1. INTRODUCTION
The stable isotope scale of water has been successfully established and maintained by the two primary reference waters: VSMOW and SLAP. In principle, only one reference material per isotope and per medium would be needed to define the isotopic scale, but two-point calibration leads to a dramatic improvement in inter-laboratory comparisons, due to various and variable scale contraction processes occurring in each measurement process.
In 1976 during a consultants’ meeting on stable isotopes at the IAEA in Vienna, δ 18O measurements of SLAP from 45 laboratories were evaluated. The data showed a rather large spread with measurements ranging between was from -54.53‰ to -56.5‰. The averagedδ 18O value for SLAP was -55.49‰, and the standard deviation was 0.55‰ (2 data points were considered as outliers (-49.2 and -53.92‰)). During this meeting, it was agreed thatδ 18O SLAP would be established at the consensus value of -55.5‰ (Gonfiantini2,3).
For deuterium, the other stable isotope of water, it is possible to (re)produce the primary reference waters based on gravimetric mixtures of isotopically pure waters. In this way, the absolute deuterium abundances of VSMOW and SLAP has been precisely determined by several authors (Hagemann4, De Wit5, Tse6).
A similar experiment for oxygen is much harder, as pure18O and 16O waters are not available. Only (Baertschi7) has performed a very extensive experiment, resulting in the absolute 18O abundance of VSMOW, with a relative precision of 0.2‰.
In this study, we take the next step, namely determination of theδ 18O of SLAP with respect to VSMOW. Instead of determining the absolute abundance of SLAP, we focus on the relative difference in δ 18O between VSMOW and SLAP, which we aim to achieve with much higher precision (≤ 0.05‰). In this way, we achieve a more accurate S.I. traceable result for the VSMOW-SLAP scale.
We quantify the difference in δ 18O between VSMOW and SLAP by gravimetrical mixing of a SLAP-like water with highly18O enriched water to mimic VSMOW and compare this with real VSMOW.
Although the real δ 18OSLAPvalue as such, does not influence the use of the VSMOW-SLAP scale, the various measurements from Gonfiantini3 from -54.53 to -56.5‰, and from Verkouteren and Klinedinst1, Barkan and Luz8, pointed out by Kaiser9, from -55.11 to -56.18‰, raise the intriguing question what the actual value is. This real value can play an important role in understanding IRMS issues, such as scale contraction caused by memory effects. Understanding such IRMS side-effects is essential to work with a well-maintained instrument and for correcting measurements accordingly. Ideally, isotopic measurements from mass spectrometers and optical spectroscopic instruments, should be very close to their actual values. This is especially important if the isotopic values for different materials have to be compared, for example δ 18O in carbonates, or in atmospheric CO2, in relation to that of water. Furthermore, recent years have seen more complex, ’second order’ isotope work, like exploiting the very small differences in behavior between 17O and 18O (expressed as 17O excess, Δ17O) (Hofmann10, Landais11) and the deviation from stochastic distribution of the rare isotopes in molecules (’clumped isotopes’) (Eiler12, Bernasconi13). Also in these fields, understanding (and correcting for) instrument-related isotope effects is crucial.