The process of replacing a stable atom in a compound with a radioisotope of the same element to enable its path through a biological or mechanical system to be traced by the radiation it emits. In some cases a different stable isotope is used and the path is detected by means of a mass spectrometer. A compound containing either a radioactive or stable isotope is called a labelled compound and the atom used is a label. If a hydrogen atom in each molecule of the compound has been replaced by a tritium atom, the compound is called a tritiated compound. A radioactive labelled compound will behave chemically and physically in the same way as an otherwise identical stable compound, and its presence can easily be detected using a Geiger counter. This process of radioactive tracing is widely used in chemistry, biology, medicine, and engineering. For example, it can be used to follow the course of the reaction of a carboxylic acid with an alcohol to give an ester, e.g.CH3COOH+C2H5OH → C2H5COOCH3+H2O To determine whether the noncarbonyl oxygen in the ester comes from the acid or the alcohol, the reaction is performed with the labelled compound CH3CO18OH, in which the oxygen in the hydroxyl group of the acid has been ‘labelled’ by using the 18O isotope. It is then found that the water product is H218O; i.e. the oxygen in the ester comes from the alcohol, not the acid.
CH3COOH+C2H5OH → C2H5COOCH3+H2O