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General | |
---|---|
Symbol | 18O |
Names | oxygen-18, 18O, O-18, Ω, Heavy oxygen |
Protons (Z) | 8 |
Neutrons (N) | 10 |
Nuclide data | |
Natural abundance | 0.2% |
Half-life (t1/2) | stable |
Isotope mass | 17.9991610 Da |
Spin | 0 |
Isotopes of oxygen Complete table of nuclides |
Oxygen-18 (18
O, Ω
[1]) is a natural,
stable isotope of
oxygen and one of the
environmental isotopes.
18
O is an important precursor for the production of
fluorodeoxyglucose (FDG) used in
positron emission tomography (PET). Generally, in the
radiopharmaceutical industry,
enriched water (H
2Ω) is bombarded with hydrogen ions in either a
cyclotron or
linear accelerator, producing
fluorine-18. This is then synthesized into FDG and injected into a patient. It can also be used to make an extremely
heavy version of water when combined with
tritium (
hydrogen-3): 3
H
218
O or T
2Ω. This compound has a density almost 30% greater than that of natural water.
[2]
The accurate measurements of 18
O rely on proper procedures of analysis, sample preparation and storage.
[3]
In ice cores, mainly
Arctic and
Antarctic, the ratio of 18
O to 16
O (known as
δ18
O) can be used to determine the temperature of precipitation through time. Assuming that atmospheric circulation and elevation has not changed significantly over the poles, the temperature of ice formation can be calculated as
equilibrium fractionation between phases of water that is known for different temperatures. Water molecules are also subject to
Rayleigh fractionation
[4] as atmospheric water moves from the equator poleward which results in progressive depletion of 18
O, or lower δ18
O values. In the 1950s,
Harold Urey performed an experiment in which he mixed both normal water and water with oxygen-18 in a barrel, and then partially froze the barrel's contents.
The ratio 18
O/16
O (δ18
O) can also be used to determine
paleothermometry in certain types of fossils. The fossils in question have to show progressive growth in the animal or plant that the fossil represents. The fossil material used is generally
calcite or
aragonite, however oxygen isotope paleothermometry has also been done of
phosphatic fossils using
SHRIMP.
[5] For example, seasonal temperature variations may be determined from a single sea shell from a
scallop. As the scallop grows, an extension is seen on the surface of the shell. Each growth band can be measured, and a calculation is used to determine the probable sea water temperature in comparison to each growth. The equation for this is:
Where T is temperature in Celsius and A and B are constants.
For determination of ocean temperatures over geologic time, multiple fossils of the same species in different stratigraphic layers would be measured, and the difference between them would indicate long term changes. [6]
In the study of plants'
photorespiration, the labeling of atmosphere by oxygen-18 allows for the measurement of oxygen uptake by the photorespiration pathway. Labeling by 18
O
2 gives the unidirectional flux of O
2 uptake, while there is a net photosynthetic 16
O
2 evolution. It was demonstrated that, under preindustrial atmosphere, most plants reabsorb, by photorespiration, half of the oxygen produced by
photosynthesis. Then, the yield of photosynthesis was halved by the presence of oxygen in atmosphere.
[7]
[8]
Fluorine-18 is usually produced by irradiation of 18O-enriched water (H218O) with high-energy (about 18 MeV) protons prepared in a cyclotron or a linear accelerator, yielding an aqueous solution of 18F fluoride. This solution is then used for rapid synthesis of a labeled molecule, often with the fluorine atom replacing a hydroxyl group. The labeled molecules or radiopharmaceuticals have to be synthesized after the radiofluorine is prepared, as the high energy proton radiation would destroy the molecules.
Large amounts of oxygen-18 enriched water are used in positron emission tomography centers, for on-site production of 18F-labeled fludeoxyglucose (FDG).
An example of the production cycle is a 90-minute irradiation of 2 milliliters of 18O-enriched water in a titanium cell, through a 25 μm thick window made of Havar (a cobalt alloy) foil, with a proton beam having an energy of 17.5 MeV and a beam current of 30 microamperes.
The irradiated water has to be purified before another irradiation, to remove organic contaminants, traces of tritium produced by a 18O(p,t)16O reaction, and ions leached from the target cell and sputtered from the Havar foil. [9]