Size-exclusion chromatography (SEC) uses a porous gel matrix (e.g. Sephadex, Sepharose). Large molecules cannot enter the pores and elute first in the void volume. Small molecules diffuse into pores and are retarded, eluting later. There is no binding — separation is purely by molecular size/hydrodynamic radius. SEC is used for final polishing, desalting, and determination of molecular weight distribution of biologics.