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Thin Layer Chromatography

What is Thin Layer Chromatography?

Chromatography is the physical process of determining the make up of complex solutions or mixtures. The components that are part of the complex mixture are separated by passing the unknown mixture; disolved with a solvent, usually distiled water, over a glass plate of adsorbent substances. The separation comes about by the different flow rates of the compounds making up the solution over the adsorption bed. A molecule's affinity for the adsorbent material will slow down the movement across the plate. There are other methods in use for chromatography such as; separation by absorbent material, or separation through ionization. General Glassblowing uses the adsorbent method in its chromatography developement. The molecules are held in a thin layer on the surface of the attractive (Adsorbent) material.

Procedure and Equipment

Thin-layer chromatography is adsorption chromatography performed on open-layers of adsorbent material supported on glass plates. It permits the analytical capabilities of column adsorption chromatography o be applied as conveniently as paper chromatography and offers other techniques possible only with TLC. The increasing popularity of TLC is accounted for by the outstanding characteristics of the method.

Separation in Minutes

Resolution is usually accomplished in 5 to 60 minutes, depending on sample complexity. Even the developing time of one and one-half to two hours or longer needed for some separations is only a fraction of that required in paper chromatography or electrophresis. This rapidity is advantageous not only in obtaining prompt analytical results from TLC analysis itself, but also in preparative fractionation for analysis by other methods and in small-scale tests to select adsorbents and solvents for use in full-scale chromatographic work.

Preparation of Adsorbent Layers

Application of adsorbents to supporting glass plates is accomplished by making a slurry and spreading it in uniform layers on the plates. The usual solvent is distilled water, but other solvents have been used, and various chemicals may be added.

Glass plates for supporting thin-layers are of various sizes. The usual length is about 20cm which accommodates the generally used developement distance of 100mm. A 20cm square plate is suitable for two-dimensional work for up to 20 samples in one-dimensional developement, while narrower plates permit savings in adsorbents and solvents when fewer samples are to be chromatogaphed.

Air drying for a few minutes after application allows the thin-layers to set before the plates are moved. Multi-shelf storage racks may then be used to transport groups of plates conveniently and to protect the thin layers during drying, activating, and storing. Active plates are stored in desiccating cabinets for protection and visability during developement.

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