurements of CO (sub 2) use molecular ion currents at mass-to-charge ratios m/z 44, 45 and 46 to derive the elemental isotope ratios n( (super 13) C)/n( (super 12) C) and n( (super 18) O)/n( (super 16) O), abbreviated (super 13) C/ (super 12) C and (super 18) O/ (super 16) O, relative to a reference. The ion currents have to be corrected for the contribution of (super 17) O-bearing isotopologues, the so-called ' (super 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 existing (super 13) C/ (super 12) C ( (super 13) R), (super 17) O/ (super 16) O ( (super 17) R) and (super 18) O/ (super 16) O ( (super 18) R) calibrations of the reference material Vienna Standard Mean Ocean Water (VSMOW) and CO (sub 2) generated from the reference material Vienna Pee Dee Belemnite (VPDB) by reaction with 100% H (sub 3) PO (sub 4) at 25 degrees C (VPDB-CO (sub 2) ). I find (super 17) R (sub VSMOW) /10 (super -6) =382.7 (sub -2.1) (super +1.7) , (super 18) R (sub VSMOW) /10 (super -6) =2005.20+ or -0.45, (super 13) R (sub VPDB-CO2) /10 (super -6) =11124+ or -45, (super 17) R (sub VPDB-CO2) /10 (super -6) =391.1 (sub -2.1) (super +1.7) and (super 18) R (sub VPDB-CO2) /10 (super -6) =2088.37+ or -0.90. I also rephrase the calculation scheme for the (super 17) O correction completely in terms of relative isotope ratio differences (delta values). This reveals that only ratios of isotope ratios (namely, (super 17) R/ (super 13) R and (super 13) R (super 17) R/ (super 18) R) are required for the (super 17) 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 relative (super 13) C/ (super 12) C difference, but not for (super 18) O/ (super 16) O. Even though inter-laboratory differences can be corrected for by a common 'ratio assumption set' and/or normalisation, the ultimate accuracy of the (super 17) O correction is hereby limited. Errors of similar magnitude can be introduced by the assumed mass-dependent relationship between (super 17) O/ (super 16) O and (super 18) O/ (super 16) O isotope ratios. For highest accuracy in the (super 13) C/ (super 12) C ratio, independent triple oxygen isotope measurements are required. Finally, I propose an experiment that allows direct measurement of (super 13) R (super 17) R/ (super 18) R.