Ionic Interaction Chromatography

Ionic interaction chromatography, or ion chromatography as it is usually called, is typically carried out employing ion exchange resins as the stationary phase. There are some silica based ion exchange materials available, but the bonded silica’s tend to be unstable in the presence of high salt concentrations and at extremes of pH. As a consequence, they have very limited areas of application. Alternatively, the polystyrene divinyl benzene cross-linked polymers, are extremely stable to a wide range of salt concentrations and can function well within the pH range of 2.0 to 12.0. An obvious application area for ion exchange chromatography is in the separation of all types of anions and cations. Metal cations and inorganic anions are all separated predominantly by ionic interactions with an ion exchange resin. Organic acids and bases, however, would be retained by mixed interactions, as dispersive and polar interactions will take place between the solute molecules and the aromatic nuclei and the aliphatic side chains of the polymer base.

The separation of simple acids and bases require that the mobile phase is buffered appropriately according to the pKa of the salts so that dissociation occurs and the ions are free to interact with the stationary phase. By employing mobile phase additives and using novel operating conditions, ionic interactions can be used to separate a far wider range of materials than simple organic and inorganic anions and cations.

Diquat and Paraquat Herbicides

An example of the use of ion interactions to separate the non-selective contact herbicides Diquat and Paraquat is given in figure 46.

Courtesy of Asahi Chemical Industry Co. Ltd.

Figure 46 The Separation of the Herbicides Diquat and Paraquat

The column was fairly large (10 cm long and 7.6 mm in diameter) and was packed with cross-linked polystyrene beads, 9 mm in diameter, carrying -COOH groups as the interacting ion moieties. At the optimum flow rate the column would give about 5,500 theoretical plates but at the flow-rate used which was 1 ml/min. the efficiency would be considerably below that. Nevertheless, it is clear that a very good separation was obtained.