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Thin Layer Chromatography and Retention Factor
One very common and useful chromatography technique that allows us to obtain small amounts of separate products in a matter of several minutes is known as thin layer chromatography. In this technique, the stationary phase consists of a solid rectangular plate. A small sample of mixture, usually in liquid form, is placed at one end of the plate. THe plate is then placed into a beaker that contains the mobile phase in liquid form. It is crucial that the level of mobile phase (also known as the eluant or eluting solvent) is below the region of the plate where the sample mixture was spotted. Otherwise, the mixture will entirely dissolve in the solvent and no separation will occur. Once the plate is placed into the eluting solvent, the solvent begins to travel up the plate. When the eluting solvent reaches the sample, the sample mixes in and is carried up the plate. The rate at which a compound of the mixture travels up depends on its affinity to the stationary phase (plastic plate). Compounds that form stronger attractions move up more slowly than compounds that form weaker attractions. Once the eluant reaches near the top, the plate is removed and analyzed. One important quantity that describes how well the compound binds to the stationary phase is known as the retention factor. The greater the retention factor for a given compound, the more strongly it bonds to the stationary phase and the slower its rate of movement is.
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