LABELLING

Definition

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 If a hydrogenatom in each molecule of thecompound has been replaced by a tritiumatom, the compound is called atritiated compound.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.