text: 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 (H) to hydrogen-1 in a substance x may be given as
  where  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  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 H  = 50‰, i.e., an enrichment of 5%;
  * δ O  = 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 .
  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 O concentration and a 0.2‰ lower H 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  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 (O) and deuterium (H) be reported relative to VSMOW, on a scale such that the  of SLAP is −55.5‰ and the  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 O, and O in the new standards from the old standards are zero within the error of measurement. There is a small but measurable deviation of H concentration in SLAP2 from SLAP— 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.
  The older two standards are now kept at the IAEA and no longer sold.