Oxygen-18

Oxygen-18 (18O) 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 creating fluorine-18. This is then synthesized into FDG and injected into a patient.

Paleoclimatology
In Arctic and Antarctic ice cores, O-18 is used to retrieve the original temperatures of the precipitation during different years by analyzing the isotope ratio of the respective annual layers of ice. This happens due to the differing weights of water with the normal isotope of oxygen, and water with oxygen-18. 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 water with oxygen-18 sunk to the bottom of the barrel and was first to freeze.