labelling: The process of replacing astable atom in a compound with a radioisotopeof the same element toenable its path through a biologicalor mechanical system to be traced bythe radiation it emits. In some casesa different stable isotope is used andthe path is detected by means of amass spectrometer. A compound containingeither a radioactive or stableisotope is called a labelled compoundand the atom used is a label. If a hydrogenatom in each molecule of thecompound has been replaced by a tritiumatom, the compound is called atritiated compound. A radioactive labelledcompound will behave chemicallyand physically in the same wayas an otherwise identical stable compound,and its presence can easily bedetected using a Geiger counter. Thisprocess of radioactive tracing iswidely used in chemistry, biology,medicine, and engineering. For example,it can be used to follow thecourse of the reaction of a carboxylicacid with an alcohol to give an ester,e.g. CH3COOH + C2H5OH →C2H5COOCH3+H2O To determine whether the noncarbonyloxygen in the ester comesfrom the acid or the alcohol, the reactionis performed with the labelledcompound CH3CO18OH, in which theoxygen in the hydroxyl group of theacid has been ‘labelled’ by using the18O isotope. It is then found that thewater product is H218O; i.e. the oxygenin the ester comes from the alcohol,not the acid.
