in analytical chemistry, technique for separating chemical substances by exploiting the differences in the rates at which they pass through a bed of a porous, semisolid substance. The method is especially useful for separating enzymes, proteins, peptides, and amino acids from each other and from substances of low molecular weight. The separation of the components of a mixture by gel chromatography is based on the differences in the molecular sizes of the components. Small molecules tend to diffuse into the interior of the porous particles so that their flow is restricted, while large molecules are unable to enter the pores and tend to flow unhindered. Thus, the components of highest molecular weight leave the bed first, followed by successively smaller molecules. The bed materials most extensively used are polyacrylamide and a polymer prepared from dextran and epichlorohydrin. The dry polymers are usually suspended in suitable agents to form a homogeneous, semisolid mixture.