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

     In 1937, H.C. Ury and J.R. Hoffman first obtained water enriched with oxygen 18 (heavy oxygen water) by hydrodistillation. The main method of separating oxygen 18 in modern times is still water distillation, and 99.8% H218O has been obtained by this method. Low temperature rectification of carbon monoxide or nitric oxide also separates oxygen 18.

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

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