Chromatography (A-level only) — AQA A-Level Chemistry
Test yourself on Chromatography (A-level only) with AQA A-Level practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Chromatography (A-level only) explained
Every chromatographic method has the same two parts: a stationary phase that stays put, and a mobile phase that moves through or over it.
Read the full explanation
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.
Types of chromatography include: thin-layer chromatography (TLC) – a plate is coated with a solid and a solvent moves up the plate column chromatography (CC) – a column is packed with a solid and a solvent moves down the column gas chromatography (GC) – a column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature.
On a thin-layer plate the stationary phase is a thin coating of silica or alumina and the mobile phase is a solvent that climbs by capillary action. In a column the same kind of solid is packed into a tube and the solvent runs down through it under gravity, so the separated components leave the bottom one after another and can actually be collected: that is why it is the preparative method. In gas chromatography the column holds a solid, or a solid coated with a non-volatile liquid, and the mobile phase is an unreactive carrier gas pushed through under pressure while the column sits in a hot oven. The high temperature keeps the sample as a vapour, and each component leaves at its own retention time.
Separation depends on the balance between solubility in the moving phase and retention by the stationary phase.
A component in chromatography partitions between two states: dissolved in the mobile phase (solvent or gas) and retained by the stationary phase. The proportion of time spent in each determines its travel distance or retention time. A substance highly soluble in the mobile phase but weakly retained by the stationary phase travels further, yielding a high Rf value. Conversely, a component strongly adsorbed onto a polar solid stationary phase, like silica, barely moves. Separation relies on each component having a unique balance of these affinities. Crucially, retention mechanisms differ: solid stationary phases retain components via adsorption (e.g., hydrogen bonding to silica), whereas liquid stationary phases retain them via dissolving (partition).
The use of mass spectrometry to analyse the components separated by GC.
Gas chromatography separates a mixture in time, but on its own it tells you only when each component left the column, and two different compounds can share a retention time. Feeding the outflow straight into a mass spectrometer removes that ambiguity. Each component is ionised as it arrives and its own mass spectrum is recorded: the molecular ion peak gives the relative molecular mass, and the fragmentation pattern acts as a fingerprint that software matches against a database of known spectra. The combination is powerful because the two techniques fail in different ways, and it works on tiny samples, which is why it is used in drug testing in sport, forensic analysis, airport screening and pollution monitoring.
Required practical 12 Separation of species by thin-layer chromatography.
Rule the baseline in pencil about a centimetre from the bottom of the plate, because ink would dissolve and travel with the solvent. Spot the samples with a capillary tube, keeping each spot small and letting it dry before respotting so the sample stays concentrated. Stand the plate in a tank with the solvent below the baseline, never above it, or the spots dissolve into the solvent instead of climbing. The lid matters: it saturates the atmosphere with solvent vapour and stops solvent evaporating off the plate, which would otherwise distort the chromatogram. When the solvent front nears the top, remove the plate and mark the front at once, before it dries. Colourless spots are revealed under ultraviolet light or with a locating agent such as ninhydrin.
Your focus
- Identify the stationary phase and the mobile phase in a described experiment.
- Explain why the components of a mixture travel at different speeds through a chromatography system.
- Calculate an Rf value from a chromatogram and state why it carries no unit.
Show all 19 objectives
- Explain why a retention time identifies a component only when a standard is run under the same conditions.
- Choose the technique suited to a stated sample, such as a volatile liquid mixture or a mixture of amino acids, and give the property of the sample that decides it.
- Explain why the solvent climbs a thin-layer plate but runs down a packed column, naming what drives it in each case.
- Explain why gas chromatography is carried out at high temperature and under pressure.
- Select the appropriate technique when a question needs the separated components to be collected rather than only detected.
- Explain separation in chromatography as a balance between solubility in the mobile phase and retention by the stationary phase.
- Distinguish between adsorption on a solid stationary phase and partition in a liquid stationary phase.
- Predict the relative Rf values or retention times of two components based on their polarities and affinities for the stationary phase.
- Explain why a fragmentation pattern can tell apart two compounds that share a relative molecular mass, when the molecular ion peak alone cannot.
- Explain why a retention time on its own is not enough to identify a component.
- Use a molecular ion peak to state the relative molecular mass of a separated component.
- Suggest an application of the combined technique and justify it from a property of the method.
- Describe the steps of a thin-layer chromatography run in the order they are carried out.
- Explain why the tank must be covered and why the solvent must start below the baseline.
- Calculate an Rf value from measurements taken on a developed plate.
- Identify a fault in a diagram of the apparatus and state the effect it would have on the chromatogram.
Chromatography (A-level only) exam tips
Marking Points
- one mark for naming the stationary phase and the mobile phase
- one mark for components travelling at different speeds because they divide differently between the two phases
- one mark for identification by comparing an Rf value or a retention time with that of a known compound
- one mark for stating that the comparison must be made under the same conditions
- one mark for identifying the stationary phase in a named technique, the coating on the plate, the packing in the column, or the solid or coated solid in a gas chromatography column
- one mark for identifying the mobile phase, a solvent moving up the plate, a solvent moving down the column, or a carrier gas under pressure
- one mark for stating that gas chromatography is run hot so that the components are carried as vapour
- one mark for stating that column chromatography allows the separated components to be collected
- State that separation depends on the balance between solubility in the mobile phase and retention by the stationary phase.
- Explain that a component with greater solubility in the mobile phase and weaker retention travels further or has a shorter retention time.
- Identify adsorption as the specific retention mechanism for solid stationary phases, such as silica in TLC.
- Contrast adsorption with partition (dissolving), which is the retention mechanism when a liquid stationary phase is used.
- one mark for the gas chromatography stage separating the components by retention time
- one mark for the mass spectrometer recording a spectrum for each separated component
- one mark for the molecular ion peak giving the relative molecular mass
- one mark for the fragmentation pattern being matched against a database to identify the component
- one mark for a pencil baseline, with the reason that ink would dissolve and run
- one mark for the solvent starting below the baseline so that the spots are not washed off
- one mark for a lid on the tank, because it saturates the atmosphere with solvent vapour and reduces evaporation from the plate
- one mark for marking the solvent front as soon as the plate is removed
- one mark for locating colourless spots under ultraviolet light or with a locating agent
Examiner 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.
- 💡Match the direction to the method: up the plate, down the column, through the gas chromatography column.
- 💡Say what the stationary phase is made of, not simply where it sits.
- 💡Use each technique's own vocabulary: plate and spot, column and fraction, retention time.
- 💡Be precise with terminology: use 'adsorption' for solid stationary phases (like TLC silica) and avoid the term 'absorption'.
- 💡When comparing two molecules, explicitly state which has the stronger affinity for the stationary phase and link this to its Rf value or retention time.
- 💡Answer in two stages, separation and then identification, and say which instrument does which.
- 💡Name the molecular ion peak explicitly whenever relative molecular mass is asked for.
- 💡A named application earns credit only when it is tied to a property of the technique, such as working on very small samples.
- 💡Measure both distances from the baseline, one to the centre of the spot and one to the solvent front, and show that in your working.
- 💡When asked what is wrong with an apparatus diagram, check the solvent level, the lid and the baseline first; those are usually the faults.
- 💡Give a reason with every precaution, because the mark is almost always for the reason rather than the instruction.
Common Mistakes
- 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
- calling the plate itself the stationary phase, when it is the thin coating of silica or alumina
- saying the solvent moves down the plate in thin-layer chromatography
- describing the carrier gas as reacting with the sample, when it must be unreactive
- calling the liquid coating in a gas chromatography column the mobile phase, when the gas is mobile and the coating is stationary
- Confusing adsorption with absorption; correct by specifying 'adsorption' when molecules bind to the surface of a solid stationary phase.
- Explaining separation using only one phase; correct by always comparing solubility in the mobile phase against retention on the stationary phase.
- Stating that components separate due to different masses; correct by attributing separation to differing relative affinities for the two phases.
- saying the mass spectrometer separates the mixture, when the column has already done that
- identifying a component from its retention time alone and calling that conclusive
- using the base peak rather than the molecular ion peak to obtain the relative molecular mass
- describing a use of the technique without saying what each of the two stages contributes
- ruling the baseline in pen, so the ink separates alongside the sample
- filling the tank so that the solvent covers the spots
- leaving the tank uncovered, so solvent evaporates from the plate and the front becomes uneven
- letting the plate dry before the solvent front has been marked, so the distance cannot be measured
- measuring from the bottom edge of the plate rather than from the baseline