Organic Chemistry MCAT Practice Exam

In size-exclusion chromatography, which type of compounds elute last?

1. Larger compounds that cannot enter the beads

2. Smaller compounds that penetrate the beads

3. Proteins with low molecular mass

4. All compounds elute at the same time

The correct answer is: Larger compounds that cannot enter the beads

In size-exclusion chromatography, larger compounds elute last because they are unable to enter the pores of the polymer beads used in the stationary phase. This characteristic allows them to bypass the internal volume of the beads and travel through the column more quickly. The design of the chromatography setup takes advantage of the size of the molecules being separated, where smaller compounds can enter the pores and thus have a longer path to travel, resulting in a longer retention time in the column. As a consequence, smaller compounds tend to elute earlier as they get 'trapped' within the beads for a portion of their travel through the column, while larger compounds stay in the mobile phase and exit the column last. This mechanism highlights the fundamental principle of size-exclusion chromatography, which is to separate molecules based on their size.