Chromatography
Introduced| English | Meaning |
|---|---|
| chromatography/krəʊməˈtɒɡrəfi/ | A separation technique based on different affinities of substances for a stationary phase and a mobile phase. |
| solvent/ˈsɒlvənt/ | The substance in which a solute dissolves to form a solution. |
| Rf value/ˌɑːˈref ˈvæljuː/ | In planar chromatography, the distance travelled by a substance's spot divided by the distance travelled by the solvent front from the same starting line. |
Separating colours and substances
- How do forensic scientists separate the dyes in an ink, or the colours in a food?
- With a simple, powerful technique: chromatography 色谱法.
Chromatography route
Watch a mixture separate as the solvent front moves.
Chromatography separates a mixture because its components:
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Different speeds separate the substances.
How chromatography works
- A spot of the mixture is placed on paper, and a solvent 溶剂 moves up through it.
- Different substances travel at different speeds, so they separate into spots.
Paper chromatography separates a mixture as the solvent rises up the paper
A more soluble substance will travel:
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More soluble components move further.
Reading a chromatogram
- Substances that are more soluble (or less attracted to the paper) travel further.
- Comparing with known substances identifies the components.
A mixture separates into coloured spots that rise different distances up the paper
A spot moves 3 cm; the solvent moves 6 cm. What is the Rf value?
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Rf = 3 ÷ 6 = 0.5.
An Rf value is always between 0 and 1.
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The spot cannot move further than the solvent.
Rf = distance moved by the spot ÷ distance moved by the ______.
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Rf compares the spot's distance to the solvent's.
Rf value 比移值
- The Rf value = distance moved by the spot ÷ distance moved by the solvent.
- It is always between 0 and 1, and is the same for a substance under the same conditions.
Worked example. A spot moves 4 cm while the solvent moves 8 cm. Rf = 4 ÷ 8 = 0.5. This value identifies the substance when compared with known samples.
You've got it
- chromatography separates a mixture as a solvent carries components at different speeds
- more soluble substances travel further up the paper
- Rf = distance of spot ÷ distance of solvent (always between 0 and 1)