Element a stable isotope of oxygen, symbol O, abbreviated as 18 O. In 1929, W.F. Geauke and H.L. Johnston discovered by molecular spectroscopy that natural oxygen is composed of three isotopes: oxygen 16, oxygen 17 and oxygen 18. Modern determination, the accurate isotopic composition of oxygen in air is oxygen 16: oxygen 17: oxygen 18=2667:1:5.5.

Water enriched with oxygen 18 (heavy oxygen water) was first obtained in 1937 by H.C. Ury and J.R. Hoffmann by water distillation. The main separation method of oxygen 18 is still water distillation. 99.8% H218O has been obtained by this method. Low temperature distillation of carbon monoxide or nitric oxide can also separate oxygen 18.

The discovery of the heavy isotope of oxygen caused the discrepancy between the chemical scale (O = 16.00000) and the physical scale (16O = 16.00000) of atomic weight. This is because the oxygen used in the chemical scale is natural oxygen, and the existence of oxygen 17 and oxygen 18 makes the chemical scale larger than the physical scale, the chemical scale is 1.000275 times of the physical scale. Chemistry and physics are interrelated, and differences in scales inevitably lead to confusion. After the two scales have been used for some time, the need for a unified scale becomes more and more felt. It was not until 1961, when chemistry and physics adopted 12C = 12 as the base atomic weight, that the two scales were unified.

Since oxygen has no long-lived radioisotope (see radioactivity), oxygen 18 is an important tracer atom and has been widely used in the study of chemical reaction mechanisms, catalytic mechanisms, and reaction processes related to life activities.