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