Chemistry, Third edition

CHROMATOGRAPHY

sively in forensic science to detect and identify trace amounts of substances. There are

many types of chromatography, probably the simplest is paper chromatography.

Paper chromatography

Black ink is a mixture of several coloured substances. It can be separated into its com-

ponents by paper chromatography. A small drop of black ink is spotted onto a rectan-

gular sheet of filter paper. When the ink has dried the paper is folded into a cylinder

shape and secured with a clip. The paper is then placed into a beaker containing a

solvent, in this case propanone (acetone), making sure that the level of propanone

is below the ink spot (Fig. 19.17). The solvent gradually rises up the filter paper by

capillary action and separates the ink into a number of coloured spots. This separation

happens because the different dyes present in the ink have different solubilities in the

water surrounding the cellulose particles of the paper (the stationaryphase) and the

solvent (the movingphase). The dyes most soluble in the moving phase are carried

furthest up the filter paper in a given period of time, whereas those most soluble in

the water on the cellulose tend to lag behind. The different dyes distribute (partition)

themselves between the water on the cellulose and the solvent moving over the cellu-

lose. When the solvent reaches the top of the filter paper the different dyes in the ink

will have separated out into different coloured spots. The paper can be taken out and

dried – the resulting pattern is called a chromatogram(see Fig. 19.18).

This technique is generally used to identify the components in a mixture. For

example, the dyes that it is thought an ink mightcontain can be spotted on the filter

paper along with the ink.

The chromatogram obtained in Fig. 19.18 shows that the ink contains compo-

nents A, B and C but not D because the ink has separated into three spots, which are

at the same height as the spots obtained for A, B and C.

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Fig. 19.17Paper chromatography. Fig. 19.18Running a chromatogram.

Column chromatography

This is used to separate mixtures of compounds, so that each compound can be col-

lected. A glass column is packed with an adsorbent, through which a solution of the

mixture is passed. Alumina(aluminium oxide) or silica gel(prepared by adding con-

centrated hydrochloric acid with sodium silicate) are commonly used as adsorbents.

The column must be uniformly packed so that solvent flows freely through it. The mix-

ture is added at the top of the column and solvent is passed through the column to wash

(orelute) the mixture down through the column. As the mixture moves through the