In thin-layer chromatography (TLC), a liquid solvent moves up a plate carrying the components of a sample across a solid stationary phase.
| Phase | In TLC | Role |
|---|---|---|
| Stationary phase | A thin coating of silica, , or alumina, | Solute particles can adsorb onto its surface. |
| Mobile phase | A liquid solvent | Moves through the stationary phase and carries dissolved components up the plate. |
The solvent may be polar or non-polar. Polar examples include water and alcohols, while alkanes provide non-polar examples. Greater solubility in the chosen solvent favours movement up the plate, whereas stronger attraction to the stationary phase reduces the distance travelled.
Developing a TLC plate
- Wear gloves when handling the TLC plate to reduce contamination from the hands.
- Draw a pencil baseline about 1 cm above the lower edge of the plate.
- Use a capillary tube to add a tiny spot of each sample or reference solution to the baseline, then allow the spots to dry.
- Add a shallow layer of solvent to a chromatography chamber or covered beaker.
- Place the plate in the chamber with the solvent level below the baseline, then replace the lid.
- Allow the solvent to rise until it is close to the top of the plate.
- Remove the plate and immediately mark the solvent front in pencil.
- Allow the plate to dry, using a fume cupboard when required for the solvent being used.
Why the practical details matter
| Choice | Reason |
|---|---|
| Use pencil for the baseline | Pencil does not dissolve in the solvent and therefore does not produce an additional chromatographic spot. |
| Apply only tiny sample spots | Large spots can spread and overlap, making the separated components difficult to distinguish. |
| Keep the solvent below the baseline | This prevents the starting samples from dissolving directly into the solvent at the bottom of the chamber. |
| Cover the chamber | The lid reduces loss of solvent by evaporation. |
| Let the solvent rise close to the top | A greater distance of travel improves the accuracy of the measurements, although an can still be calculated if the solvent front stops below the top. |
Exam Tip: In a method question, state explicitly that the starting spots are above the solvent level and that both the baseline and final solvent front are marked in pencil.