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

Rationale: SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ O value is -55.5‰, although several expert laboratories measure significantly more negative δ O values. The real δ O 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. This study describes the quantification of δ O with respect to δ O .

Methods: SLAP-like water was quantitatively mixed with highly O-enriched water to mimic VSMOW. The O concentration was determined using an electron ionization quadrupole mass spectrometer. The isotopic composition of the SLAP-like and VSMOW-like waters was measured using an optical spectrometer, alongside original VSMOW and SLAP.

Results: This study resulted in a much more negative δ O value for SLAP than expected. The averaged outcome of seven independent experiments is δ O  = -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 δ O value.

Conclusions: Although this finding as such does not influence the use of the VSMOW-SLAP scale, it raises the intriguing question of 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 δ O from and to the VPDB scale, various fractionation factors, and Δ O. The absolute O abundance for SLAP was calculated as (1887.98 ± 0.43) × 10 based on the absolute O abundance of VSMOW and the presented δ O in this paper.