An important constraint in developing models of p-process nucleosynthesis is that the abundances of many of the p-process nuclides are not well known. A recent review of the p-process has identified six p-process nuclides that are of particular significance to p-process theorists [M. Arnould and S. Goriely, Phys. Rep. 384, 1 (2003)]. These nuclides are 92,94 Mo, 96,98 Ru, 138 La, and 180 Ta m . The absence of accurate abundances for these isotopes is due to the fact that the isotopic

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... of the elements concerned have not been corrected for isotope fractionation induced by the thermal ionization mass spectrometric instruments used to measure them. To remedy this deficiency, a VG 354 mass spectrometer was calibrated using gravimetric mixtures of enriched isotopes to enable the absolute isotopic compositions of these elements to be obtained. Although the isotopic abundances of 92,94 Mo, 138 La, and 180 Ta m have previously been reported, the absolute abundances of 96,98 Ru are reported for the first time in this article, with a significant reduction in the magnitude of the values as compared to existing abundances.