The fundamental principle of separation by size exclusion in a column is represented in Figure 17.1. The column is packed with semisolid particles of a polymer whose structure is crosslinked to form a gel and whose pore distribution has been controlled during the synthesis of the polymer. Molecules that are smaller than the pore size can enter inside the pores and therefore have a longer path and a longer transit time than larger molecules which cannot enter the pores. Molecules larger than the pore size cannot enter the pores and elute before smaller molecules. This condition is called total exclusion because of the fact that the largest molecules are rejected from entering the pores, as shown in Figure 17.1. Molecules that can enter the pores will have an average residence time in the particles that depends on the molecular size and shape.