The absolute δ 18 O value for SLAP with respect to VSMOW reveals a much
RATIONALE: SLAP is one of the two calibration materials for the
isotopic water scale. By consensus the established δ
18O value is -55.5‰, although there are firm
indications that δ 18O SLAP is
significantly more negative. The real δ 18O
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. In this
study quantification of δ 18O
SLAP with respect to δ 18O
VSMOW 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 real VSMOW and SLAP.
RESULTS: This study resulted in a much more depleted δ
18O value for SLAP than expected. The averaged outcome
of 7 independent experiments is δ 18O
SLAP -56.33 ± 0.03‰. There is a large discrepancy
between the actual isotopic measurements of even the most carefully
operating groups and the true δ 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 of δ 18O from and to the VPDB
scale, various fractionation factors, and the Δ
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 presented
δ 18O SLAP in this paper.