Thin-Layer Chromatography

Thin-layer chromatography (TLC) is a quick, cheap way to separate a small sample, check its purity or identify its components.The stationary phase is a thin layer of a solid, usually silica (SiO₂) or alumina (Al₂O₃), on a flat glass, metal or plastic plate. The mobile phase is a liquid solvent.Method:
1. Draw a baseline in pencil about 1 cm from the bottom of the plate. (Ink would dissolve in the solvent and separate too.)
2. Put small spots of the mixture, and of known reference substances, on the baseline.
3. Stand the plate in a beaker containing a small depth of solvent. The solvent level must be below the baseline, so the spots do not dissolve into the solvent in the beaker.
4. Cover the beaker with a lid, so the air inside is saturated with solvent vapour and the solvent does not evaporate from the plate.
5. When the solvent has nearly reached the top, remove the plate and mark the solvent front in pencil straight away.6. Many organic compounds are colourless, so the spots must be made visible. Either shine UV light on a plate containing a fluorescent dye (the spots appear dark), or use a developing agent such as iodine, which stains the spots.Each component moves a different distance, depending on its balance between solubility in the solvent and adsorption onto the solid. A pure substance gives a single spot; a mixture gives several. Comparing spots with the reference substances, or calculating R_f values, identifies the components.TLC uses tiny amounts, so it is used to test purity and follow reactions, not to collect product.