In Size Exclusion Chromatography, why do larger molecules elute first?

• A. They are not trapped in small pores
• B. They diffuse faster
• C. They interact with stationary phase
• D. They have higher charge

Answer: (A)

In Size Exclusion Chromatography, separation hinges on whether a molecule can enter the pores of the stationary phase beads. Larger molecules are too big to fit into these pores, so they stay in the spaces between beads and travel through the column via the shorter path (the void volume). This means they move through the column more quickly and elute first. Smaller molecules, on the other hand, can enter the pores, which increases their path length and residence time in the column, causing them to elute later. In SEC, interactions with the stationary phase are minimized, and diffusion or charge differences don’t drive the separation, so the key factor is whether the molecule can access the pores.