Retardation factor

In chromatography, the retardation factor (R) is defined as the fraction of an analyte in the mobile phase of a chromatographic system.. In planar chromatography in particular, the retardation factor (R_f) is defined as the ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front. Ideally, the values for R_F are equivalent to the R values used in column chromatography.

It is important to note that, although the term retention factor is sometimes used synonymously with retardation factor (R_f), in regards to planar chromatography, the term is not defined in this context. However, in column chromatography, the retention factor (k) is defined as the ratio of time an analyte is retained in the stationary phase to the time it is retained in the mobile phase.

General definition

In chromatography the retention factor, R, is the fraction of the sample in the mobile phase at equilibrium, defined as:

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

Planar chromatography

The retardation factor in planar chromatography, R_f 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, it can only move in the direction of the solvent flow, and cannot move faster than the solvent. R_f 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 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.

Relationship with retention factor

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

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

and conversely:

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

See also

• Paper chromatography
• Thin layer chromatography