affinity chromatography

Meaning: Affinity chromatography purifies a molecule by its specific, reversible binding to a ligand attached to the stationary phase (beads in a column). The mixture flows through; the target binds and the other components wash out. The target is then eluted by adding free ligand, or by changing pH or salt concentration to weaken the binding. Examples: an enzyme on a substrate analog, an antibody on its antigen, a His-tagged protein on nickel beads.

On the MCAT: A passage describes purifying a tagged recombinant protein, then asks which components elute first and which last. Molecules lacking affinity elute early; the target elutes late, after the eluting agent is added. Questions also ask what to attach to the beads: the binding partner of the target. Contrast it with size-exclusion chromatography (the largest molecules elute first) and ion-exchange chromatography (charge-based, eluted by salt or pH change).

Common confusion: Students treat it like a size or polarity separation. Here the separation rests on specific binding, so molecules of similar size or charge can still be resolved. The ligand is fixed on the column, not added to the sample, and the target is usually the last to leave.