Planar Chromatography
The retardation factor in planar chromatography, Rf can be mathematically described by the following ratio:
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 Rf value will always be in the range 0 to 1; if the substance moves, it can only move in the direction of the solvent flow, and cannot move faster than the solvent. Rf values are only useful if they are between these two extremes. One can choose a mobile phase with different characteristics (particularly polarity) in order to control how far the substance being investigated migrates.
An Rf 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.
Read more about this topic: Retardation Factor