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of chromatography were virtually complete and since then, despite the plethora of publications that have appeared on the subject, the vast majority has dealt with applications of the technique and only a minority with fundamental aspects of the subject and novel instrumentation concepts. Today, chromatography is an extremely versatile technique; it can separate gases, and volatile substances by GC, involatile chemicals and materials of extremely high molecular weight (including biopolymers) by LC and if necessary very inexpensively by TLC. All three techniques, (GC), (LC) and TLC have common features that classify them as chromatography systems. Chromatography has been defined as follows, Chromatography is a separation process that is achieved by distributing the components of a mixture between two phases, a stationary phase and a mobile phase. Those components held preferentially in the stationary phase are retained longer in the system than

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 the interactive character of the molecule as a whole will be determined by the net effect of all the sites. If the dispersive sites dominate, the overall property of the molecule will be dispersive which the biotechnologists call "hydrophobic" or "lyophobic". If dipoles and polarizable groups dominate in the molecule, then the overall property of the molecule will be polar, which

of Supelco Inc.   Figure 19 The Separation of a Mixture by Exclusion Chromatography   Until relatively recently, silica has been the most commonly used exclusion media for the separation of high molecular weight hydrocarbons and polymers. However, it was not so successful in the separation of polymeric materials of biological origin. More recently the micro-reticular macroporous polystyrene gels have been introduced and found to be very useful for the separation of biopolymers by size exclusion. These materials have even replaced many of the traditional applications of silica gel. In summary, in all types of chromatography, solute retention is controlled by either the magnitude and probability of interaction and/or by the amount of stationary phase that is available to them. However, even if, by appropriate choice of the phase system, the solutes are separated, unless the peak dispersion is contained to allow the individual solutes to be

molecular eight fatty acids can be derivatized to form volatile substances that can be separated by GC. The derivatization must be highly reproducible and usually proceed to completion in order to maintain adequate accuracy. The capillary columns in GC can have much higher efficiencies than their LC counterpart and thus GC can more easily handle multicomponent mixtures such as essential oils. On the other hand, only LC can separate the peptides, polypeptides, proteins and other large biopolymers that are important in biotechnology

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