Thin-layer chromatography
Thin-layer chromatography
- Chromatography separates a mixture using two phases — one still, one moving.
- In thin-layer chromatography (TLC) the spots travel up a plate.
- Each substance has its own $R_{\text{f}}$ value.
The set-up
- the stationary phase stays still (e.g. aluminium oxide on a plate).
- the mobile phase moves (a solvent travelling up the plate).
- the baseline is where the spots start; the solvent front is the highest the solvent reaches.
Practice
In TLC, the stationary and mobile phases are:
The plate coating stays still; the solvent (mobile phase) travels up, carrying the spots.
The $R_{\text{f}}$ value
$$R_{\text{f}} = \frac{\text{distance moved by the spot}}{\text{distance moved by the solvent front}}$$
- It is always between 0 and 1.
- A substance that sticks more to the stationary phase moves less → a smaller $R_{\text{f}}$.
Practice
The Rf value is:
Rf = spot distance / solvent-front distance; it lies between 0 and 1.
Practice
A substance that sticks strongly to the stationary phase has:
Strong adsorption means it travels a shorter distance, giving a smaller Rf.
You've got it
Key idea
- TLC: a stationary phase (plate) and a mobile phase (solvent moving up)
- $R_{\text{f}} = \dfrac{\text{spot distance}}{\text{solvent-front distance}}$, always between 0 and 1
- sticks more to the stationary phase → smaller $R_{\text{f}}$