affinity

Meaning: Affinity is how strongly a molecule is attracted to another substance through intermolecular forces such as hydrogen bonding, dipole interactions and London dispersion forces. In a separation, each compound partitions between the stationary and mobile phases, and its relative affinity for the two sets its speed. On silica or alumina (normal phase) the stationary phase is polar, so polar compounds bind more strongly and move slowly. In affinity chromatography, the column carries a specific ligand, so the target protein binds and other molecules wash through. Elsewhere, affinity can mean binding strength (low Kd means high affinity) or electron affinity.

Equation: Rf = dcompound / dsolvent, where dcompound is the distance the spot travels from the origin (m), dsolvent is the distance the solvent front travels (m), and Rf is dimensionless. Higher affinity for the stationary phase gives a lower Rf.

On the MCAT: A passage describes TLC, column or affinity chromatography. You decide which compound elutes first or has the larger Rf by comparing polarity, or how to release a bound protein (add free ligand, or change pH or salt).

Common confusion: Students assume the more polar compound travels farther because "like dissolves like." On a polar stationary phase with a less polar solvent, the polar compound is held more tightly, so its Rf is lower and it elutes later. Identify which phase is polar first.