chromatography
Summary
Chromatography separates a mixture because each component partitions differently between a stationary phase and a moving mobile phase. In normal-phase systems (polar silica), less polar compounds stick less, travel farther and elute first, so they have a higher Rf.
Detail
Meaning: Chromatography separates a mixture by repeated partitioning of each component between a stationary phase (such as polar silica or alumina) and a mobile phase (a liquid solvent or a gas) that flows past it. A compound that binds the stationary phase more strongly moves more slowly. Common forms are thin-layer (TLC), column, gas and high-performance liquid chromatography. Ion-exchange separates by charge, size-exclusion by size, and affinity by specific binding to an attached ligand.
Equation: Rf = dcompound / dsolvent front, where each d is the distance from the origin line in meters (measured to the center of the spot for the compound). Rf is unitless and falls between 0 and 1.
On the MCAT: A passage shows a TLC plate or silica column and asks which compound travels farthest or elutes first. On normal-phase silica, the less polar compound has the higher Rf. A more polar solvent raises Rf for all the spots. In size-exclusion columns, larger molecules elute first because they cannot enter the bead pores. In ion-exchange, a molecule with charge opposite to the resin is retained longer.
Common confusion: Students memorize "nonpolar moves first" as a universal rule. It holds for normal-phase systems only. In reverse-phase chromatography the stationary phase is nonpolar, so polar compounds elute first. Size-exclusion also runs against intuition: small molecules are slowed, not large ones.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Principles of Instrumental Analysis (Skoog)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.