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Concept
Oxygen-18
Summary
Oxygen-18 (18O, Ω) is a natural, stable isotope of oxygen and one of the environmental isotopes.
18O is an important precursor for the production of fluorodeoxyglucose (FDG) used in positron emission tomography (PET). Generally, in the radiopharmaceutical industry, enriched water (H218O) 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): 3H218O or T218O. This compound has a density almost 30% greater than that of natural water.
The accurate measurements of 18O rely on proper procedures of analysis, sample preparation and storage.
In ice cores, mainly Arctic and Antarctic, the ratio of 18O to 16O (known as δ18O) 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 as atmospheric water moves from the equator poleward which results in progressive depletion of 18O, or lower δ18O 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 18O/16O (δ18O) 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. 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.
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