Retardation factor

In chromatography, the retardation factor, R, is the fraction of the sample in the mobile phase at equilibrium, defined as:^[1]

${\displaystyle \ R={\frac {\mbox{quantity of substance in the mobile phase}}{\mbox{total quantity of substance in the system}}}}$ 

The retardation factor, R_F, is commonly used in paper chromatography and thin layer chromatography (TLC) for analyzing and comparing different substances. It can be mathematically described by the following ratio:^[2]

${\displaystyle \ R_{F}={\frac {\mbox{migration distance of substance}}{\mbox{migration distance of solvent front}}}}$ 

An R_F 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. For example, if particular substance in an unknown mixture travels 2.5 cm and the solvent front travels 5.0 cm, the retardation factor would be 0.50. One can choose a mobile phase with different characteristics (particularly polarity) in order to control how far the substance being investigated migrates.

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 R_F exactly from experiment to experiment.

In terms of retention factor (k), retardation factor (R) is defined as follows:

${\displaystyle \ R={\frac {1}{1+k}}}$ 

based on the definition of k:^[3]

${\displaystyle \ k={\frac {1-R}{R}}}$