What is the point of paper chromatography?

Paper chromatography can show that different substances are in an apparently homogeneous substance. In general, a sample of a substance is applied near the bottom of a paper chromatography strip. The bottom is then positioned so that the very bottom of the paper is touching a small amount of a liquid solvent; care must be taken not to immerse the sample itself, or it will dissolve into the solvent. When the liquid touches the paper,...

Paper chromatography can show that different substances are in an apparently homogeneous substance. In general, a sample of a substance is applied near the bottom of a paper chromatography strip. The bottom is then positioned so that the very bottom of the paper is touching a small amount of a liquid solvent; care must be taken not to immerse the sample itself, or it will dissolve into the solvent. When the liquid touches the paper, it slowly moves upwards. If the components of the sample can be dissolved by the liquid, they will start to move as well. The higher the solubility of the molecule in the sample, the further up the paper it will move. Those with lower solubility will not be carried as far up the paper. When the liquid reaches the top, it is removed and dried, bands of the different components can be seen.

Although it is not used often in research labs, in my first job post-college I used paper chromatography. The doctor for whom I worked, P.S. Rao, was researching a way to differentiate a type of enzyme released from damaged muscle into types released from heart muscle, as opposed to that released from skeletal muscle. Since even a needle used to take a blood sample would cause a rise in the overall level of the enzyme (creatine phosphokinase, or CPK), the total alone could not be used to show that someone had suffered a heart attack. He had realized that a blood sample treated with specific chemicals, then run on paper chromatography, would show an extra band when the patient had had a heart attack. This is now run as a purely chemical test kit (which we were also developing), and is a standard, easy test to determine whether someone has had a heart attack or not.