Isotopes of oxygen

Depending on the terrestrial source, the standard atomic weight varies within the range of [15.99903, 15.99977] (the conventional value is 15.999).

16O has high relative and absolute abundance because it is a principal product of stellar evolution and because it is a primary isotope, meaning it can be made by stars that were initially hydrogen only.

This quickly (half-life around 110 minutes) beta decays to 18O making that isotope common in the helium-rich zones of stars.

[12] Since physicists referred to 16O only, while chemists meant the natural mix of isotopes, this led to slightly different mass scales.

Solid samples (organic and inorganic) for oxygen isotopic ratios are usually stored in silver cups and measured with pyrolysis and mass spectrometry.

[15] Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.

For example, it was proven that the oxygen released in photosynthesis originates in H2O, rather than in the also consumed CO2, by isotope tracing experiments.

Furthermore, the 17O(n,α)14C reaction is a further undesirable result of an elevated concentration of heavier isotopes of oxygen.

[22] Oxygen-15 and nitrogen-13 are produced in air when gamma rays (for example from lightning) knock neutrons out of 16O and 14N:[23] 15O decays to 15N, emitting a positron.