Thin-layer chromatography
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Thin-layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures.
General
Thin-layer chromatography is performed on a sheet of glass, plastic, or aluminium foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide (alumina), or cellulose, called TLC plates. This layer of adsorbent is known as the stationary phase.
Operation
On a TLC plate, a small line is drawn using a pencil, and spots of the compound are placed on the line, using a glass capillary. After the sample has been applied on the plate, the TLC plate is immersed until the pencil line in a solvent or solvent mixture (known as the mobile phase), which is drawn up the plate via capillary action. Because different analytes ascend the TLC plate at different rates, separation is achieved. The mobile phase has different properties from the stationary phase. For example, with silica gel, a very polar substance, non-polar mobile phases such as heptane are used. The mobile phase may be a mixture, allowing chemists to fine-tune the bulk properties of the mobile phase.
After the analytes stop ascending, the spots are visualized. Often this can be done simply by projecting ultraviolet light onto the sheet using an ultraviolet lamp. Sometimes, a bit of help is required, the sheets are treated with a phosphor, and dark spots appear on the sheet where compounds absorb the light impinging on a certain area.
