Thin Layer Chromatography - Ion Exchange Materials

Ion Exchange Materials

There are four classes of ion exchange materials commonly used in TLC, the inorganic ion exchangers, the ion exchange resins, the ion exchange celluloses and the ion exchange bonded phases. The inorganic ion exchange materials are largely the zeolites (the aluminosilicates), apatite and hydroxy apatite. However, some of the phosphates, molibdates, tungstates and vanadates of the tetravalent metals have also been shown useful for ion exchange separations. However, the most popular ion exchange substrates used in TLC are the ion exchange resins and the ion exchange cellulose's.

Ion exchange resins are made from polystyrene cross-linked during polymerization with divinyl benzene. The resin is subsequently treated with a suitable reagent to form ionogenic groups on the surface (e.g. by sulfonation). The ion exchange groups can range from strongly acidic to strongly basic. Typical strong acid groups are -SO3H and -PO(OH)2, acid groups would be typically -COOH and weak acid exchange groups, -C6H5OH. An example of a strong basic ion exchange group would be -NR3OH. These ion exchange phases are used with similar mobile phases as those used in LC ion exchange separations and, thus, require to be buffered appropriately for the resin that is selected and the solutes that are to be separated.

Ion exchange celluloses are prepared by attaching ionic groups to the cellulose chain in a similar manner to the way such groups are attached to polystyrene resins. The major difference between an ion exchange resin and an ion exchange cellulose is that, in the former, the ion exchange groups are attached to a largely dispersive matrix whereas with ion exchange cellulose, the groups are attached to a largely polar matrix. Thus, in separations carried out on ion exchange resins, the dominant interactive forces that control retention will be ionic and dispersive. In contrast, the dominant forces that will control retention on a ion exchange cellulose will be ionic and polar.

Ion exchange bonded phases are another, probably the most important type of TLC ion exchange material. These bonded materials are usually silica based and formed by directly reacting appropriate substituted silanes with a silica gel base. The silane must contain a suitable reactive group that can either be converted to an ion exchange radical or permit an ion exchange radical to be attached to it. Carboxylic acids and amino groups are frequently used to provide ionogenic properties to bonded phases. Unfortunately, silica based bonded phases are very unstable at extremes of pH due to the silica matrix being eroded and the bonded material rapidly eliminated. Consequently, ion exchange bonded phases have a very limited pH range over which they can be employed effectively. As a result, ion exchange resins are preferentially chosen as the stationary phase, both in LC and TLC, for separations where ionic interactions dominate and control the level of retention.