Retardation factor
Encyclopedia
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 (Rf) 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 RF 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 (Rf), 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.
the retention factor, R, is the fraction of the sample in the mobile phase at equilibrium, defined as:
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.
and conversely:
Chromatography
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
, 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 (Rf) 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 RF 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 (Rf), in regards to planar chromatography, the term is not defined in this context. However, in column chromatography
Column chromatography
Column chromatography in chemistry is a method used to purify individual chemical compounds from mixtures of compounds. It is often used for preparative applications on scales from micrograms up to kilograms.The main advantage of column chromatography is the relatively low cost and disposability...
, 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 chromatographyChromatography
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
the retention factor, R, is the fraction of the sample in the mobile phase at equilibrium, defined as:
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.
Relationship with retention factor
In terms of retention factor (k), retardation factor (R) is defined as follows:and conversely: