The absolute δ¹⁸ O value for SLAP with respect to VSMOW reveals a much lower value than previously established

SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ¹⁸ O value is -55.5‰, although several expert laboratories measure significantly more negative δ¹⁸ O_SLAP values. The real δ¹⁸ O_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 δ¹⁸ O_SLAP with respect to δ¹⁸ O_VSMOW .

SLAP-like water was quantitatively mixed with highly ¹⁸ O-enriched water to mimic VSMOW. The ¹⁸ 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.

This study resulted in a much more negative δ¹⁸ O value for SLAP than expected. The averaged outcome of seven independent experiments is δ¹⁸ O_SLAP  = -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 δ¹⁸ O value.

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 δ¹⁸ O from and to the VPDB scale, various fractionation factors, and Δ¹⁷ O. The absolute ¹⁸ O abundance for SLAP was calculated as (1887.98 ± 0.43) × 10^-6 based on the absolute ¹⁸ O abundance of VSMOW and the presented δ¹⁸ O_SLAP in this paper.