Стандарт изотопного состава океанической воды VSMOW/SLAP

Standard defining the isotopic composition of ocean water

Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water, that is, a particular sample of water whose proportions of different isotopes of hydrogen and oxygen are accurately known. VSMOW is distilled from ocean water and does not contain salt or other impurities. 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. 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 (39 °F) and boiling point of 101.4 °C (215 °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.

All measurements are reported with their standard uncertainty. 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. Deuterium (2H / 1H) – 155.76 ± 0.05 ppm, about 1 in 6420 hydrogen atoms
Tritium (3H / 1H) – 18.5 ± 3.6 TU = , measured on 16 September 1976, about 1 in 5.401016 hydrogen atoms
Oxygen 18 (18O / 16O) – 2005.20 ± 0.45 ppm, about 1 in 499 oxygen atoms
Oxygen 17 (17O / 16O) – 379.9 ± 0.8 ppm, about 1 in 2640 oxygen atoms

The concentrations of 17O, and 18O are indistinguishable between VSMOW and VSMOW2, and between SLAP and SLAP2. The specification sheet gives the standard errors in these measurements. The concentration of 2H is unchanged in VSMOW2 as well, but is slightly increased in SLAP2. The IAEA reports:

δ2HSLAP2/VSMOW = −427.5 ± 0.3‰, (Compare −428‰ for SLAP.) On 6 July 2007, the tritium concentration was 3.5 ± 1.0 TU in VSMOW2, and 27.6 ± 1.6 TU in SLAP2.

δ 2H GISP −189.5 ± 1.2‰
δ 18O GISP −24.66 ± 0.09‰
δ 17O GISP −12.71 ± 0.1‰

The VSMOW–SLAP scale is recommended by the USGS, IUPAC, and IAEA for measurement of deuterium and 18O concentrations in any substance. For 18O, a scale based on Vienna Pee Dee Belemnite can also be used. 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. Variations in isotopic content are useful in hydrology, meteorology, and oceanography. Different parts of the ocean do have slightly different isotopic concentrations: δ 18O values range from –11.35‰ in water off the coast of Greenland to +1.32‰ in the north Atlantic, and δ 2H concentrations in deep ocean water range from roughly –1.7‰ near Antarctica to +2.2‰ in the Arctic. Variations are much larger in surface water than in deep water.

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. Waters with different isotopic compositions had slightly different triple points. Thus, the International Committee for Weights and Measures specified in 2005 that the definition of the kelvin temperature scale would refer to water with a composition of the nominal specification of VSMOW. The decision was welcomed in 2007 by Resolution 10 of the 23rd CGPM. The triple point is measured in triple point cells, where the water is held at its triple point and allowed to reach equilibrium with its surroundings. Using ordinary waters, the range of inter laboratory measurements of the triple point can be about 250 μK. With VSMOW, the inter laboratory range of measurements of the triple point is about 50 μK. After the 2019 redefinition of the SI base units, the kelvin is defined in terms of the Boltzmann constant, which makes its definition completely independent of the properties of water. The defined value for the Boltzmann constant was selected so that the measured value of the VSMOW triple point is identical to the prior defined value, within measurable accuracy. Triple point cells remain a practical method of calibrating thermometers.