Vienna Standard Mean Ocean Water
Published and distributed by the Vienna-based International Atomic Energy Agency in 1968, the standard and its essentially identical successor, VSMOW2, continue to be used as a reference material.
[5] To resolve the confusion, November 1966 meeting of the Vienna-based International Atomic Energy Agency (IAEA) recommended the preparation of two water isotopic standards: Vienna SMOW (VSMOW; initially just "SMOW" but later disambiguated[5]) and Standard Light Antarctic Precipitation (SLAP).
[7] A standard with oxygen-18 and deuterium concentrations between that of VSMOW and SLAP, called Greenland Ice Sheet Precipitation (GISP), was also prepared.
[7] The IAEA began distributing samples in 1968, and Gonfiantini (1978) compiled analyses of VSMOW and SLAP from 45 laboratories around the world.
[2] Due to confusion over multiple water standards, the Commission on Isotopic Abundances and Atomic Weights recommended in 1994 that all future isotopic measurements of oxygen-18 (18O) and deuterium (2H) be reported relative to VSMOW, on a scale such that the δ18O of SLAP is −55.5‰ and the δ2H of SLAP is −428‰, relative to VSMOW.
About 300 liters was prepared from a mixture of distilled waters, from Lake Bracciano in Italy, the Sea of Galilee in Israel, and a well in Egypt, in proportions chosen to reach VSMOW isotopic ratios.
The IAEA also published a successor to SLAP, called SLAP2, derived from melted water from four Antarctic drilling sites.
Measurements of particular combinations of oxygen and hydrogen isotopes are unnecessary because water molecules constantly exchange atoms with each other.
Except for tritium, which was determined by the helium gas emitted by radioactive decay, these measurements were taken using mass spectroscopy.
That is, SLAP was measured to contain approximately 5.55% less oxygen-18 and 42.8% less deuterium than does VSMOW, and these figures were used to anchor the scale at two points.
[9] The physical samples, which are distributed by the IAEA and U.S. National Institute of Standards and Technology, are used to calibrate isotope-measuring equipment.
[28] In 1954, the International Committee for Weights and Measures (CIPM) established the definition of the Kelvin as 1/273.16 of the absolute temperature of the triple point of water.
Thus, the International Committee for Weights and Measures specified in 2005[29] that the definition of the kelvin temperature scale would refer to water with a composition of the nominal specification of VSMOW.
[33] After the 2019 revision of the SI, the kelvin is defined in terms of the Boltzmann constant, which makes its definition completely independent of the properties of water.
