A technique for analysing or separating mixtures of gases, liquids, or dissolved substances, such as mixtures of amino acids or chlorophyll pigments. The original technique (invented by the Russian botanist Mikhail Tsvet (1872–1919) in 1906) is a good example of column chromatography. A vertical glass tube is packed with an adsorbing material, such as alumina. The sample is poured into the column and continuously washed through with a solvent (a process known as elution). Different components of the sample are adsorbed to different extents and move down the column at different rates. In Tsvet’s original application, plant pigments were used and these separated into coloured bands in passing down the column (hence the name chromatography). The usual method is to collect the liquid (the eluate) as it passes out from the column in fractions.

In general, all types of chromatography involve two distinct phases—the stationary phase (the adsorbent material in the column in the example above) and the moving phase (the solution in the example). The separation depends on competition for molecules of sample between the moving phase and the stationary phase. The form of column chromatography above is an example of adsorption chromatography, in which the sample molecules are adsorbed on the alumina. In partition chromatography, a liquid (e.g. water) is first absorbed by the stationary phase and the moving phase is an immiscible liquid. The separation is then by partition between the two liquids. In ion-exchange chromatography the process involves competition between different ions for ionic sites on the stationary phase (see ion exchange). Gel filtration is another chromatographic technique in which the size of the sample molecules is important.

See also affinity chromatography; gas-liquid chromatography; paper chromatography; thin-layer chromatography.