Spread-layer chromatography

This is invariably referred to in various literature as open-column chromatography drop chromatography strip-chromatography spread-layer chromatography surface chromatography . 

Technique of thin-layer chromatography. Preparation of the plate. In thin-layer chromatography a variety of coating materials is available, but silica gel is most frequently used. A slurry of the adsorbent (silica gel, cellulose powder, etc.) is spread uniformly over the plate by means of one of the commercial forms of spreader, the recommended thickness of adsorbent layer being 150-250 m. After air-drying overnight, or oven-drying at 80-90 °C for about 30 minutes, it is ready for use. 

Thin layer chromatography is in principle similar to paper chromatography when used in the ascending method, i.e. the solvent creeps up the stationary phase by capillarity. The adsorbent (e.g. silica, alumina, cellulose) is spread on a rectangular glass plate (or solid inert plastic sheet). Some adsorbents (e.g. silica) are mixed with a setting material (e.g. CaSC>4) by the manufacturers which causes the film to set on drying. The adsorbent can... 

A sheet of high-quality filter paper containing adsorbed water serves as the stationary phase in paper chromatography. However, thin-layer chromatography, which employs a layer of silica gel or other material spread on a glass or plastic plate, has often supplanted paper chromatography because of its rapidity and sharp separations (Fig. 3-5).16/96a 98-10°... 

In paper chromatography we use filter paper, marketed for this purpose. It comes usually in the form of a 2-5 cm-wide tape, from which a strip of the necessary length can easily be cut. The more modern technique of thin layer chromatography (TLC), makes use of thin sheets of aluminium oxide, silica-gel, cellulose or some other material, supported by a metal sheet or a polymer. Chromatographic thin layers can be prepared in the laboratory from commercially available adsorbents. A thick suspension of these is made with water (usually a 2 1 w/w mixture of water adsorbent is made up) and this is then spread on a metal plate with a suitable spreader device. Techniques vary from device to device, and the instructions of the manufacturer should be followed whenever thin layer plates are to be prepared. Ready-made thin layer sheets are also available commercially. These contain the active material spread on a plastic support. Thin-layer chromatographic materials, especially ready-made plates, are much more expensive than chromatographic paper, but normally offer faster and sharper separations than the paper. The procedures described in Section VI.20 can be carried out both on a slow chromatographic paper (e.g. Whatman No. 1) or on a cellulose thin layer (e.g. Whatman cellulose). 

Figure 1.3 Thin-layer chromatography, (a) Thin-layer of adsorbent containing a binder so that it adheres to the glass plate, (b) Glass plate the plates ate made up by spreading a thidc slurry of adsorbent on the plate, followed by drying in an oven nowadays pre-ptepared plate may be bought with the adsorbent on a plastic backing, (c) In the early days of TLC, gas jars were used but nowadays customised TLC tanks are available the inside of the tank is lined with filter paper soaked in eluent in order that the atmosphere in the tank is saturated with eluent vapour, (d) Eluent the plate is "spotted at a point which will be just above the level of the eluent, (e) The TLC plate. Many means of visualisation are possible (absorption of iodine, use of fluorescent agent on the adsorbent, concentrated sulphuric acid spray etc.) here a mixture is run against a reference standard of a key component in the mixture however it is possible to run many samples simultaneously.

Thin-layer chromatography is a separation technique in which the components of a lipid mixture are differently distributed between a solid stationary phase, spread as a thin layer on a plate made of inert material, and a solvent mobile phase. Depending on their type, the components are retained with different strengths on the layer to give distinctive spots or bands. The migration of a band is presented quantitatively by the corresponding Rf value. The stronger the retention, the lower is the Rf value. 

Figure 12.1 Thin-layer Chromatography (a) Device for spreading a layer of adsorbent (b) Chamber for chromatographing with a solvent (aluent) at the bottom (c) A developed plate showing mixture (A+B) having separated into its constituents B and A spots marked (A+B), B and A on the starting line refer to those of the mixture and known compounds B and A respectively.

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