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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 light of a UV lamp."

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 onto an LC column. In general, the sample is passed through a short length of tube packed with an adsorbent which selectively removes the material of interest. The adsorbent is then washed and the adsorbed material extract with a small amount of suitable solvent and the solvent then inject onto the column. An example of the use of this technique is in the determination of tetrahydrocannabinol carboxylic acid in urine. This substance appears in the urine of those subjects that have recently smoked marihuana. The tetrahydrocannabinol carboxylic acid can be extracted from the urine

of sample preparation is the subject of Book 18 but an example of one of the more complex sample preparation methods will be given to illustrate some of the procedures that may be necessary.   Liquid extraction is a clumsy procedure, particularly when used on the micro scale which is often necessary in sample preparation. An alernative procedure is solid phase extraction. The procedure is relatively simple and involves the use of a short tube packed with an appropriate adsorbent such as silica, reversed phase silica or, for some applications, macro porous polymer beads. The adsorbent must be capable of removing the substances of interest from the liquid medium. Extracting trace materials from water (e.g., pollution analysis) a reversed phase would be appropriate. Then the substances could be displaced into solvents such as n-hexane, methylene dichloride etc. A diagram of a simple solid phase extraction tube is shown in figure 34

nbsp; Courtesy of Supelco Inc.   Figure 34 A Solid Phase Extraction Tube   The extraction tubes are usually made of an inert plastic such as polypropylene and have a range of capacities of 1, 2, or 5 ml. The tube is one fifth filled with adsorbent and contained by plastic frits at either end. The upper part of the tube, above the packing, acts as a funnel or container for the liquid to be extracted. The liquid sample is allowed to percolate through the adsorbent bed. Sometimes the lower end of the tube is connected to a vacuum or the top to a gas supply to increase the flow of sample through the bed. The adsobed material is then desorbed with an appropriate solvent, the sample diluted to a known volume and an aliquot used

component mixture containing widely differing chemical species. Furthermore, the analysis can be carried out, at one extreme, on a very 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

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