thin-layer chromatography, in analytical chemistry, technique for separating dissolved chemical substances by virtue of their differential migration over glass plates or plastic sheets coated with a thin layer of a finely ground adsorbent, such as silica gel or alumina, that is mixed with a binder such as starch or plaster of paris. The technique, which has become a standard analytical tool in food and pharmaceutical laboratories, is especially useful for separating the components of naturally occurring substances, notably those found in animal and vegetable tissues called lipids and the volatile and fragrant components of plants and flowers known as terpenes.
For thin-layer chromatography, a sample of the mixture to be separated is deposited at a spot near one end of the plate and a suitable solvent is allowed to rise up the plate by capillary action. The components of the sample become separated from one another because of their different degrees of attachment to the coating material on the plate or sheet. The solvent is then allowed to evaporate, and the location of the separated components is identified, usually by application of reagents that form coloured compounds with the substances. Thin-layer chromatography has a distinct advantage over paper chromatography in that the thin-layer chromatographic plate or sheet is able to withstand strong solvents and colour-forming agents.