A-Level Chemistry AQA Notes

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Chromatography

Chromatography is a separation technique used for separating and identifying the species present in a mixture.
The mobile phase is the substance in chromatography that carries the soluble components of the mixture
The stationary phase is the substance in chromatography that holds back the components that are attracted to it.
Each component in the mixture has a different level of solubility in the mobile phase and retention by the stationary phase, resulting in the separation of these components

Thin Layer Chromatography

In thin layer chromatography a plate is coated with a solid and a solvent moves up the plate
- An organic solvent is used as the mobile phase
- A sheet coated with a thin layer of silica gel or alumina acts as the stationary phase
Thin layer chromatography can be used to separate and identify amino acids by their Rf values.

Different amino acids have different R groups, so they all have different affinities for the solvent. Therefore, moving up the plate at different rates depending on their affinity.
Amino acids are colourless, so to make them visible a developing agent is used, such as ninhydrin or ultraviolet light
Rf values are affected by temperature, a spot of a pure substance is often added to the baseline for comparison. If the correct conditions are used the Rf values can be compared to database values

Column Chromatography

In column chromatography (CC) a column is packed with a solid and a solvent moves down the column
- Silica or aluminium oxide is packed into a narrow column and acts as the stationary phase
- The mixture is dissolved in a solvent which is poured through the column. This acts as the mobile phase

Gas Chromatography

In gas chromatography (GC) a column is packed with a solid or with a solid coated by a liquid, and a gas is passed through the column under pressure at high temperature.
- Stationary phase is a solid or liquid coating inside a long-coiled tube
- An inert gas such as N2 acts as the mobile phase

The time taken for a component to leave the coil is called the retention time. This can be compared with standards to identify different substances.
A chromatograph shows these retention times as a series of peaks with the area under each peak being proportional to the amount of that component present
Mass spectrometry can be used to analyse the components separated by GC
The mass spectrum of each component can be compared to spectra in a database, allowing the components to be identified with greater certainty.

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