Organic Chemistry MCAT Practice Exam

Which factor improves separation in size-exclusion chromatography?

1. Longer columns

2. Higher temperature

3. Smaller bead size

4. Faster mobile phase

The correct answer is: Longer columns

In size-exclusion chromatography, the primary goal is to separate molecules based on their size as they pass through a column filled with porous beads. The larger molecules are unable to enter the smaller pores of the beads and thus travel through the column more quickly, while smaller molecules get trapped in the pores and take longer to elute. When considering factors that improve separation, longer columns play a significant role. A longer column increases the available surface area for the interaction between the stationary phase (the beads) and the mobile phase (the solvent carrying the analytes). This extended length allows for more opportunities for the molecules to interact with the pore structure of the beads, thereby leading to better resolution and separation of different sized molecules. Essentially, longer columns provide more 'time' for the separation to occur, allowing for a clearer distinction between the elution times of different sized species. While other factors, such as bead size and temperature, can influence the chromatographic separation, they do not enhance separation as effectively as increasing the column length does. Smaller bead size could create more surface area and potentially enhance resolution on its own, but it also increases resistance and may lead to band broadening if not optimized. Higher temperatures can influence the viscosity of the mobile phase, potentially improving flow