In TLC, the retention factor (Rf) is calculated as which ratio?

• A. Distance of sample / Distance of solvent
• B. Distance of solvent / Distance of sample
• C. Distance of sample / Distance of mobile phase
• D. Distance of solvent / Distance of stationary phase

Answer: (A)

In TLC, you judge how far a compound travels relative to how far the solvent itself moves. The retention factor is the distance moved by the compound from the origin divided by the distance moved by the solvent front from the origin. This gives a unitless value (0 to 1) that reflects how strongly the compound interacts with the stationary phase versus the mobile phase: a smaller value means more retention on the plate, a larger value means the compound rides the solvent further up. In practice, you measure from the origin to the center of the compound’s spot and from the origin to the solvent front. The ratio of these two distances is the Rf.