Chromatography (AQA A-Level Chemistry): Revision Notes

7.11.1 Chromatography

What is chromatography?

• Chromatography is a separation technique
• It is used to separate, and subsequently identify, molecules and compounds that making up a mixture. As a process, chromatography separates a mixture by dissolving it in a solvent (known as the mobile phase) and passing it through a solid (called the stationary phase). The substance to be separated is the mobile phase, but as they move through the chromatogram they may partition themselves into a liquid stationary phase or adsorb onto the solid stationary phase.

There are many types of chromatography:

• 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 a solid coated by a liquid)
• A gas is then passed through the column under pressure at high temperature.

Simply (mostly accurate): • The mixture = Mobile phase • The chromatography plate = Stationary phase Retention time:

• This is the amount of time it takes a substance to pass through the stationary phase.

• It depends on three things: Size/mass of the substance.

• Larger particles pass along a chromatogram more slowly (vise versa) Substance's solubility in the mobile phase vs solubility in liquid stationary phase (partition).

• Particles that are more soluble in the liquid stationary phase in GC will partition themselves into this phase and move along the chromatogram more slowly. - Particles that are very soluble in the mobile phase will move quickly through the chromatogram. Binding of substance to the solid stationary phase (caused by adsorption).

• Particles that have a greater attraction to a solid stationary phase in TLC and CC will adsorb more onto that phase

• This means they'll move more slowly through the chromatogram