TLC, or Thin-Layer Chromatography, is a commonly used technique in synthetic chemistry to check for purity and the progress of reactions. TLC uses a stationary phase, typically a silica gel bound to a plastic, glass or aluminum backing, and a mobile phase, typically common organic solvents such as ethyl acetate or hexane, to separate components of a reaction or sample.

The movement of analytes on a TLC plate is measured by the Rf value. Rf stands for retardation factor. The Rf value is defined as the ratio of the distance an analyte moved to the distance the solvent front moved. The Rf value of analytes in a reaction or sample can and should be adjusted by changing the mobile phase in order to give optimum separation. In TLC, the optimum Rf value is totally dependent on the reaction or sample you are working with. For checking reaction progress, you merely need to see the individual reaction components separate from each other. For separation of a product from the leftover reactants, the Rf of the product is preferred to be lower in order to keep the analyte on a column longer.