Chromatography

Chromatography is a family of laboratory techniques for separating mixtures of chemicals into their individual compounds. The basic principle of chromatography is that different compounds will stick to a solid surface, or dissolve in a film of liquid, to different degrees.

To understand chromatography, suppose that all the runners in a race have sticky shoe soles, and that some runners have stickier soles than others. The runners with the stickier shoes will not be able to run as fast. All other things being equal, the runners will cross the finish line in the exact order of their shoe stickiness—the least sticky first and the stickiest last. Even before the race is over, they will spread out along the track in order of their stickiness.

Similarly, different chemical compounds will stick to a solid or liquid surface to varying degrees. When a gas or liquid containing a mixture of different compounds is made to flow over such a surface, the molecules of the various compounds will tend to stick to the surface. If the stickiness is not too strong, a given molecule will become stuck and unstuck hundreds or thousands of times as it is swept along the surface. This repetition exaggerates even tiny differences in the various molecules' stickiness, and they become spread out along the "track," because the stickier compounds move more slowly than the less-sticky ones do. After a given time, the different compounds will have reached different places along the surface and will be physically separated from one another. Or, they can all be allowed to reach the far end of the surface—the "finish line"—and be detected or measured one at a time as they emerge.

Using variations of this basic phenomenon, chromatographic methods have become an extremely powerful and versatile tool for separating and analyzing a vast variety of chemical compounds in quantities from picograms (10^-12 gram) to tons.

Chromatographic methods all share certain characteristics, although they differ in size, shape, and configuration. Typically, a stream of liquid or gas (the mobile phase) flows constantly through a tube (the column) packed with a porous solid material (the stationary phase). A sample of the chemical mixture is injected into the mobile phase at one end of the column, and the compounds separate as they move along. The individual separated compounds can be removed one at a time as they exit (or "elute from") the column.

Because it usually does not alter the molecular structure of the compounds, chromatography can provide a non-destructive way to obtain pure chemicals from various sources. It works well on very large and very small scales; chromatographic processes are used both by scientists studying micrograms of a substance in the laboratory, and by industrial chemists separating tons of material.

The technology of chromatography has advanced rapidly in the past few decades. It is now possible to obtain separation of mixtures in which the components are so similar they only differ in the way their atoms are oriented in space, in other words, they are isomers of the same compounds. It is also possible to obtain separation of a few parts per million of a contaminant from a mixture of much more concentrated materials.