37.1 Thin-Layer Chromatography (TLC)

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What is Thin-Layer Chromatography?

Chromatography is a technique used to separate mixtures. Think of it like a race — different substances in a mixture travel at different speeds, so they end up at different places at the end of the race. This lets us identify what is in the mixture.

Thin-layer chromatography (TLC) is one type of chromatography. It is used to separate and identify the different substances (called components) in a mixture. It is quick, cheap, and needs only a very tiny amount of sample.

In TLC, substances move across a flat surface. Some substances travel far; others barely move. The distance each substance travels tells us something important about its chemical identity.

The Key Terms You Need to Know

(a) Stationary Phase

The stationary phase is the part of the system that does not move. In TLC, the stationary phase is a thin layer of solid material coated onto a flat backing (usually a glass plate, a plastic sheet, or aluminium foil). A common example of this solid material is aluminium oxide (Al₂O₃), though silica gel (SiO₂) is also widely used.

• The stationary phase is polar (meaning it has a slight separation of positive and negative charge across it).
• Substances in the mixture interact with — or stick to — the stationary phase to different degrees. The more a substance is attracted to the stationary phase, the less it moves.

(b) Mobile Phase

The mobile phase is the part of the system that moves. In TLC, the mobile phase is a liquid solvent that travels up the plate by a process called capillary action (the ability of a liquid to flow up a narrow surface on its own, against gravity).

• The mobile phase can be polar (e.g. water, ethanol) or non-polar (e.g. hexane, ethyl acetate).
• As the solvent moves up the plate, it carries the substances in the mixture with it.
• How far a substance travels depends on how much it dissolves in the solvent (its solubility in the mobile phase) compared to how much it sticks to the stationary phase.

(c) Rf Value

The Rf value (short for Retention Factor) is a number that tells you exactly how far a substance has travelled compared to how far the solvent has travelled. It is calculated using this formula:

• Both distances are measured from the baseline (where you spotted the sample — see below).
• Rf values are always between 0 and 1 (or 0 and 100 if expressed as a percentage). A substance cannot travel further than the solvent, so Rf can never be greater than 1.
• Rf values are specific to a given substance under the same conditions (same stationary phase, same mobile phase, same temperature). This means Rf values can be used to identify unknown substances by comparing them to known ones.

(d) Solvent Front and Baseline

• The baseline is a pencil line drawn near the bottom of the TLC plate before the experiment begins. The sample mixture is spotted (applied as a tiny dot) onto this line. It is drawn in pencil (not pen) because ink would dissolve in the solvent and interfere with the results. All distance measurements start from the baseline.

• The solvent front is the furthest point that the solvent (mobile phase) has reached on the plate by the time the experiment is stopped. You must mark the solvent front with a pencil as soon as you remove the plate from the solvent tank, because the solvent evaporates quickly and the front becomes invisible.