Chromatography

Chromatography is the name given to a group of similar, very useful seperation techniques that involve a stationary phase and a moving, or mobile, phase. The seperation of dye pigments by chromatography can be seen if a drop of ink is placed on a sheet of paper. The spot grows our –tword by capillary action or wetting, each dye moving with the solution at a different, but characteristic, rate. The result is a dark disk surrounded by colored rings. Wetted paper fibers form the stationary phase, and the ink solution is the mobile phase. The rate at which each dye moves depends upon how strongly the dye is attracted to the wet fşbers in the paper.

 The Russian botanist Mikhail Tswett was the first to seperate chemical substances by chromatography. He described the use of column chromatography in 1906. Tswett disvolved the pigments from plant leaves with the liquid petroleum ether.  After packing a glass tube or column with powdered chalk (calcium carbonate, CaCO3 ), he poured the solution of plant pigments into the top of the column. When he washed, or eluted, the column by pouring in more pure petroleum ether after the solution, it began to show distinct yellow and gren bands. These bands, each containing a pure pigment, became well separated as they moved down the column, so that the pure pigments could be obtained.

 Vapor phase chromatography (VPC), also called gas chromatography (GC), is a more recent separation method. Here the mobile phase consists of a mixture of gaseous or vaporized substance plus a gas such as helium, called the carrier, that acts as an eluting agent. The stationary phase consists of a solid or a liquid adhering to a solid, packed in a column. As the gas passes through the column, substances in the mixture are attracted differently to the stationary phase and thus are separated.

 Vapor phase chromatography is a rapid method of separating mixtures and has become important in chemical synthesis, the preparation of complicated substances from simpler ones. A synthesis may require many chemical reactions, each one followed by the separation of product. With vapor phase chromatography, a synthesis that formerly took months might instead be complated in days. Vapor phase chromatography can also be used to detect minute quantities of sunstances. For example, as little as a picogram of amphetamine can be detected in urine using vapor phase chromatography with a special detector.