Thin-layer chromatography
| English | Chinese | Pinyin |
|---|---|---|
| chromatography | 色谱 | sè pǔ |
| stationary phase | 固定相 | gù dìng xiāng |
| mobile phase | 流动相 | liú dòng xiāng |
| baseline | 基线 | jī xiàn |
| solvent front | 溶剂前沿 | róng jì qián yán |
Separating on a plate
- Chromatography 色谱 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.
TLC route
Follow a spot up a plate and use Rf to identify substances.
In thin-layer chromatography, the ratio of distances moved by a spot and the solvent is the ______ value.
Always between 0 and 1.
A more soluble component travels further up a TLC plate.
It spends more time in the moving solvent.
The set-up
- the stationary phase 固定相 stays still (e.g. aluminium oxide on a plate).
- the mobile phase 流动相 moves (a solvent travelling up the plate).
- the baseline 基线 is where the spots start; the solvent front 溶剂前沿 is the highest the solvent reaches.
Thin-layer chromatography and the Rf value
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
- It is always between 0 and 1.
- A substance that sticks more to the stationary phase moves less → a smaller $R_{\text{f}}$.
A developed TLC plate under UV light: each spot is a separated component.
The Rf value is:
Rf = spot distance / solvent-front distance; it lies between 0 and 1.
A substance that sticks strongly to the stationary phase has:
Strong adsorption means it travels a shorter distance, giving a smaller Rf.
Using an Rf value
Worked example. A spot moves 3.0 cm while the solvent front moves 7.5 cm. Rf = 3.0 ÷ 7.5 = 0.40 — compared with knowns to identify the compound.
You've got it
- 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 $R_{\text{f}}$