Put a dot of black washable felt-tip ink on damp paper, and it spreads out into bands of colour. Black ink is a mixture of dyes, and paper can pull them apart.
In paper chromatography, you put a small spot of each sample on a pencil line near the bottom of a strip of paper, then stand the strip in a little solvent, such as water, with the line just above the surface. The solvent soaks up the paper and carries the molecules of each spot along with it. The line is drawn in pencil because graphite stays put in the solvent.
Each molecule spends part of its time stuck to the paper and part dissolved in the moving solvent. Paper is made of cellulose, chains of glucose covered in -OH groups. A dye that makes strong hydrogen bonds with those groups sticks to the paper more and moves slowly. A dye that dissolves well in the solvent travels further. When the solvent nearly reaches the top, you take the paper out and mark how far the solvent got: the solvent front. The finished paper is a chromatogram.
Rf values
How far a dye travels depends on how long the paper stays in the solvent, so chemists compare each spot with the solvent front. The Rf value is the distance the spot moved divided by the distance the solvent moved, both measured from the pencil line. In the figure, the red spot moved 3.0 cm and the solvent 6.0 cm, so the red dye's Rf is 3.0 ÷ 6.0 = 0.50. An Rf value always lies between 0 and 1.
On the same paper with the same solvent, a substance always gives the same Rf value, so a spot can be matched to a known substance run beside it. A pure substance gives a single spot, and a mixture gives a spot for each substance in it. That makes chromatography a second test of purity. Two substances can sometimes share an Rf value in one solvent, so chemists check with a second solvent too.
Chromatography at work
Labs use the same idea in machines. In gas chromatography, a sample is turned into a gas and carried by a stream of helium through a thin tube, often 30 m long, coiled up inside an oven. Each kind of molecule sticks to the lining of the tube by a different amount, so the molecules come out of the far end one kind at a time. Joined to a mass spectrometer, a machine that weighs molecules, it can name each one.
Drug-testing labs check athletes' urine for banned substances this way. Food labs use chromatography with a liquid solvent to find out which colourings a food holds, and to catch dyes banned from food. In 2005 an orange-red dye for oils and waxes, Sudan I, turned up in chilli powder used in a Worcestershire sauce, and hundreds of foods made with the sauce were taken off shop shelves in the UK.