37.1 Thin-layer chromatography

The stationary phase stays fixed (for example alumina on a solid support); the mobile phase is the solvent that moves through it. A sample travels from the baseline, while the solvent front marks the furthest solvent position.

A compound repeatedly partitions between phases according to polarity and intermolecular attraction. Its Rf is distance travelled by the spot divided by distance travelled by the solvent front.

If a spot moves 3.0 cm and the solvent front moves 6.0 cm, Rf = 0.50. Measure both distances from the baseline.

Rf has no units and should be between 0 and 1; do not measure from the edge of the paper or compare values from different solvent systems without care.

Rf is the distance travelled by a solute spot divided by the distance travelled by the solvent front, with both distances measured from the baseline.

Rf has no units and lies between 0 and 1. It helps compare a substance with a reference only when the stationary phase, solvent, temperature and measurement method are the same.

A spot that travels 2.4 cm while the solvent front travels 8.0 cm has Rf = 0.30. A second spot at 6.4 cm has Rf = 0.80, not 0.80 cm.

Do not measure from the paper edge or use the solvent-front distance as the denominator for one spot and the baseline distance for another.

A solute travels farther when it is relatively soluble in the mobile phase and interacts weakly with the stationary phase. Strong attraction to the stationary phase holds it back and lowers Rf.

Polarity, hydrogen bonding and the solvent composition control the balance. “More soluble” means relative to the other phase, not simply soluble in an absolute sense.

A polar compound may remain closer to polar alumina while a less strongly adsorbed compound moves with the solvent and gives a larger Rf.

A larger Rf does not mean a larger molecule or a higher boiling point; it describes phase interactions in that chromatographic system.