Chromatography - Definition A method of separation and analysis of a mixture of soluble chemical substances.

What can chromatography be used for? - identify additives in foods - compare fibres found at a crime scene - identify drugs and alcohol - test water samples for pollution - detect bombs in airports - detecting pesticides or insecticides in food - fingerprinting

Paper Chromatography – how does it work? Chromatography lets us separate inks and dyes according to the size of their particles. As the solvent (water) rises through the paper it dissolves the sample mixture, which will then travel up the paper. Small particles travel further than large particles, due to the differences in solubility and their attractions with the paper.

Molecules in dyes and inks Many common dyes and inks are water soluble and spread apart into the component dyes using water as a solvent. If the dye or ink you are testing does not spread out using water, it may be “permanent” ink. In this case, you will have to use a different solvent such as alcohol.

What are food additives and why are they put in our food? We need to make food last longer – so we find ways to preserve it. A substance used to preserve food is known as a food additive. Food additives can also be used to improve taste or appearance of food. Additives that have been approved for use in Europe have been given E numbers to identify them.

Detecting Additives Food additives can be detecting by a technique known as chromatography. This technique separates out the different components within the food additives based on how well they dissolve in a particular solvent. Their solubility determines how far they travel across a surface (chromatography paper). Once the compounds have been separated out using chromatography, they can be identified by comparing with known substances. Alternatively a mass spectrometer can be used.

Chromatography TASK Write step-by-step instructions on how to carry out the separation of food dyes using this technique. Copy this picture into your books to help you explain the steps. Beaker Separated ink particles Spot of dye to be separated Solvent

Retention factor (Rf) values The Rf factor is used to compare the components of various samples. The Rf values of suspect samples can be compared with known samples. Rf = distance from the base line to the spot distance from the base line to the solvent front Solvent front the point at which the water stopped moving up the paper If two substances have the same Rf value, they are likely (but not necessarily) the same compound. If they have different Rf values, they are definitely different compounds. Spot the point at which a band or spot of colour is Base line the line where the original sample was placed