Isotope markers in compounds are used to identify the source of environmental pollutants,...

Isotope markers in compounds are used to identify the source of environmental pollutants, investigate leaks in underground tanks and pipelines, and trace the

theft of oil and other liquid products. Both radioactive and isotopic markers are used.

Deuterium is typically used as a marker for organic compounds, by replacingthree or more  hydrogen atoms on the molecule in a reactor containing heavy water. However, isotopes of carbon and oxygen can also be used. The detection limit of tracers using a combination of gas chromatography and mass spectrometry is about 100 ppb in crude oil and about 20 ppb in refined products.

Explain now such markers might be used in chemical processes.