Chromatography Types: TLC, CC & GC (Part 3/4) Revision Notes | A Level AQA Chemistry

1. Why Chromatography Matters at A-Level

You already know from earlier in this series that all chromatography techniques share one core idea: components in a mixture distribute themselves between a stationary phase and a mobile phase, and those that interact more strongly with the stationary phase travel more slowly. What changes between TLC, column chromatography, and gas chromatography is the physical form of those two phases — and that change has enormous consequences for what each technique can do.

AQA expects you to be able to describe how each technique works, interpret the data it produces, and evaluate which technique is most appropriate for a given purpose. This lesson builds each technique from the ground up, then brings them together for direct comparison.

2. Thin-Layer Chromatography (TLC)

Picture a small rectangular plate — typically glass or aluminium — coated with a thin, uniform layer of silica gel or alumina. That coating is the stationary phase, and it is polar. A tiny spot of the mixture is placed near the bottom of the plate, which is then stood upright in a shallow pool of solvent (the mobile phase). The solvent climbs the plate by capillary action, carrying the components of the mixture with it.

Components that interact more strongly with the polar stationary phase (through hydrogen bonding or dipole interactions) are held back and travel a shorter distance. Components that are more soluble in the mobile phase travel further. When the solvent front approaches the top of the plate, the plate is removed and the solvent front is marked immediately to minimise error from evaporation.

Many spots are colourless and invisible to the naked eye. To visualise them, the plate is placed under ultraviolet (UV) light — compounds that absorb UV appear as dark spots on a fluorescent background — or the plate is developed with a chemical locating agent such as iodine vapour or ninhydrin spray (for amino acids).

Worked example: Calculating and Using an Rf Value

A TLC plate is run with a solvent. The solvent front travels 8.0 cm from the baseline. A component travels 5.6 cm. Calculate the Rf value and suggest how it could be used to identify the compound.

1. $R_f = \dfrac{\text{distance travelled by component}}{\text{distance travelled by solvent front}}$ — This ratio is always between 0 and 1. A value close to 1 means the component strongly favours the mobile phase; close to 0 means it strongly favours the stationary phase.
2. $R_f = \dfrac{5.6 \text{ cm}}{8.0 \text{ cm}}$ — Both distances are measured from the same baseline (the original spot position), so units cancel.
3. $R_f = 0.70$ — 5.6 ÷ 8.0 = 0.70. This is a dimensionless number — do not include units.
4. Compare Rf = 0.70 with a data table of known Rf values run under identical conditions. — If a known standard also gives Rf = 0.70 on the same plate with the same solvent, the compound is likely that substance. Running a reference standard alongside the unknown on the same plate is the most reliable approach.

Answer: $R_f = 0.70$ (no units). Identify by comparing with known standards run under identical conditions on the same plate.

TLC is fast, cheap, and requires only a small sample. It is widely used to monitor the progress of a reaction — if the spot for the starting material disappears and a new spot appears, the reaction has proceeded. It can also check the purity of a product: a pure compound produces a single spot, whereas a mixture produces multiple spots.

3. Column Chromatography (CC)

Column chromatography scales up the principle of TLC. Instead of a flat plate, a vertical glass column is packed with a polar stationary phase — typically silica gel or alumina — and the mixture is loaded onto the top. Solvent (the mobile phase) is then poured through continuously, and components elute (wash out) from the bottom of the column at different times.

The component that interacts least with the stationary phase elutes first; the component that interacts most strongly elutes last. Fractions are collected from the bottom of the column in separate containers, and each fraction can then be analysed (for example by TLC) to confirm which component it contains.

Unlike TLC, column chromatography is a preparative technique — it actually separates and collects the individual components in usable quantities, rather than merely identifying them. This makes it the method of choice when a chemist needs to purify a synthesised compound after a reaction.

4. Gas Chromatography (GC)

Gas chromatography works on the same partition principle, but both phases are fundamentally different from TLC and CC. The mobile phase is an inert carrier gas — typically nitrogen or helium — and the stationary phase is a high-boiling-point liquid coated onto an inert solid inside a long, coiled capillary column held inside an oven.

The sample is injected as a liquid into a heated injection port, where it vaporises instantly. The carrier gas sweeps the vapour through the column. Components partition between the carrier gas and the liquid stationary phase: those that are more soluble in the stationary phase spend more time there and take longer to reach the detector. Those that are less soluble in the stationary phase — or that are more volatile — travel through faster.

The detector — most commonly a flame ionisation detector (FID) — produces an electrical signal each time a component reaches it. This signal is plotted against time to produce a gas chromatogram: a series of peaks, each representing one component. The position of each peak (its retention time) identifies the component; the area under each peak is proportional to the amount of that component present.

Reading the chromatogram above: three peaks appear at approximately 2, 5, and 8 minutes. Each peak corresponds to one component eluting from the column. The peak at 5 minutes has the largest area, indicating that component is present in the greatest proportion. To identify each component, the retention times are compared with those of known reference compounds run under identical conditions.

Quantitative analysis is possible because peak area is proportional to the amount (or concentration) of each component. By calculating the fraction of the total peak area that each individual peak represents, the percentage composition of the mixture can be determined.

Worked example: Calculating Percentage Composition from a GC Chromatogram

A GC chromatogram shows three peaks with areas of 240, 360, and 120 arbitrary units. Calculate the percentage of the mixture represented by each component.

1. $\text{Total area} = 240 + 360 + 120 = 720 \text{ arbitrary units}$ — The total area represents 100% of the mixture. Adding all three peak areas: 240 + 360 = 600, then 600 + 120 = 720.
2. $\% \text{ component 1} = \dfrac{240}{720} \times 100 = 33.3\%$ — 240 ÷ 720 = 0.3333..., multiplied by 100 gives 33.3% (to 3 significant figures).
3. $\% \text{ component 2} = \dfrac{360}{720} \times 100 = 50.0\%$ — 360 ÷ 720 = 0.500, multiplied by 100 gives 50.0%. This is the largest peak and the most abundant component.
4. $\% \text{ component 3} = \dfrac{120}{720} \times 100 = 16.7\%$ — 120 ÷ 720 = 0.1667, multiplied by 100 gives 16.7%. Sanity check: 33.3 + 50.0 + 16.7 = 100.0%. ✓

Answer: Component 1: 33.3%, Component 2: 50.0%, Component 3: 16.7%

5. GC Coupled with Mass Spectrometry (GC-MS)

Gas chromatography separates the components of a mixture, but on its own it can only identify them by comparing retention times with known standards. When GC is coupled to a mass spectrometer (GC-MS), each component that elutes from the column is fed directly into the mass spectrometer, which produces a fragmentation pattern — a mass spectrum — unique to that compound.

This combination is extremely powerful: GC provides the separation and the retention time, while the mass spectrometer provides an unambiguous molecular fingerprint. GC-MS is used in forensic science (identifying drugs or accelerants), environmental monitoring (detecting trace pollutants), and food analysis (checking for adulterants or contaminants).

AQA expects you to know that GC-MS allows identification of components even when no reference retention time is available, because the mass spectrum can be matched against a library of known spectra. This makes GC-MS far more definitive than GC alone for unknown samples.

6. Comparing All Three Techniques

Now that each technique has been covered individually, it is worth drawing them together. The key differences come down to the nature of the phases, the type of output, and the purpose each technique serves.

7. Exam Technique and Common Mistakes

Chromatography questions on AQA papers tend to fall into three categories: describe how a technique works, interpret data from a chromatogram, or evaluate which technique is most suitable for a given purpose. Each requires a different approach.

When asked to describe how TLC works, examiners want a sequential account: spot applied to plate, plate placed in solvent, solvent rises by capillary action, components separate based on their relative affinities for stationary and mobile phases, solvent front marked, Rf values calculated. Avoid vague phrases like "the components move at different speeds" without explaining why.

In normal-phase TLC, a less polar component is less strongly attracted to the polar stationary phase, so it spends a greater fraction of time moving with the mobile phase and usually travels further, giving a higher Rf value under the same conditions.

When asked to evaluate which technique to use, structure your answer around the purpose (analytical vs preparative), the nature of the sample (volatile or non-volatile), and whether quantitative data is needed. A conclusion is required for full marks on an evaluate question.

Worked example: How to Read a Chromatogram Safely

1. Use retention time or Rf only under stated conditions.
2. Use peak area to compare relative amount, not retention time.
3. Treat a single TLC spot as evidence of purity, not absolute proof.