What type of molecules elute last in size-exclusion chromatography?

In size-exclusion chromatography, separation occurs based on the size of the molecules rather than their charge or hydrophobicity. The stationary phase consists of porous gel beads that allow smaller molecules to enter the pores, while larger molecules are excluded from entering these spaces. As a result, larger molecules navigate through the column more quickly because they cannot enter the pores and thus have a shorter pathway to travel. Conversely, smaller molecules spend more time within the pores, resulting in delayed elution.

Therefore, small molecules elute last because they are retained in the porous structure longer than larger molecules, making them arrive at the detector after the larger ones. This fundamental principle of size-exclusion chromatography underpins the correct choice.