Lesson 17 Background Information

The first use of chromatography has been attributed to the Italian-born Russian botanist Mickhail Semenovich Tsvett, who, in 1906, tickled a mixture of plant pigments down a tube of powdered calcium carbonate. The various substances in the mixture attached with different degrees of strength to the powder particles. As the mixture moved down the tube, the pigments began to separate from one another and their individual colors became clearly visible. Tsvett called this new technique “chromatography’ (in Greek, “writing in color”).

Although Tsvett’s technique has been modified considerably since it was invented, the fundamental process is still used to separate complex mixtures, including colorless ones, In 1944, two British biochemists, Archer J.P. Martin and Richard L.M. Synge, made a modification to the technique, inventing paper chromatography. In this technique, a solution is separated as it moves up a piece of absorbent paper. Paper chromatography is easier and faster to use than Tsvett’s technique was, and only very small quantities of solution are required. Martin and Synge were awarded the Nobel Prize in chemistry in 1952 for the use of paper chromatography to analyze protein fragments.

Martin went on to develop gas chromatography. In this technique, which is frequently used to analyze volatile substances, the components of a mixture are carried at different speeds by an inert gas and separate out on an adsorbent surface. (The term: adsorb” and “absorb” are often confuses. In adsorption a thin layer of molecules from a substance attaches to the surface of a solid. Absorption involves one substance penetrating into the inner structure of another.)

Chromatography works because substances in a mixture move at different rates through an absorptive material or over an adsorptive material. In paper chromatography, these rates of movement are the result of repeated sorption) the process of being taken up and held by either absorption or adsorption) and desorption (the reverse of sorption) as a sample is carried by the solvent through the paper. The solvent moves up the paper by the process of capillary action. The solutes move with the solvent at different speeds, repeatedly being sorbed into the desorbed from the paper as they travel through it. The extent to which the solutes move up the paper is determined by the solubility of the solute molecules. As the solution moves up the paper, the various components of the solution separate out and occupy distinct areas on the paper the various components of the solution separate out and occupy distinct areas on the paper.

The use of chromatography has several advantages over that of other separation techniques. Chromatography can be used to separate many soluble and volatile substances. In addition, the technique does not damage complex organic organic molecules, which makes it a valuable tool in the analysis of biological material.

NOTE:

1. Colors do not always separate according to the rules that govern the mixing of colors. For example: because mixing yellow and blue inks results in green ink, you may assume that green ink will separate into yellow and blue inks. Chromatography does not separate colors; it separates solutes.

2. Chromatography is often used to separate colorless substances. The position of these substances on the paper can be detected either by incorporating radioactive isotopes into them or by developing the chromatograms with substances that make the colorless solutes visible.