How TLC separates a mixture
Thin-layer chromatography (TLC) separates substances as a solvent moves up a plate coated with a solid stationary phase.
Each component has two competing tendencies: it can be carried upwards by the moving solvent or held back by the fixed coating. Differences in these interactions cause the components to move at different rates and become separated.
| Phase | Meaning in TLC | Effect on movement |
|---|---|---|
| Stationary phase | The solid coating fixed to the TLC plate | Stronger retention by the coating reduces the distance a component travels. |
| Mobile phase | The solvent moving through the stationary phase | Greater solubility in the solvent allows a component to be carried further up the plate. |
Because TLC uses a solid stationary phase, retention can occur through adsorption onto the surface of that solid.
A component that is strongly retained by the stationary phase moves relatively slowly, whereas one that is more readily carried in the mobile phase travels further.
Using TLC to analyse mixtures
A mixture can produce several spots because its components separate during development. Known substances can be run alongside an unknown so that the positions of their spots can later be compared.
A suitable practical application is the comparison of aspirin, ibuprofen, paracetamol and caffeine with a tablet containing a mixture of analgesic ingredients.
Exam Tip: Explain TLC separation using both phases: a substance may be carried readily by the mobile solvent or retained more strongly on the stationary phase.