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fact, despite the many advances that have taken place in LC techniques over the past years, the use of TLC for routine analyses continues to grow. However, samples containing multiple components cannot be separated by TLC due to restricted plate capacity. In TLC all the solutes must be contained by the plate whereas in LC, as the solutes are eluted from the column, the component capacity is much greater. Thin Layer Chromatography Chambers A diagram of two simple thin layer chromatography development chambers is shown in figure 29. Figure 29 The Normal Method of Thin Layer Plate Development

by the computer and stored on disk. The data may be partially processed 'on the fly' or processed at the completion of the analysis. Thin Layer Chromatography Apparatus Thin layer chromatography appears to have been first developed and utilized by Schraiber in 1939 (11). Schraiber working with Izmailov at the Khar'kov Chemistry and Pharmacy Research Institute employed the techniques for the analysis of pharmaceuticals. In her own words, " It occurred to us that a thin layer of the sorbent could be used in lieu of a strip of paper; also we felt that the flat bed could be considered as a cut-out of the adsorbent column. We believed that in carrying out the separation process in such a layer, the process would be accelerated significantly. In our work, we deposited a drop of the solution being investigated on the flat adsorbent layer and observed the separation into concentric circular zones which could become visible because of their fluorescence in

, the primary classification of 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

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

, more solute molecules in the stationary phase will randomly acquire sufficient energy (EA) to leave the stationary phase and enter the gas phase. Thus, the distribution coefficient of all solutes with respect to the stationary phase will be reduced as the temperature rises and it will be seen in due course that this will cause the band velocity of all the solutes to be increased. Elution Development in Thin Layer Chromatography The development processes that take place on a thin layer plate is complicated by the frontal analysis of the mobile phase itself. The mobile phases used to elute the solutes in TLC are usually multi-component, containing at least three individual solvents. If the plate is not pre-conditioned with solvent, there is an elaborate modification of the plate surface which is depicted, for a ternary solvent mixture, in Figure 3. The edge of the plate is dipped into a tray of the solvent mixture which begins to migrate along the plate, driven

it needs to encompass all types of detecting systems ranging from elaborate electronic devices to the human eye or even the sense of smell. Tswett in his pioneering chromatographic separation of some plant pigments used the human eye to determine the nature of the separation and, even today, as one of the more common separation techniques is thin layer chromatography, the human eye is still one of the more frequently used detectors. Similarly, essential oil chemists smell the eluent from a gas chromatography (GC) column in organoleptic assessment. The detector, as well as being an essential supporting device for the gas chromatograph has also played a critical role in the development of the technique as a whole. There has been a synergistic interaction between column development and detector development. The need to develop higher column� efficiencies has demanded higher detector sensitivities which has provoked the development of more sensitive detectors. In turn, the more