Overview

Oxygen-18 (18O) is one of the three stable isotopes of oxygen. It is heavier than the dominant 16O isotope and occurs naturally in Earth’s reservoirs (atmosphere, oceans, rocks and organisms) at low abundance—approximately two parts per thousand of oxygen atoms. Because it is nonradioactive and chemically identical to other oxygen isotopes, 18O is widely used as an environmental and laboratory tracer.

Properties and occurrence

Mass difference between 18O and lighter isotopes leads to subtle differences in physical behaviour during phase changes and chemical reactions. These differences produce measurable variations in isotope ratios, commonly expressed in delta notation (δ18O), which compare sample ratios to a standard. Natural processes such as evaporation, condensation and biochemical reactions fractionate isotopes and create characteristic signatures in water, ice, carbonates and organic matter.

Measurement and fractionation

Isotope ratios are determined principally by isotope ratio mass spectrometry (IRMS) and, increasingly, by spectroscopic techniques. Two main fractionation types affect 18O distributions: equilibrium fractionation, which depends on temperature, and kinetic fractionation, associated with rates of evaporation or diffusion. These effects make 18O a temperature and process proxy in environmental studies.

Uses and applications

  • In paleoclimatology, δ18O records from ice cores, marine carbonates and speleothems are interpreted to infer past temperatures and ice volume changes.
  • In hydrology and ecology, 18O tracks water sources and cycling, and is used in studies of metabolism and water turnover in organisms as a safe, nonradioactive tracer.
  • In nuclear medicine production, 18O-enriched water is irradiated in cyclotrons to produce fluorine-18, the radioisotope used in PET imaging.

History and notable distinctions

The identification and use of oxygen isotopes grew with early 20th-century developments in mass spectrometry. Oxygen-18 must be distinguished from 17O and 16O when interpreting isotope data; combined analysis of multiple isotope systems refines reconstructions of environmental change. Because 18O is stable, it records integrated physical and biological processes rather than indicating radioactivity.

Practical considerations

Samples for 18O analysis require careful handling to avoid contamination and isotopic exchange. Interpretation of δ18O values often relies on calibration with local standards and models of fractionation. For further technical and reference material see general isotope resources: stable isotope information and foundational descriptions of oxygen isotope systems.