Introduction to Chromatography: TLC, GC and HPLC

Chromatography is a family of separation techniques used to split a mixture into its individual components and, in many cases, to identify or measure them. Every chromatographic method relies on the same underlying idea: a stationary phase that stays fixed and a mobile phase that moves through or over it. Components of a mixture interact differently with the two phases, so they travel at different rates and separate from one another.

At A-level, three techniques are commonly compared: thin-layer chromatography (TLC), gas chromatography (GC) and high-performance liquid chromatography (HPLC). Although they use different equipment, all three separate mixtures by exploiting differences in how strongly each component is attracted to the stationary phase versus the mobile phase.

In TLC, the stationary phase is a thin layer of solid, usually silica or alumina, coated onto a glass or plastic plate. A small spot of the mixture is placed near the bottom of the plate on a pencil baseline, and the plate is stood in a shallow layer of solvent, the mobile phase, inside a sealed tank. As the solvent rises up the plate by capillary action, it carries the components of the mixture with it. Components that stick more strongly to the solid layer move more slowly, while components that dissolve more readily in the solvent move further.

Once the solvent front has travelled most of the way up the plate, the plate is removed and each spot's position is measured. The result is expressed as a retention factor, \( R_f \), given by \( R_f = \dfrac{d_s}{d_f} \), where \( d_s \) is the distance travelled by the spot and \( d_f \) is the distance travelled by the solvent front, both measured from the baseline. Because \( R_f \) always lies between 0 and 1, it allows a component to be compared against a known reference run on the same plate under the same conditions.

Worked example: A spot travels 3.2 cm from the baseline while the solvent front travels 8.0 cm. Using \( R_f = \dfrac{d_s}{d_f} \), \( R_f = \dfrac{3.2}{8.0} = 0.40 \). This value can then be compared with reference \( R_f \) values measured under the same solvent and plate conditions to help identify the substance.

Gas chromatography separates mixtures that can be vaporised without decomposing. The stationary phase is a viscous liquid coated onto the inside of a long, coiled column, and the mobile phase is an inert carrier gas such as helium or nitrogen that pushes the vaporised sample through the column. The column sits inside an oven, and its temperature can be controlled precisely.

Components with weaker attraction to the liquid coating spend more time in the gas phase and pass through the column faster, while components that interact more strongly with the coating take longer. As each component leaves the column, a detector records a signal, producing a gas chromatogram, a plot of detector response against time. Each peak's position along the time axis is called its retention time, and the area under a peak relates to the relative amount of that component present.

Because GC can only be used on compounds that vaporise cleanly at achievable temperatures, it works best for volatile, thermally stable substances. It is often coupled directly to a detector such as a mass spectrometer, letting each separated component be identified as it leaves the column; this pairing is explored further in mass spectrometry.

HPLC follows the same principle as TLC but uses a liquid mobile phase pumped at high pressure through a column packed with fine solid particles as the stationary phase. Because the particles are so small, HPLC columns offer far more surface area for separation than a TLC plate, giving sharper, more precise separations.

As with GC, a detector records a signal as each component leaves the column, producing a chromatogram with peaks at characteristic retention times. HPLC is especially useful for mixtures containing large, non-volatile or heat-sensitive molecules, such as many pharmaceuticals and biological compounds, that would decompose if they were vaporised for GC.

All three techniques separate components by the balance between their attraction to a stationary phase and their movement with a mobile phase, but they differ in the phases involved and the type of sample each suits best.

Technique	Stationary phase	Mobile phase	Best suited to
TLC	Solid coating on a plate	Liquid solvent	Quick, qualitative checks on small samples
GC	Liquid coating in a column	Inert carrier gas	Volatile, thermally stable mixtures
HPLC	Packed solid particles in a column	Liquid solvent under pressure	Large, non-volatile or heat-sensitive molecules

A chromatogram tells you how many components a mixture contains and roughly how much of each is present, but on its own it rarely confirms exact structure. That is why chromatographic separation is usually paired with a structural technique afterwards, such as IR spectroscopy to identify functional groups or proton NMR to map out the hydrogen environments in each separated compound.

Chromatography always separates mixtures using a stationary phase and a mobile phase, but the phases and equipment differ between TLC, GC and HPLC. TLC gives a retention factor \( R_f = \dfrac{d_s}{d_f} \) from a static plate, while GC and HPLC give a retention time from a chromatogram peak, with GC suited to volatile samples and HPLC suited to larger or heat-sensitive ones. Recognising which technique fits a given sample, and how to read the resulting \( R_f \) value or retention time, is the core skill this topic builds.

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