Chromatography techniquesAQA A-Level Chemistry: Revision notes
Section 1
The principle of chromatography
Chromatography separates and identifies the components of a mixture. It uses two phases:
- the stationary phase: a solid, or a liquid held on a solid
- the mobile (moving) phase: a liquid solvent or a gas that moves through the stationary phase
Separation depends on the balance between solubility in the moving phase and retention by the stationary phase. A component that is more soluble in the moving phase and less strongly retained travels further (TLC) or faster (CC and GC).
Always write both halves of the balance: solubility in the moving phase and retention by the stationary phase.
Section 2
Thin-layer chromatography (TLC)
In TLC, a plate is coated with a solid (such as silica) and a solvent moves up the plate.
Required practical 12:
- draw a pencil line near the bottom (pencil does not dissolve)
- spot the mixture and standards on the line
- stand the plate in a covered beaker with solvent below the line
- remove before the solvent reaches the top and mark the solvent front at once
- locate colourless spots with ultraviolet light or a developing agent, then measure distances
The solvent level must start below the pencil line, otherwise the spots dissolve into the beaker.
Section 3
Column and gas chromatography
Column chromatography (CC): a column is packed with a solid and a solvent moves down the column. Components emerge at different times and are collected.
Gas chromatography (GC): a column is packed with a solid or a solid coated with a liquid, and a gas is passed through under pressure at high temperature. Components with a stronger retention take longer to emerge, giving a longer retention time.
GC is often linked to mass spectrometry (GC–MS): each separated component passes into the mass spectrometer, which gives its M(r) and fragmentation pattern, so the component can be identified.
GC separates; mass spectrometry identifies. Say both.
Section 4
Rf values and retention times
Rf = distance moved by the spot ÷ distance moved by the solvent front. It is between 0 and 1 and has no units.
Worked example: a spot moves 4.5 cm and the solvent front 7.5 cm, so Rf = 4.5 ÷ 7.5 = 0.60.
Substances are identified by comparing Rf values (TLC) or retention times (CC, GC) with standards run under the same conditions. A match suggests the same substance but does not prove it: different compounds can have the same Rf or retention time, so confirm with another technique such as mass spectrometry.
Measure the spot distance to the centre of the spot and from the same baseline as the solvent front.
Must Know
- Chromatography separates and identifies components of mixtures
- TLC: solid-coated plate, solvent up; CC: packed column, solvent down; GC: gas under pressure at high temperature through a column
- Separation depends on the balance between solubility in the moving phase and retention by the stationary phase
- Rf = distance of spot ÷ distance of solvent front; compare with standards
- Retention times are compared with standards, and GC is analysed by mass spectrometry
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chromatography techniques
- A student separates the dyes in a black ink by thin-layer chromatography (TLC). A line is drawn near the bottom of a plate coated with silica, spots of the ink and of three known dyes are placed on the line, and the plate is stood in a shallow depth of solvent in a covered beaker.Explain why the different dyes in the ink travel different distances up the plate.2 marks
- In a TLC experiment the solvent front moved 7.5 cm from the baseline. A pure dye P moved 4.5 cm from the baseline. A sample of an unknown ink, run on the same plate, gave two spots, at 4.5 cm and 1.8 cm from the baseline.Calculate the Rf value of the second spot in the ink and state, with a reason, which of the two substances in the ink is more strongly retained by the stationary phase.2 marks
- A forensic chemist analyses a mixture of volatile organic compounds using gas chromatography linked to mass spectrometry (GC–MS). The column is packed with a solid coated with a liquid, and an inert gas is passed through it under pressure at high temperature. The mixture gives three peaks, with retention times of 2.1, 3.8 and 5.2 minutes. A standard of compound S, run under identical conditions, has a retention time of 3.8 minutes.Explain how gas chromatography separates the components of the mixture and how mass spectrometry is used after the separation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).