Thin-layer chromatography
Introduced| English | Meaning |
|---|---|
| chromatography/krəʊməˈtɒɡrəfi/ | A separation technique based on different affinities of substances for a stationary phase and a mobile phase. |
| stationary phase/ˈsteɪʃənəri feɪz/ | The phase that remains fixed in a chromatography system and interacts differently with components of a mixture. |
| mobile phase/ˈməʊbaɪl feɪz/ | mobile phase |
| baseline/ˈbeɪslaɪn/ | The starting line on a chromatogram where sample spots are placed before the solvent moves. |
| solvent front/ˈsɒlvənt frʌnt/ | The furthest position reached by the mobile solvent during a chromatographic separation. |
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.
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Always between 0 and 1.
A more soluble component travels further up a TLC plate.
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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:
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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:
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Rf = spot distance / solvent-front distance; it lies between 0 and 1.
A substance that sticks strongly to the stationary phase has:
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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 $R_{\text{f}}$