Chromatography (Oxford AQA International A Level (IAL) Chemistry): Revision Note

Chromatography

• Chromatography is a technique that enables the separation of mixtures and includes:

  • Thin-layer chromatography (TLC)

  • Column chromatography (CC)

  • Gas chromatography (GC) - sometimes called gas-liquid chromatography (GLC)

• All of these chromatography techniques require a mobile phase and a stationary phase

• The mobile phase:

  • Contains the mixture / sample (components)

  • Is also referred to as the solvent or eluent

  • Is a liquid or a gas

• The stationary phase:

  • Separates the sample

  • Is a solid or a liquid

Chromatography Phases Summary Table

Retention

• All chromatography applies the principle that components in a mobile phase flow through a stationary phase at different rates

• The rate of separation depends upon:

  • The solubility of the components in the mobile phase

  • The retention of the components by the stationary phase

Retention factor

• In thin-layer chromatography, the retention factor is used as a measure of solubility

• The retention factor is known as the R_f value and is calculated by:

R_f =

Measurements to calculate an R_f value

• Each component in a mixture has a unique R_f value

  • An R_f value cannot be greater than 1, because it is a ratio of how far the component travelled

• More soluble components have a higher R_f value

  • This means that they travel further in a given amount of time

Retention time

• In gas chromatography, the retention time is used as a measure of solubility

• This is the time that elapses from the moment of injection to when a component exits the chromatography tube

• More soluble components have a lower retention time

  • This means that they travel quicker over the length of the chromatography tube