TLC - Retention Factor (Rf)

The retention factor, or R_f, 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 R_f is 0.75:

The R_f for a compound is a constant from one experiment to the next only if the chromatography conditions below are also constant:

• solvent system
• adsorbent
• thickness of the adsorbent
• amount of material spotted
• temperature

Since these factors are difficult to keep constant from experiment to experiment, relative Rf values are generally considered. “Relative R_f” means that the values are reported relative to a standard, or it means that you compare the R_f values of compounds run on the same plate at the same time.

The larger an R_f 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 R_f 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 R_f value than a polar compound run on the same plate.

The R_f 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 R_f 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 R_f exactly from experiment to experiment.