Chromatography is a type of process used by scientists to separate mixed substances. Chromatography literally means ‘colour writing’. The word chromatography was first used by a botanist, who developed a process to separate plant pigments. Although chromatography was first developed to separate colours, the term ‘chromatography’ now includes a broad range of techniques exploiting the differing properties of chemicals to help scientists identify the components of a mixture.

Chromatography takes advantage of differences in the way component molecules in a mixture behave. Chromatography is used to find to purify a mixture, or isolate a particular molecule, known as the ‘analyte’. In this regard, there are two principal objectives of chromatography: preparatory and analytical. Preparatory chromatography refers to processes aimed at separating the components of relatively large quantities of a substance, essentially purifying it. The other form, analytical chromatography, is used in determining what is in the subject mixture, and in what concentrations.

There are two theories of chromatography - ‘retention chromatography’ and ‘plate chromatograpy’ theories. These theories of chromataography are practically applied in a number of ways. ‘Paper chromatography’ is one of the simplest chromatography procedures. This procedure belongs to a class of techniques known as ‘capillary action chromatography’, and will be familiar to many who were taught the method at school. A drop of the subject mixture is introduced to a strip of ‘chromatography paper’, which is then dipped in a suitable solvent. As the chromatography paper absorbs the solvent, it carries with it the subject mixture, and the component substances separate as each one travels at a different rate – known as the ‘retention rate’.

There are other, more sophisticated, chromatography procedures. For example, ‘column chromatography’ procedures, which are similar in some ways to paper chromatography. A transparent column is filled with a solid medium, through which the subject mixture is carried in an added solvent, either by gravity or under pressure. The components of the mixture separate as they travel through the medium, and so can be identified.

LSL