Radiochemical purity (RCP) may be defined as "the proportion of the total radioactvity in the sample which is present as the desired radiolabelled species".
Radiochemical purity is important in radiopharmacy since it is the radiochemical form which determines the biodistribution of the radiopharmaceutical. Radiochemical impurities will have different patterns of biodistribution which may obscure the diagnostic image obtained and render the investgation meaningless.
Measurement of RCP requires the use of a method to separate the different labelled chemical specieswhich may be present in the radiopharmaceutical preparation.
The most commonly used method is chromatography which separates the different species on thebasis of their differing affinities for a variety of liquid or solid phases.
The most simple and widely performed procedure uses thin-layer chromatography (TLC) in which the various compounds may be separated because they aredifferentially distributed between a liquid (mobile) phase and a solid (stationary) support mostlyconsisting of silica gel, normally bound to glass-fibre sheets. (Instant thin-layer chromatography or ITLC).
Liquid Chromatography (LC) is an increasingly popular alternative to TLC in which the components are separated down a column by elution with a suitable mobile phase. LC can be performed using High Pressure Liquid Chromatography (HPLC) a very efficient separation mode which requires expensive specialised equipment but the same principles apply to Solid-Phase Extraction (SPE) a low-tech version of liquid chromatography which requires very little in the way of equipment.
An alternative to chromatography is to use electrophoresis in which the components in a radiopharmaceutical may be separated by their different mobilities in an electric field.