Paper Chromatography

Separations: Chromatography of M&M and Ink Dyes

Almost all substances we come into contact with on a daily basis are impure; that is, they are mixtures. Similarly, compounds synthesized in the chemical laboratory are rarely produced pure. As a result, a major focus of research in chemistry is designing methods of separating and identifying components of mixtures.

Many separation methods rely on physical differences between the components of a mixture. For example, filtration takes advantage of substances being present in different states (solid vs. liquid); centrifugation relies on differences in density; and distillation makes use of differences in boiling points of the various components. Chromatography exploits differences in solubility and adsorption.

The word chromatography, which is derived from two Greek words literally meaning "color writing", was coined at the beginning of this century when the method was first used to separate colored components of plant leaves. Today, the name is a bit misleading, because most forms of chromatography do not depend on color. Several types of chromatography are commonly used, among which are paper chromatography, thin-layer chromatography or TLC, liquid-liquid chromatography, gas chromatography, and high performance liquid chromatography or HPLC.

Chromatography is so useful that some form can be found in most scientific laboratories around the world. For example, in forensic chemistry crime laboratories, the FBI maintains a library of chromatograms of inks that are used commercially. In the first case in which chromatography of inks were used, a man in Miami falsified travel and expense vouchers. However, the ink pen he used had ink that wasn't available commercially until 3 years after the trips had taken place.

The theory behind chromatography is to allow a mixture of different chemicals to be distributed or partitioned between a stationary phase and a mobile phase (eluent or solvent) . The mobile phase...