How does chromatography separate and identify the components of a mixture?
Chromatography as a method of separation using a stationary and a mobile phase. Thin-layer and column chromatography and the calculation of Rf values. Gas chromatography and the meaning of retention time. The combination of gas chromatography with mass spectrometry for identification.
A focused answer to the AQA A-Level Chemistry 3.3.16 specification points on chromatography. Covers the principle of stationary and mobile phases, thin-layer and column chromatography, Rf value calculation, gas chromatography and retention time, and the use of GC-MS.
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What this dot point is asking
AQA wants you to explain how chromatography separates mixtures using a stationary and mobile phase, describe thin-layer and column chromatography, calculate Rf values, explain gas chromatography and retention time, and describe how GC-MS identifies components.
The principle
All chromatography relies on two phases:
- The stationary phase stays fixed (e.g. silica on a TLC plate, or the liquid coating in a GC column).
- The mobile phase moves through it (a solvent in TLC, an inert carrier gas in GC).
Components distribute themselves between the two phases depending on their relative attraction (adsorption or solubility). A component strongly attracted to the mobile phase and weakly to the stationary phase travels furthest. The separation works because the components share an equilibrium between the two phases: a molecule that spends more of its time dissolved in (or adsorbed onto) the stationary phase moves slowly and lags behind, while one that spends more time in the mobile phase moves quickly. The greater the difference in these attractions, the better the separation, which is why the choice of solvent (in TLC) or column coating (in GC) is adjusted to spread the components out.
Thin-layer and column chromatography
In TLC, a small spot of mixture is placed near the bottom of a plate coated with silica (stationary phase). The plate stands in solvent (mobile phase), which rises and carries the components different distances.
Rf values are constant for a given compound, stationary phase and solvent, so they help identify components by comparison with standards run on the same plate. Column chromatography uses the same principle with the stationary phase packed in a vertical column; the mobile phase is poured through, and components elute (leave the bottom of the column) at different times, so it can be used to collect and purify a chosen component rather than just analyse a mixture.
Gas chromatography
In GC the sample is vaporised and carried by an inert gas (mobile phase) through a column coated with a liquid stationary phase. Each component takes a characteristic time to pass through, called the retention time, recorded as a peak on the chromatogram. The area under each peak is proportional to the amount of that component.
GC-MS
Try this
Q1. Name the two phases in any form of chromatography. [2 marks]
- Cue. A stationary phase and a mobile phase.
Q2. What does GC-MS add beyond gas chromatography alone? [1 mark]
- Cue. Identification of each separated component from its mass spectrum.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksIn a thin-layer chromatography experiment, the solvent front moves and a component spot moves . Calculate the value, and explain why values must be compared against standards run on the same plate to identify a compound.Show worked answer →
(no units, always less than 1).
depends on the stationary phase, the solvent (mobile phase) and the temperature, so the same compound can give different values under different conditions. Comparing the unknown's spot against known standards run on the same plate, in the same solvent, removes these variables, so a matching is reliable evidence of identity.
Markers reward the correct of and the explanation that is condition-dependent, so standards must be run alongside on the same plate.
AQA 20213 marksExplain how gas chromatography separates the components of a mixture, and state what advantage combining it with mass spectrometry (GC-MS) provides.Show worked answer →
In gas chromatography the sample is vaporised and carried by an inert gas (the mobile phase) through a column coated with a liquid stationary phase. Each component spends a different proportion of time dissolved in the stationary phase depending on its attraction to it, so each leaves the column after a characteristic retention time and appears as a separate peak.
GC-MS adds identification: each separated component passes into a mass spectrometer, which records its mass spectrum, allowing the component to be identified (by molecular mass and fragmentation), not just separated.
Markers reward the mobile and stationary phases, separation by retention time, and that the mass spectrometer identifies each component.
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Sources & how we know this
- AQA A-level Chemistry (7405) specification — AQA (2015)