Organic Chemistry MCAT Practice Exam

What can be estimated using size-exclusion chromatography?

1. Molecular mass of compounds

2. Optical activity of compounds

3. Purity of the compounds

4. Concentration of chiral centers

The correct answer is: Molecular mass of compounds

Size-exclusion chromatography (SEC), also known as gel permeation chromatography or gel filtration chromatography, is a technique primarily used to separate molecules based on their size. In this method, a sample is passed through a column filled with porous beads; the pores allow smaller molecules to enter the beads, thus taking longer to elute from the column compared to larger molecules, which cannot enter the beads and pass through the column more quickly. Because of this selective separation based on size, SEC can provide an estimate of the molecular mass of compounds. As larger molecules elute faster than smaller ones, the retention time in the column can be correlated to the size and, consequently, the molecular mass of the substances present in the sample. Calibration with standards of known molecular weights helps in quantifying the molecular mass of unknown compounds. In contrast, other choices do not align with the primary function of this technique. Optical activity generally relates to the configuration of chiral centers and is measured using different methods, such as polarimetry. Purity of compounds can involve various analysis methods, including high-performance liquid chromatography (HPLC), which may offer more specific insights into the composition of mixtures. Concentration of chiral centers also requires distinct analytical tools, often focused on specific interactions with