chromatography is based on the physical nature of the mobile phase. The mobile phase can be a gas or a liquid which gives rise to the two basic forms of chromatography, namely, gas chromatography (GC) and liquid chromatography (LC). The stationary phase can also take two forms, solid and liquid, which provides two subgroups of GC and LC, namely; gas–solid chromatography (GSC) and gas–liquid chromatography (GLC), together with liquid solid chromatography (LSC) and liquid chromatography (LLC). The different forms of chromatography are summarized in Table 1. Most thin layer chromatography techniques are considered liquid-solid systems although the solute normally interacts with a liquid-like surface coating on the adsorbent or support or, in some cases an actual liquid coating. Table 1 The Classification of Chromatography chromatography systems

Liquid Chromatography Applications Liquid chromatography lends itself particularly to the separation of highly polar and large molecular weight materials that have very low volatility and, thus, can not be separated by gas chromatography. In addition, however, the technique is also used in trace analysis (e.g., drugs and drug metabolites in blood) where a concentration procedure is necessary and the sample is eventually regenerated in a small volume of liquid that can be injected directly

. The separation of the enantiomers of hexabarbital on this stationary phase by the direct injection of blood serum is shown in figure 56. Chromatogram A was obtained after 20 injections of serum and chromatogram B after 60 consecutive injections of blood serum. It is seen that here is very little column deterioration and that, although the tail of the major peak has become a little extended after 60 injections, the column could still be used very effectively for the analysis. Liquid Chromatography Applications Liquid chromatography has been used in an extremely wide range of analytical methods and it is impossible to give a comprehensive set of examples that would illustrate its wide applicability. The following are a few LC analyses that may indicate the scope of the technique and give the reader some idea of its importance and versatility. An example of the use of reversed phase chromatography (employing a C8 column) for the separation of some benzodiazepines is

costly and complex instrument, and at the other, on a simple, inexpensive thin layer plate. The first scientist to recognize chromatography as an efficient method of separation was the Russian botanist Tswett (1), who used a simple form of liquid-solid chromatography to separate a number of plant pigments. The colored bands he produced on the adsorbent bed evoked the term chromatography for this type of separation (color writing). Although color has little to do with modern chromatography, the name has persisted and, despite its irrelevance, is still used for all separation techniques that employs the essential requisites for a chromatographic separation,viz. a mobile phase and a stationary phase. The technique, as described by Tswett was largely ignored for a along time and it was not until the late 1930s and early 1940s that Martin and Synge(2) introduced liquid-liquid chromatography by supporting the stationary phase, in this case

Stoll (2) in 1913. Willstatter and Stoll repeated Tswett's experiments without heeding his warning not to use too "aggressive " adsorbents as these would cause the chlorophylls to decompose. As a consequence, the experiments of Willstatter et al. failed and their published results, rejecting the work of Tswett, impeded the recognition of chromatography as a useful separation technique for nearly 20 years. In the late 1930s and early 1940s Martin and Synge introduced a form of liquid-liquid chromatography by supporting the stationary phase, in this case water, on silica gel in the form of a packed bed and used it to separate some acetyl amino acids. They published their work in 1941 (3) and in their paper recommended the replacement of the liquid mobile phase with a suitable gas which would accelerate the transfer between the two phases and provide more efficient separations. Thus, the concept of gas chromatography was born. In the same paper in 1941, Martin and Synge suggested the

evaluated at, or close to that temperature where the separation ratio remains constant and independent of solvent composition, then the potential advantages that can be gained from an optimized solvent mixture will never be disclosed. Any evaluation of either a particular stationary phase, or solvent mixture, for the separation of closely eluting solutes must be carried out over a range of temperatures.       Optimum Operating Conditions for Chiral Separations in Liquid Chromatography Thermodynamic reasoning need not be used exclusively to examine a chromatographic problem but can also be employed together with other aspects of chromatography theory to achieve a practical goal. The following example shows how thermodynamics can be used with appropriate optimizing equations to identify the optimum conditions for particular difficult types of separation. In gas chromatography (GC), chiral selectivity is controlled by choice of stationary phase and operating