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. Chromatography is used extensively in forensics, from analyzing body fluids for the presence of illicit drugs, to fiber analysis, blood analysis from a crime scene, and at airports to detect residue from explosives.

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 separation surface and be detected or measured one at a time as they emerge.

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1. What is chromatography, and give an example of how it would be useful in forensics?

2. Why are glass fragments found on a suspect particularly incriminating?

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