The absolute δ18O value for SLAP with respect to VSMOW reveals a much lower value than previously established.

Abstract
RATIONALE: SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ18O value is -55.5‰, although several expert laboratories measure significantly more negative δ18OSLAP values. The real δ18OSLAP value as such does not influence the isotopic water scale, however knowledge of the size of isotopic scale contraction in stable isotope measurements is vital for second order isotopes. In this study quantification of δ18OSLAP with respect to δ18OVSMOW is described.

METHODS: SLAP-like water was quantitatively mixed with highly 18O enriched water to mimic VSMOW. The 18O concentration was determined using an electron ionization quadrupole mass spectrometer. The isotopic composition of the SLAP-like and VSMOW-like waters were measured with an optical spectrometer, alongside original VSMOW and SLAP.
RESULTS: This study resulted in a much more negative δ18O value for SLAP than expected. The averaged outcome of 7 independent experiments is δ18OSLAP -56.33 ± 0.03‰. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating isotope laboratories and the true δ18O value.

CONCLUSIONS: Although this finding as such does not influence the use of the VSMOW-SLAP scale, it raises the intriguing question what we actually measure with our instruments, and why even a fully corrected measurement can be so far off. Our result has consequences for issues like the transfer of δ18O from and to the VPDB scale, various fractionation factors, and the Δ17O. The absolute 18O abundance for SLAP was calculated at (1887.98 ± 0.43) x 10-6 based on the absolute 18O abundance of VSMOW and the presented δ18OSLAP in this paper.