ion-exchange chromatography

A chromatographic method in which the components of mixtures are separated by differences in their acid-base behaviour. The chromatogram column is filled with a charged resin that is easily able to exchange ions for any charged molecules that are passed through the column. Amino acid separation is often achieved by this method and the resin commonly used is sulphonated polystyrene in which the sulphonic acid groups are charged with sodium ions. If an acidic mixture of amino acids is added to the column then, because the amino acids are mostly cations at acid pH, they will tend to displace the sodium ions. The more basic amino acids, e.g. histidine, will be held more tightly to the resin than the more acid amino acids, e.g. glutamic acid. By gradually increasing the pH and sodium concentration of the mobile phase the amino acids are displaced from the resin. They emerge from the column in order of decreasing acidity.