18O2

18O2 is the diatomic molecule of oxygen in which both atoms are the heavier isotope 18O. It is one of the isotopologues of molecular oxygen (O2), alongside the more common 16O16O and the mixed isotopologues such as 16O18O and 18O17O.

In nature, the majority of oxygen atoms are 16O. 18O has an abundance of about 0.204% and 17O about 0.037%. Consequently, the fraction of O2 molecules that are 18O18O is approximately the square of the atomic abundance of 18O, giving around 4 parts per million (ppm) of atmospheric O2 as 18O18O. The remaining O2 is dominated by 16O16O, with smaller fractions of mixed isotopologues.

The presence of two 18O atoms makes 18O2 heavier and slightly alters its spectroscopic and kinetic properties compared with the main isotopologue. These differences affect vibrational and rotational transitions, enabling detection and quantification by isotope-sensitive spectroscopic techniques and mass spectrometry. The data from 18O2 measurements are used in studies that require isotopic tracers or precise isotope ratios.

Production and measurement: 18O2 can be produced by isotopic enrichment methods, such as exchange reactions with 18O-enriched water or other enrichment techniques, often followed by purification. It is typically measured and quantified using isotope ratio mass spectrometry or laser-based spectroscopy methods, which can distinguish 18O-containing molecules from the abundant 16O-containing ones.

Applications: 18O2 serves as a tracer in ecological and physiological experiments to study gas exchange, photosynthesis, and respiration. In Earth science, analyses of 18O2 and related isotopes contribute to partitioning atmospheric O2 production and consumption and aid in reconstructing biosphere productivity and climate records. Safety considerations for oxygen handling apply similarly to other O2 gases.