The retention factor, or Rf, is defined as the distance traveled by the compound divided by the distance traveled by the solvent.
For example, if a compound travels 2.1 cm and the solvent front travels 2.8 cm, the Rf is 0.75:
The Rf for a compound is a constant from one experiment to the next only if the chromatography conditions below are also constant:
Since these factors are difficult to keep constant from experiment to experiment, relative Rf values are generally considered. Relative Rf means that the values are reported relative to a standard, or it means that you compare the Rf values of compounds run on the same plate at the same time.
The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When comparing two different compounds run under identical chromatography conditions, the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a mixture, you can predict that a compound of low polarity will have a larger Rf value than a polar compound run on the same plate.
The Rf 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. If two substances have the same Rf value, they are likely (but not necessarily) the same compound. If they have different Rf values, they are definitely different compounds. 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.
Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2010. The information on these pages is available for academic use without restriction.