Compounds labelled with radioisotopes such as14C,3H,32P,35S and125I, are essential tools for research and especially in the life sciences. Syntheses can be directed to isotopic labelling or
to non-isotopic labelling. In isotopic labelling a compound is labelled with an isotope of an element already present in the
compound whereas non-isotopic labelling is achieved with an isotope is foreign to the material or compound being labelled.
This paper reviews the properties of the more important and commonly used radioisotopes in research and methods currently
employed for the synthesis of radiolabelled compounds. The relative ease of measurement of radioactivity in large numbers
of samples, when compared for example with the measurement of mass, and the great sensitivity with which very small quantities
of radioactive compounds can be accurately measured, has resulted in the widespread need and use of radiolebelled compounds.
Methods for the practicable labelling of compounds for use as radiotracers all into three main categories, namely, chemical
systhesis, biochemical methods and isotope exchange reactions. Examples are chosen to illustrate these methods for the labelling
of amino acids, peptides, proteins, carbohydrates, lipids, neurochemicals, nucleic acids, steroids and miscellaneous compounds
of interest in biological research. Most methods of labelling employed lead to specific labelling, that is, to modecules in
which the position(s) of the radioactive atom(s) is known with certainty. However, isotope exchange reactions which are especially
useful for labelling molecules with tritium are often non-specific. The routine use of tritium nuclear magnetic resonance
spectroscopy has resolved any uncertainties in the specificity of labelling with tritium. Examples are given illustrating
the effectiveness of this valuable analytical technique. A review of the synthesis of radiolabelled compounds would not be
complete without reference to the special properties of the labelled compounds themselves which affect their useful shelf-life,
self-decomposition, purification and analysis; factors which all need to be clearly understood in order to use radiolabelled
compounds with confidence in scientific research.