A redetermination of absolute values for 17RVPDB-CO2 and 17RVSMOW.

In a companion paper in this issue we presented a review of the current state of (17)O-corrections for CO(2) mass spectrometry and considered an approach (including algebraic formulae) of how to determine absolute values for (17)R(VPDB-CO2) and (17)R(VSMOW). Here we present the results of experiments conducted to determine these values. Two oxygen gases (one depleted in heavy isotopes and the other isotopically normal oxygen) were analysed to obtain the relative (17)O content. Samples of both gases were converted into CO(2), and the resulting CO(2) samples were analysed as well. Possible experimental and analytical errors are carefully considered and eliminated as far as feasible. Much attention was paid to understanding and dealing with cross-contamination effects occurring in the mass spectrometer. Based on the data obtained, the absolute values are calculated to be: (17)R(VPDB-CO2) = 0.00039511 +/- 0.00000094 and (17)R(VSMOW) = 0.00038672 +/- 0.00000087 (expanded uncertainties). Both values are on the original scale of Craig (Geochim. Cosmochim. Acta 1957; 12: 133-149) with (13)R(VPDB-CO2) = 0.0112372. A (17)O-correction algorithm incorporating the newly determined value for (17)R(VPDB-CO2) and lambda = 0.528 by Meijer and Li (Isot. Environ. Health Stud. 1998; 34: 349-369) is constructed. A computational test is performed to demonstrate the degree of delta(13)C bias relative to the previously known correction algorithms. delta(13)C values produced by the constructed algorithm are in the middle of the values produced by the other algorithms. We refrain, however, from giving any recommendation concerning which (17)O-correction algorithm to use in order to obtain delta(13)C data in the most accurate way. The present work illuminates the need to reconsider recommendations concerning the correction algorithm.