AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Chromatography can be used to separate and identify the components in a…
Full specification wording
Chromatography (A-level only): Chromatography can be used to separate and identify the components in a mixture.
Every chromatographic method has the same two parts: a stationary phase that stays put, and a mobile phase that moves through or over it.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every chromatographic method has the same two parts: a stationary phase that stays put, and a mobile phase that moves through or over it. A mixture placed on the stationary phase is carried along by the mobile phase, and each component travels at its own speed because each divides differently between the two phases. That is the separation. Working out what the components are is a second job, done by comparison: the distance a spot travels relative to the solvent front gives an Rf value on a thin-layer plate, and the time a component takes to leave a gas chromatography column gives a retention time. Both mean something only against a known compound run under identical conditions, or against a database, because temperature, solvent and stationary phase all change the number.
What earns the marks
- Naming the stationary phase and the mobile phase.
- Components travelling at different speeds because they divide differently between the two phases.
- Identification by comparing an Rf value or a retention time with that of a known compound.
- Stating that the comparison must be made under the same conditions.
Common mistakes and how to put them right
Saying chromatography separates by mass, which is what a mass spectrometer does.
Quoting an Rf value greater than one, which is impossible because a spot cannot overtake the solvent.
Giving an Rf value a unit.
Identifying a component from a retention time measured under different conditions.
Exam tips
- Name both phases in the first line of any chromatography answer, because the marks are usually attached to them.
- State the conditions whenever you claim an identification; a retention time is evidence only against a matching standard.
- Rf values have no units, so do not invent one.
Try the lesson
The start of the Chromatography (A-level only): Chromatography can be used to separate and identify the components in a mixture. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the purpose of chromatography when it is applied to a mixture?
Listen
Every chromatographic method has the same two parts: a stationary phase that stays put, and a mobile phase that moves through or over it. A mixture placed on the stationary phase is carried along by the mobile phase, and each component travels at its own speed because each divides differently between the two phases. That is the separation. Working out what the components are is a second job, done by comparison: the distance a spot travels relative to the solvent front gives an Rf value on a thin-layer plate, and the time a component takes to leave a gas chromatography column gives a retention time. Both mean something only against a known compound run under identical conditions, or against a database, because temperature, solvent and stationary phase all change the number. A strong answer earns marks for these points. naming the stationary phase and the mobile phase. components travelling at different speeds because they divide differently between the two phases. identification by comparing an Rf value or a retention time with that of a known compound. stating that the comparison must be made under the same conditions. Exam tip: Name both phases in the first line of any chromatography answer, because the marks are usually attached to them.
Key words
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stationary and mobile phases
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different speeds of travel
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Rf value
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retention time
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See every definition
- stationary and mobile phases
- Every chromatographic method has a stationary phase that stays put and a mobile phase that moves through or over it.
- different speeds of travel
- Each component travels at its own speed because each divides differently between the two phases.
- Rf value
- The distance a spot travels relative to the solvent front on a thin-layer plate, carrying no unit.
- retention time
- The time a component takes to leave a gas chromatography column.
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