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separating a mixture of compounds can be carried out by gas chromatography. identifying compounds once they have been separated then uses techniques like mass spectrometry. The pattern of peaks is like a fingerprint for each unknown compound. it is matched against unknown compounds on a computer database

Method of gas chromatography

the sample mixture is vaporised

a "carrier " gas moves through the vapour through the coiled column

The compounds in the sample have different attractions to the material in the column. The compounds with stronger attractions will take longer to get through the column. we say that they have a longer retention time

the compounds with weak attractions to the material in the column leave it first. they have shorter retention times

the unknown substances are then identified by comparing the chromatograph with the results for known substances. the analysis must have taken place in exactly the same conditions to compare retention times

Mass spectrometry

to ensure that we identify the unknown substances the gas chromatography apparatus can be attached directly to a mass spectrometer. This identifies substances very quickly and accurately and can detect very small quantities in the sample

a mass spectrometer can be used to find the relative molecular mass of a compound by using the molecular ion peak. the peak with the largest mass corresponds to an ion with just one electron removed. the mass of an electron is really small. the mass of an electron can often be ignored because of this. the molecular ion peak is always found at the right, it is always the last peak

paper chromatography works because some compounds in a mixture dissolves better than others in particular solvents. the solubility of substances determines how far they travel across the paper.

Food scientists can identify different foods to see what additives have been used. for example, food colourings can be detected by paper chromatography . they can use results from known compounds to positively analyse them

once the compounds in food have been seperated using chromatography they can be identified. this is done by comparing the chromatogram with others obtained from known substances. To do this we must use the same solvent at the same temperature

industries need rapid and accurate methods for analysing their products the pros and cons for Modern instrumental methods are :

highly accurate and sensitive

Quicker

Enable very small samples to be analysed

Is usually very expensive

takes special training to use

Gives results that can often be interpreted only by comparison with data from known substnaces

take a strip of chromatography paper

Draw a line 2cm above the bottom

draw three pencil crosses on the base line

dip a clean capillary tube into the coloured liquid

Carefully place some of the colouring liquid on one of the crosses ( keep it as small as possible)

Get new colourings from other groups and repeat the dots on the other crosses (use a fresh capillary tube)

Pour water into a tall beaker to a depth of 0.5 cm

Place the chromatography paper in the beaker

cover with a lid

Leave the chromatogram to develop

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