The absolute ¹⁸O 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 delta O-18 value is -55.5 parts per thousand, although several expert laboratories measure significantly more negative delta O-18(SLAP) values. The real delta O-18(SLAP) 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 delta O-18(SLAP) with respect to delta O-18(VSMOW). Methods: SLAP-like water was quantitatively mixed with highly O-18-enriched water to mimic VSMOW. The O-18 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 delta O-18 value for SLAP than expected. The averaged outcome of seven independent experiments is delta O-18(SLAP) = -56.33 +/- 0.03 parts per thousand. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating isotope laboratories and the true delta O-18 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 delta O-18 from and to the VPDB scale, various fractionation factors, and Delta O-17. The absolute O-18 abundance for SLAP was calculated as (1887.98 +/- 0.43) x 10(-6) based on the absolute O-18 abundance of VSMOW and the presented delta O-18(SLAP) in this paper.