Separations Based on Ionic Interactions Ionic materials are not volatile under the conditions normally employed in GC, so, ionic interactions cannot be exploited in GC stationary phases to control retention. However, they are very important in LC, and ion exchange chromatography (the name given to LC separations that employ ionic interactions to control retention) is widely used to analyze ion mixtures. The use of ionic interactions to separate some alkali and alkaline earth cations is shown in figure 16.   Courtesy of Whatman Inc. Figure 16 The Separation of Cations by Ion-Exchange Chromatography The column used was IonPacCS12 (a proprietary cation exchange column) and the mobile phase was a 20nM solution of methanesulfonic acid in water. The flow rate was 1 ml/min. and 25ml of sample was injected. The separation almost exclusively involved ionic interactions as any

Ionic Interactions Polar compounds have no net charge on the molecule, in contrast, ions  possess a net charge that can interact strongly with ions having anopposite charge (usually called counter-ions). In ion exchange chromatography solutes carrying a net charge are retained by ionic interactions with counter ions situated in the stationary phase. The retentive mechanism involves simple electric forces between opposite charged ions. Ionic interactions are always accompanied by dispersive interactions and, in most cases, are also associated with polar interactions. Nevertheless, the dominant forces controlling retention in ion exchange chromatography result from ionicinteractions.  Ionic interaction is depicted diagrammatically in figure 5. Figure 5. Ionic and Dispersive Interactions

Ionic Forces Polar compounds possessing dipoles, have no net charge. In contrast, ions possess a net charge and consequently can interact strongly with ions having an opposite charge. Ionic interactions are exploited in ion exchange chromatography where the counter ions to the ions being separated are situated in the stationary phase. In a similar manner to polar interactions, ionic interactions are always accompanied by dispersive interactions and usually, also with polar interactions. Nevertheless, in ion exchange chromatography, the dominant forces controlling retention usually result from ionic interactions. Ionic interaction is depicted diagramatically in figure 10. Figure 10 Ionic and Dispersive Interactions A molecule can have many interactive sites comprised of the three basic types, dispersive, polar and ionic. Large molecules (for example biopolymers) may have hundreds of different interactive sites throughout the molecule and

polar and ionic giving rise to dispersive interactions, polar interactions and ionic interactions. Polar forces have been further divided into sub groups ranging from 'strong dipole-dipole interactions' (hydrogen bonding) to 'weak dipole-dipole interactions ((p)-(p) interactions). This type of division is questionably useful as it tends to 'confuse' more than 'explain' when dealing with chromatographic retention. Division into the two groups, dipole-dipole interactions and dipole-induced dipole interactions, however, is appropriate, as it describes two physically different types of polar interaction that is very pertinent to chromatographic retention. However, the arbitrary division of polar forces into numerous groups of different strength has not proved to be particularly helpful. No significant attempt has been made to partition dispersive or ionic interactions and such division is certainly not necessary when dealing with chromatographic retention. Most molecular interactions will

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

, a polyethylene glycol stationary phase will separate aromatic hydrocarbons largely by dipole-induced dipole interactions combined with some dispersive interactions. This effect is also demonstrated in the lower chromatogram in figure 2. Molecules can exhibit multiple interactive properties. For example, phenyl ethanol possesses both a dipole as a result of the hydroxyl group and is polarizable due to the aromatic ring. Phenyl acetic acid contain groups that will provide dispersive interactions(the methylene group and the aromatic ring), induced dipole interactions (the aromatic ring) polar interactions (the carbonyl group and also ion interaction (the acid group). Ionic interactions are discussed below.  Complex molecules such as biopolymers can contain many different interactive groups. Dipole-induced dipole interactions are depicted in figure 4