retardation factor

Meaning: In thin-layer chromatography (TLC), a sample spot is placed near the bottom of a plate and a solvent (mobile phase) rises by capillary action. Each compound partitions between the mobile phase and the stationary phase. Compounds that interact strongly with the stationary phase move less and have a small Rf. Compounds that dissolve well in the mobile phase move farther and have a large Rf. Measure the compound from the origin line to the center of its spot, and the solvent from the origin line to the solvent front.

Equation: Rf = d(compound) / d(solvent front), where both distances are in meters (or any one length unit, since Rf is a dimensionless ratio) measured from the origin line.

On the MCAT: A passage shows a TLC plate with several spots, or gives Rf values for a mixture. You rank the compounds by polarity or predict how Rf changes if the solvent is made more polar. With silica or alumina (polar stationary phase), polar compounds such as alcohols, acids and amines have lower Rf than nonpolar compounds such as alkanes or ethers. A more polar solvent raises the Rf of all the compounds, because it competes with them for the plate.

Common confusion: Students invert the ratio or expect Rf above 1. The solvent front always travels at least as far as the compound, so Rf is at most 1. Also, "polar travels farther" is wrong on normal-phase plates. In reverse-phase TLC (nonpolar stationary phase), the order flips.