Oxygen-18

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Oxygen-18, 18O
General
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Isotopes of oxygen
Complete table of nuclides

Oxygen-18 (18
O, Ω

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]

Paleoclimatology

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]

Plant physiology

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]

18F production

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

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

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]

See also

References

  1. ^ Capilla, José E.; Arevalo, Javier Rodriguez; Castaño, Silvino Castaño; Teijeiro, María Fé Díaz; del Moral, Rut Sanchez; Diaz, Javier Heredia (September 19, 2012). "Mapping Oxygen-18 in Meteoric Precipitation over Peninsular Spain using Geostatistical Tools" (PDF). cedex.es. Valencia, Spain: Ninth Conference on Geostatistics for Environmental Applications. Retrieved May 8, 2017.
  2. ISBN 978-0-486-65622-9
    .
  3. ISSN 2516-712X
    .
  4. ^ Kendall, C.; Caldwell, E.A. (1998). "Chapter 2: Fundamentals in Isotope Geochemistry". Isotope Tracers in Catchment Hydrology. Elsevier Science B.V., Amsterdam.
  5. S2CID 28224399
    .
  6. ^ Kendall, C.; McDonnell, J.J. (1998). Isotope Tracers in Catchment Hydrology. Elsevier Science B.V., Amsterdam.
  7. PMID 16661044
    .
  8. PMID 16661426
    .
  9. ^ http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/46/048/46048804.pdf [bare URL PDF]


Lighter:
oxygen-17 		Oxygen-18 is an
isotope of oxygen 		Heavier:
oxygen-19
nitrogen-19, fluorine-18
 Decay chain
of oxygen-18 		Decays to:
stable
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