Oxygen comes in three variants, but the O is so rare that it is very difficult to detect (~0.04% abundant). The ratio of O/O in water depends on the amount of evaporation the water experienced (as O is heavier and therefore less likely to vaporize). As the vapor tension depends on the concentration of dissolved salts, the O/O ratio shows correlation on the salinity and temperature of water. As oxygen gets built into the shells of calcium carbonate secreting organisms, such sediments provide a chronological record of temperature and salinity of the water in the area.
Oxygen isotope ratio in atmosphere varies predictably with time of year and geographic location; e.g. there is a 2% difference between O-rich precipitation in Montana and O-depleted precipitation in Florida Keys. This variability can be used for approximate determination of geographic location of origin of a material; e.g. it is possible to determine where a shipment of uranium oxide was produced. The rate of exchange of surface isotopes with the environment has to be taken in account.
The oxygen isotopic signatures of solid samples (organic and inorganic) are usually measured with pyrolysis and mass spectrometry. Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.