A group of laboratory separation techniques based on selective adsorption by which components of complex mixtures (vapors, liquids, solutions) can be identified. Its discoverer, Tswett (1906), named the procedure chromatography because the plant pigments used in his early experiments produced bands of characteristic color. Since the 1930s, the method has been widely applied in many variations to the analysis of colorless mixtures such as hydrocarbons, metallic salts, etc. Separation is due to redistribution of the molecules of the mixture between the thin phase (adsorption layer) and the bulk phase (adsorbent) with which it is in contact. Because the thin phase sometimes approaches molecular dimensions, the size and shape of the molecules of the mixture are of great significance. Chromatography involves the flow of a mobile (gas or liquid) phase over a stationary phase (which may be a solid or a liquid). Liquid chromatography is used for soluble substances and gas (vapor-phase) chromatography for volatile substances. As the mobile phase moves past the stationary phase, repeated adsorption and desorption of the solute occurs at a rate determined chiefly by its ratio of distribution between the two phases. If the ratio is large enough, the components of the mixture will move at different rates, producing a series of bands (chromatographs) by which their identity can be determined.See Liquid Chromatography; Gas Chromatography; Paper Chromatography; Thin-layer Chromatography; Ion-exchange Chromatography; Gel Filtration.
