Vienna Standard Mean Ocean Water

Water samples made up of different isotopes of hydrogen and oxygen have slightly different physical properties. As an extreme example, heavy water, which contains two deuterium (²H) atoms instead of the usual, lighter hydrogen-1 (¹H), has a melting point of 3.82 °C (38.88 °F) and boiling point of 101.4 °C (214.5 °F). Different rates of evaporation cause water samples from different places in the water cycle to contain slightly different ratios of isotopes. Ocean water (richer in heavy isotopes) and rain water (poorer in heavy isotopes) roughly represent the two extremes found on Earth. With VSMOW, the IAEA simultaneously published an analogous standard for rain water, Standard Light Antarctic Precipitation (SLAP), and eventually its successor SLAP2. SLAP contains about 5 % less oxygen-18 and 42.8 % less deuterium than VSMOW.

A scale based on VSMOW and SLAP is used to report oxygen-18 and deuterium concentrations. From 2005 until its redefinition in 2019, the kelvin was specified to be 1/273.16 of the temperature of specifically VSMOW at its triple point.

History and background

Abundances of a particular isotope in a substance are usually given relative to some reference material, as a delta in parts per thousand (‰) from the reference. For example, the ratio of deuterium (²H) to hydrogen-1 in a substance x may be given as

${\displaystyle \delta {}^2{\mathrm{H}}_{x/\text{reference}}=\left(\frac{{(}^2\mathrm{H}{/}^1\mathrm{H}{)}_x}{{(}^2\mathrm{H}{/}^1\mathrm{H}{)}_{\text{reference}}}-1\right)\times 1000\text{ ‰}}$,

where ${\displaystyle {(}^2\mathrm{H}{/}^1\mathrm{H}{)}_x}$ denotes the absolute concentration in x.

In 1961, pursuing a standard for measuring and reporting deuterium and oxygen-18 concentrations, Harmon Craig of the Scripps Institution of Oceanography in San Diego, California, proposed an abstract water standard. He based the proportions on his measurements of samples taken by Epstein & Mayeda (1953) of ocean waters around the world. Approximating an average of their measurements, Craig defined his "standard mean ocean water" (SMOW) relative to a water sample held in the United States' National Bureau of Standards called NBS-1 (sampled from the Potomac River). In particular, SMOW had the following parameters relative to NBS-1:

• δ ²H _SMOW/NBS-1 = 50 ‰, i.e., an enrichment of 5 %;
• δ ¹⁸O _SMOW/NBS-1 = 8 ‰, i.e., an enrichment of 0.8 %.

Later, researchers at the California Institute of Technology defined another abstract reference, also called "SMOW", for oxygen-18 concentrations, such that a sample of Potsdam Sandstone in their possession satisfied δ¹⁸O _{sandstone/SMOW} = 15.5 ‰.

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) and Standard Light Antarctic Precipitation (SLAP). Craig prepared VSMOW by mixing distilled Pacific Ocean water with small amounts of other waters. VSMOW was intended to match the SMOW standard as closely as possible. Craig's measurements found an identical ¹⁸O concentration and a 0.2 ‰ lower ²H concentration. The SLAP standard was created from a melted firn sample from Plateau Station in Antarctica. A standard with oxygen-18 and deuterium concentrations between that of VSMOW and SLAP, called Greenland Ice Sheet Precipitation (GISP), was also prepared. The IAEA began distributing samples in 1968, and Gonfiantini (1978) compiled analyses of VSMOW and SLAP from 45 laboratories around the world. The VSMOW sample was stored in a stainless-steel container under nitrogen and was transferred to glass ampoules in 1977.

The deuterium and oxygen-18 concentrations in VSMOW are close to the upper end of naturally occurring materials, and the concentrations in SLAP are close to the lower end. 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. Therefore, SLAP is defined to contain 94.45 % the oxygen-18 concentration and 57.2 % the deuterium concentration of VSMOW. Using a scale with two defined samples improves comparison of results between laboratories.

In December 1996, because of a dwindling supply of VSMOW, the IAEA decided to create a replacement standard, VSMOW2. Published in 1999, it contains a nearly identical isotopic mixture. 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. Deviations of ¹⁷O, and ¹⁸O in the new standards from the old standards are zero within the error of measurement. There is a small but measurable deviation of ²H concentration in SLAP2 from SLAP—δ²H_SLAP2/VSMOW is defined to be −427.5 ‰ instead of −428 ‰—but not in VSMOW2 from VSMOW. The IAEA recommends that measurements still be reported on the VSMOW–SLAP scale.

SLAP is kept at the IAEA, and VSMOW is sold by the U.S. Geological Survey.