Chromatography
| English | Chinese | Pinyin |
|---|---|---|
| chromatography | 色谱法 | sè pǔ fǎ |
| solvent | 溶剂 | róng jì |
| Rf value | 比移值 | bǐ yí zhí |
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:
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:
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?
Rf = 3 ÷ 6 = 0.5.
An Rf value is always between 0 and 1.
The spot cannot move further than the solvent.
Rf = distance moved by the spot ÷ distance moved by the ______.
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)