6.9.1Organic Chemistry and Analysis
Thin Layer Chromatography, TLC
5 min read 5 parts Lesson 28 of 33
Section 1 of 5Principles of TLC
TLC separates substances by how strongly they are held by a thin solid layer compared with how well the solvent carries them.
Stationary phase
- A thin layer of silica or alumina is coated on a glass or plastic plate; it is a polar solid.
Mobile phase
- A solvent, or mixture of solvents, travels up the plate by capillary action, carrying the components with it.
Separation
- Components that are more strongly adsorbed on the plate move more slowly than those that dissolve better in the solvent.
Result
- Different components end up at different heights, giving separate spots on the plate.
Section 2 of 5Carrying Out TLC
1
Draw a pencil line about 1 cm from the bottom of the plate, since pencil does not dissolve in the solvent.
2
Place small spots of each sample and of known reference compounds on the line, allowing each to dry.
3
Stand the plate in a covered container with the solvent just below the line, so the spots are not washed off.
4
Remove the plate when the solvent front is near the top, and mark the front immediately with a pencil.
5
Reveal colourless spots using a UV lamp, iodine vapour or a developing reagent such as ninhydrin, depending on the compound.
Section 3 of 5Rf Values
Rf Values
Definition
- Rf = distance moved by the spot ÷ distance moved by the solvent front, measured from the baseline.
Range
- Rf is always between 0 and 1 and has no units, since it is a ratio of two lengths.
Identification
- A spot with the same Rf as a reference compound, under identical conditions, is likely the same substance.
Interpretation
- Polar compounds stick to the silica more and have lower Rf, while less polar ones travel further.
| Spot | Distance moved / cm | Solvent front / cm | Rf |
|---|---|---|---|
| A | 2.4 | 8.0 | 0.30 |
| B | 3.2 | 8.0 | 0.40 |
| C | 6.0 | 8.0 | 0.75 |
Section 4 of 5Worked Example
Question
- A spot travels 3.2 cm while the solvent front travels 8.0 cm.
- Calculate the Rf value and state what happens to Rf if the solvent is made less polar.
- Rf = distance moved by spot ÷ distance moved by solvent front.
- Rf = 3.2 ÷ 8.0 = 0.40.
- A less polar solvent is poorer at dissolving polar compounds, so they move less and Rf values usually fall.
Answer Rf = 0.40, and values for polar compounds fall in a less polar solvent.
Section 5 of 5Exam Checklist & Tips
6.9.1 – Key Points for the Exam
- Stationary phase The solid layer on the plate that holds components.
- Mobile phase The solvent that moves through the stationary phase.
- Solvent front The furthest point reached by the solvent.
- Rf value Distance moved by the spot divided by the distance moved by the solvent front.
Common Mistakes
- Drawing the baseline in ink, which would run up the plate with the solvent.
- Submerging the baseline in the solvent so the samples dissolve into the pool.
- Measuring the distance to the edge of the spot rather than to its centre.
- Forgetting to mark the solvent front before it dries.