Chromatography

Chromatography

1. Paper Chromatography

   • Used to see what coloured materials make up/are made up of
   • The material to be separated is dissolved in a solvent and spotted on to chromatography paper, known colours are spotted alongside on a 'Start Line'
   • The paper is dipped into a solvent (e.g. ethanol) which is absorbed into the paper and rises up it. The distance moved by the solvent is marked on carefully with a pencil and the distance moved by each centre of the coloured spots is also measured, these can be compared with known substances but identical paper and solvent must be used
     • A pencil is used as is doesn't contain any dyes so it will not dissolve in the solvent, so the results will be unaffected.
   • Rf = distance moved by dissolved substance / distance moved by solvent
     • The distance a substance moves, compared to the distance the solvent front moves is called the reference value, Rf. Rf values of zero and one are no good as these are too insoluble and too soluble.
   • Any colour which horizontally matches up with another is likely to be the same colour

2. High performance liquid chromatography (HPLC)

   • Separates components in some mixtures
     • Components pass along a chromatography column, each taking a specific time to pass through (Retention Time)
       • Pure substance will always have the same retention time if done under the same conditions
       • This can be used to check the purity of a sample, as a pure substance should have only one peak
   • Advantages
     • Faster
     • Better separation than paper
     • Easy to read results
   • Disadvantages
     • Expensive to operate
     • Speed of separation can lead to lack of sensitivity
     • Trained staff needed to operate

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