Retardation factor

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In chromatography, a retardation factor (R_f) (also known as retention factor) describes the ratio of time spent in the stationary phase relative to time spent in the mobile phase. It can be mathematically described by the following ratio:

$\ R_f = \frac{\mbox{migration distance of substance}}{\mbox{migration distance of solvent front}}$

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 in the range 0 to 1: if the substance moves at all, it moves along the direction the solvent ("mobile phase") does less, but cannot move further than the solvent does. R_f values are only useful if they are between these two extremes. One can choose a mobile phase with different characteristics (especially 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.