size-exclusion

Meaning: Size-exclusion chromatography (SEC, also called gel filtration) packs a column with porous beads. Molecules small enough to enter the pores wander through the bead interiors, so their path is longer and they elute later. Molecules too large for the pores are excluded, travel mainly through the gaps between beads, and elute first. The elution order runs from largest to smallest. Separation depends mainly on molecular size and shape, not on a chemical attraction to the stationary phase. Biochemistry uses the same method to separate proteins.

On the MCAT: A passage describes a mixture of polymers or proteins of different molecular weights run through a column. You predict which component elutes first, or match peaks on a chromatogram to molecules by retention time or elution volume. Questions may also ask you to compare SEC with ion-exchange, affinity or normal-phase chromatography, where polarity, charge or specific binding sets the order.

Common confusion: Students expect small molecules to move faster and come out first. In SEC the order is reversed, because small molecules sample the pore volume and large ones skip it. Molecules too large for any pore all come out together near the start, so SEC cannot resolve them from one another.