In
chromatography
In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...
, the retardation factor (''R'') is the fraction of an
analyte
An analyte, component (in clinical chemistry), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest substances are referred to as analytes, such as 24 karat gold, NaCl, water, etc. ...
in the mobile phase of a chromatographic system.
In
planar chromatography
In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system (a ...
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.
Although the term retention factor is sometimes used synonymously with retardation factor in regard to planar chromatography the term is not defined in this context. However, in
column chromatography
Column chromatography in chemistry is a chromatography method used to isolate a single chemical compound from a mixture. Chromatography is able to separate substances based on differential adsorption of compounds to the adsorbent; compounds move th ...
, the retention factor or capacity 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,
which is inversely proportional to the retardation factor.
General definition
In chromatography, the retardation factor, ''R'', is the fraction of the sample in the mobile phase at equilibrium, defined as:
:
Planar chromatography
The retardation factor, ''R
F'', is commonly used in
paper chromatography
Paper chromatography is an analytical method used to separate coloured chemicals or substances. It is now primarily used as a teaching tool, having been replaced in the laboratory by other chromatography methods such as thin-layer chromatography ...
and
thin layer chromatography
Thin-layer chromatography (TLC) is a chromatography technique used to separate non-volatile mixtures.
Thin-layer chromatography is performed on a sheet of an inert substrate such as glass, plastic, or aluminium foil, which is coated with a t ...
for analyzing and comparing different substances. It can be mathematically described by the following ratio:
:
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.
Relationship with retention factor
In terms of retention factor (''k''), retardation factor (''R'') is defined as follows:
:
and conversely:
:
References
{{chromatography
Chromatography