The absolute δ18O value for SLAP with respect to VSMOW reveals a much lower value.
Anita Th. Aerts-Bijma , Albert C. van Buuren, Dipayan Paul, Harro A.J. Meijer
Centre for Isotope Research (CIO), Energy and Sustainability Research Institute Groningen, University of Groningen, Groningen, The Netherlands
Corresponding author: a.t.aerts-bijma@rug.nl
Abstract
RATIONALE : SLAP is one of the two calibration materials for the isotopic water scale. By consensus the establishedδ 18O value is -55.5‰, although there are firm indications that δ 18OSLAP is significantly more negative. The realδ 18OSLAP 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 ofδ 18OSLAP with respect toδ 18OVSMOW is described.
METHODS:  SLAP-like water was quantitatively mixed with highly18O enriched water to mimic VSMOW. The18O 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 real VSMOW and SLAP.
RESULTS:  This study resulted in a much more depletedδ 18O value for SLAP than expected. The averaged outcome of 7 independent experiments isδ 18OSLAP -56.33 ± 0.03‰. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating groups and the trueδ 18O 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 ofδ 18O from and to the VPDB scale, various fractionation factors, and theΔ 17O. The absolute 18O abundance for SLAP was determined at 1.88798 (43) x 10-3 based on the absolute 18O abundance of VSMOW and the presentedδ 18OSLAP in this paper.
Key Words
Water isotopic scale, SLAP, VSMOW, δ 18O, calibration, IRMS, optical spectroscopy, electron ionization quadrupole mass spectrometer (EI-QMS), 18O abundance, VSMOW-CO2, VPDB-CO2