In chromatography, a retardation factor (R_f) (also known as retention factor) is a ratio defined as follows:

For example, if particular substance in an unknown mixture travels 2.5 cm and the solvent front travels 5.0 cm, the retention factor would be 0.5. An R_f value will always be less than 1 since the solvent front always travels further than does the substance under investigation. An R_f value is characteristic for any given compound (provided that the same stationary and mobile phases are used).

It can provide corroborative evidence as to the identity of a compound. If the identity of a compound is suspected but not yet proven, an authentic sample of the compound, or standard, is spotted and run on a TLC plate side by side (or on top of each other) with the compound in question. Note that this identity check must be performed on a single plate, because it is difficult to duplicate all the factors which influence Rf exactly from experiment to experiment.

See also

• Paper chromatography
• Thin layer chromatography