TLC is an acronym for thin-layer chromatography. The most common use of TLC is for determining the purity of drug substances in the pharmaceutical industry. TLC has been used for this purpose for many years, but in most cases it is now preferable to use high-performance liquid chromatography.
A plastic or glass plate is coated with the stationary phase for this type of chromatography. A small amount of the sample solution to be analyzed is "spotted" near the bottom of the plate using a capillary tube or a micro-syringe. The sample solvent is allowed to evaporate, and the starting position is marked. The plate is then placed in a developing chamber with the bottom edge in contact with the mobile phase solvent. The solvent traverses up the plate, carrying the components of the sample mixture with it. Under ideal conditions, compounds will be separated into distinct spots on the plate by the time the solvent has traversed two thirds of the way up. The plate is then removed from the developing chamber and allowed to dry.
At this point the components of the mixture are separated, but will usually be invisible to the human eye because most pharmaceutical substances as well as most TLC stationary phases are white. In order to see the separated spots, it is common to spray the dried plate with solutions of iodine, sulfuric acid or ninhydrin. These substances react with many organic compounds to form substances more readily visible to the human eye. Alternatively, TLC plates containing fluorescent material in the stationary phase can be used. This type of plate is viewed under ultraviolet light in order to see the separated compounds as dark spots on a bright background. When compared to a known substance, the migration distance and size of the spots of the separated sample give useful information about its identity and purity.
See also: chromatography, high-performance liquid chromatography.