A chemist uses thin-layer chromatography (TLC) to investigate a sample of an unknown organic compound. After running the chromatogram, only one spot is visible. The chemist then calculates the Rf value of this spot and compares it to values in a reference table of known compounds. Explain how thin-layer chromatography separates the components of a mixture and describe how the Rf value of a spot is determined and used to identify an unknown compound.

Thin‑layer chromatography separates components by the differential adsorption of the mixture onto the stationary phase (usually silica gel) and the differential solubility in the mobile phase (solvent). When the sample is spotted on the plate, the solvent rises by capillary action. Components that are more attracted to the stationary phase move more slowly, while those that are more soluble in the solvent move faster.

The Rf value of a spot is calculated as:

Rf = (distance travelled by the spot from the origin) ÷ (distance travelled by the solvent front).

Because the same solvent system and plate are used for all runs, the Rf value is characteristic of a compound. The chemist compares the Rf of the unknown spot with the Rf values of known reference compounds run under identical conditions. If the Rf matches that of a reference compound, the unknown is identified as that compound. A single spot on the plate indicates that the sample contains only one component and is therefore pure.