Chromatography: Monosaccharides (AQA A Level Biology): Revision Note

Chromatography: monosaccharides

• Chromatography is a technique that can be used to separate a mixture into its components

• Chromatography relies on differences in the solubility of the different chemicals (called ‘solutes’) within a mixture

• All chromatography techniques use two phases:

  • The mobile phase

  • The stationary phase

• The components in the mixture separate as the mobile phase travels over the stationary phase

• Differences in the solubility of each component in the mobile phase affect how far each component can travel

• Those components with higher solubility will travel further than the others

• This is because they spend more time in the mobile phase and are therefore carried further up the paper than the less soluble components

Paper chromatography

• Paper chromatography is one specific form of chromatography

• In paper chromatography:

  • The mobile phase is the solvent in which the sample molecules can move, which in paper chromatography is a liquid, e.g. water or ethanol

  • The stationary phase in paper chromatography is the chromatography paper

Paper chromatography method

1. A spot of the mixture (that you want to separate) is placed on chromatography paper (the stationary phase) and left to dry

2. The chromatography paper is then suspended in a solvent (the liquid phase)

3. As the solvent travels up through the chromatography paper, the different components within the mixture begin to move up the paper at different speeds

   • Larger molecules move more slowly than smaller ones

   • This causes the original mixture to separate into different spots or bands on the chromatography paper

4. This produces what is known as a chromatogram

Using paper chromatography to separate monosaccharides

• Paper chromatography can be used to separate a mixture of monosaccharides

• Mixtures containing coloured molecules, such as chlorophyll, do not have to be stained as they are already coloured

• Mixtures of colourless molecules, such as a mixture of monosaccharides, have to be stained first

• A spot of the stained monosaccharide sample mixture is placed on a line at the bottom of the chromatography paper

• Spots of known standard solutions of different monosaccharides are then placed on the line beside the sample spot

• The chromatography paper is then suspended in a solvent

• As the solvent travels up through the chromatography paper, the different monosaccharides within the mixture separate at different distances from the line

• The unknown monosaccharides can then be identified by comparing and matching them with the chromatograms of the known standard solutions of different monosaccharides

  • If a spot from the monosaccharide sample mixture is at the same distance from the line as a spot from one of the known standard solutions, then the mixture must contain this monosaccharide