Principles of Chromatography (VCE SSCE Chemistry): Revision Notes

Principles of Chromatography

What is chromatography?

Chromatography is an analytical technique that works by exploiting differences in how soluble different compounds are. This powerful method allows chemists to:

• Separate the different substances present in a mixture
• Identify numerous inorganic and organic substances
• Analyse contaminants in water, toxic gases in air, impurities in food, and drugs in blood

How chromatography works

All chromatography methods rely on two essential components:

• A stationary phase (the material that stays in place)
• A mobile phase (the material that moves)

You can perform a simple chromatography experiment using chalk and ink. When you dip a stick of chalk into water-soluble black ink and then stand the chalk in a beaker containing a small amount of water, the water carries the ink up the chalk. As this happens, the ink separates into bands of different colours. Each band contains one of the substances present in the ink mixture. This pattern of bands or spots is called a chromatogram.

In this chalk-and-ink experiment:

• The stationary phase is the chalk
• The mobile phase is the water

The separation process

The different substances in the ink are called components. As the mobile phase (solvent) sweeps the components upwards over the stationary phase, the components repeatedly go through two processes:

1. Adsorption - sticking onto the solid stationary phase
2. Desorption - dissolving back into the mobile phase

The rate at which each component moves depends mainly on:

• How strongly the component adsorbs onto the stationary phase
• How readily the component dissolves in the mobile phase (its solubility)

For example, if water (a polar solvent) is the mobile phase, a more polar component will move faster up the stationary phase than a less polar component. This is because the polar component dissolves more readily in the polar water and bonds less strongly to the stationary phase.

Paper and thin-layer chromatography

In the laboratory, two common types of chromatography are used for qualitative analysis:

Paper chromatography uses high-quality absorbent paper (similar to filter paper) as the stationary phase.

Thin-layer chromatography (TLC) is very similar to paper chromatography, but the stationary phase is a thin layer of fine powder, such as alumina (aluminium oxide) or silica (silicon dioxide), spread on a glass or plastic plate.

Setting up paper and thin-layer chromatography

The procedure for both techniques is similar:

1. Place a small spot of the sample solution at one end of the paper or plate. This position is called the origin.
2. Place the paper or plate in a container with solvent. The origin must sit slightly above the level of the solvent (not submerged in it). This ensures the components are carried up the paper or plate rather than dissolving directly into the liquid.
3. As the solvent rises up the paper or plate by capillary action, it carries the components with it. Different components move at different rates, causing them to separate.
4. The solvent continues to move until it reaches the solvent front - the furthest point of solvent travel.

Testing purity

Paper or thin-layer chromatography can quickly and cheaply determine the purity of a sample. You compare the chromatogram of the sample with one of the pure material. Any impurities appear as extra spots on the chromatogram.

Identifying the components of a mixture

The components in a mixture can be identified using chromatography in two main ways:

1. Including standards of known chemicals on the same chromatogram
2. Calculating the retardation factor ($R_f$) of the sample

Method 1: Using standards

A standard is a substance with an identity and concentration that are precisely known. To use this method, you need to know which chemicals might be present in your sample.

Method 2: Calculating $R_f$ values

Another way to identify components is by comparing how far they travel relative to the solvent front. This is expressed as the retardation factor ($R_f$):

$R_f = \frac{\text{distance the component travelled from the origin}}{\text{distance the solvent front travelled from the origin}}$

Key points about $R_f$ values:

• $R_f$ values are always less than 1 (because a component cannot travel further than the solvent front)
• The component most strongly adsorbed onto the stationary phase moves the shortest distance and has the lowest $R_f$ value
• Each component has a characteristic $R_f$ value under specific conditions

By comparing the $R_f$ values of components in a mixture with $R_f$ values of known substances determined under identical conditions, you can identify the components present.

Making components visible

In chromatograms of plant pigments and food dyes, the components are easily visible because they are coloured. However, most compounds are colourless and must be made visible:

Comparing paper and thin-layer chromatography

The choice between paper and thin-layer chromatography depends on the sample being analysed:

Important notes about identification

Qualitative versus quantitative analysis

Understanding mixtures involves two types of analysis:

Qualitative analysis - determining what chemical components are present in a sample (the chemical composition)

Quantitative analysis - measuring the amount of different chemical components present in a sample (the concentrations)

Paper and thin-layer chromatography are primarily used for qualitative analysis, though they can provide some quantitative information when used with standards of known concentration.

Remember!