Chromatography

Picture a dot of green leaf extract on paper, with the bottom edge dipped in solvent. As the solvent climbs the paper by capillary action, it drags the pigments along with it. This is a tug of war between two phases: the mobile phase (the solvent, moving) and the stationary phase (the paper, fixed). Every component in the mixture is being pulled toward the mobile phase while also being held back by attraction to the stationary phase. The paper is slightly polar, since it's made of cellulose full of -OH groups. A component that is very polar sticks to the paper tightly and barely moves. A component that is nonpolar has little attraction to the paper and rides along with the solvent, traveling far. That's why a leaf extract splits into bands: chlorophylls, carotenes, and xanthophylls each have different polarities, so each settles at a different height once the solvent stops moving.

To compare runs done on different papers or over different times, chemists use the retention factor, Rf, defined as the distance the spot traveled divided by the distance the solvent front traveled. Rf has no units and always falls between 0 and 1. A component with Rf close to 1 traveled almost as far as the solvent, meaning it strongly preferred the mobile phase, so it is relatively nonpolar given a polar stationary phase. A component with Rf close to 0 barely left the starting line, meaning it clung to the stationary phase.

Chromatography sits alongside the other separation and identification tools in this unit: it depends on the same intermolecular forces, like hydrogen bonding and dipole interactions, that explain boiling points and solubility elsewhere in the unit. Where gas laws and spectroscopy tell you about bulk behavior or concentration, chromatography tells you how many distinct substances are present and gives a rough polarity ranking of each.