If the flow of mobile phase is monitored by a detector and recorder/VDU system, such as is used in gas
and high-performance liquid chromatography, then tR can be used as a measure of VR.
In paper and thin-layer chromatography, the separation process is halted at a stage which leaves the
separated components in situ on the surface in the form of spots. The rate at which a solute has moved
is then determined by its retardation factor, Rf, which is defined as
It is inversely related to D and clearly cannot be greater than 1. Distances are measured from the point
of application of the sample. As both tR and Rf are related to D they will depend on the conditions under
which a chromatogram is run. Valid comparisons between samples and between samples and standards
can be made only if experimental conditions are identical. In many cases this is difficult to achieve and
it is common practice to run samples and standards sequentially or simultaneously to minimize the
effects of variations.
Chromatographic Performance
The ideal chromatographic process is one in which the components of a mixture form narrow bands
which are completely resolved from one another in as short a time as possible. The performance of a
particular chromatographic system can be assessed in the following ways.
Efficiency and Resolution
The width of a band or peak is a measure of the efficiency of the process whilst resolution is assessed by
the ability to resolve the peaks of components with similar tR or Rf values.
Efficiency, N, for column separations is related to retention time and peak width measured in terms of
the standard deviation, assuming an ideally Gaussian-shaped peak (p. 16), i.e.
In practice it is easier to measure baseline width or the width at one half of the peak height, so N is
generally calculated using one of the alternative formulae:
or