Ion exchange

Meaning: Ion exchange chromatography separates charged molecules by electrostatic attraction to oppositely charged groups fixed on a solid resin, the stationary phase. A cation exchange resin carries negative groups (such as sulfonate, SO₃⁻) and holds positively charged analytes. An anion exchange resin carries positive groups (such as quaternary ammonium, NR₄⁺) and holds negatively charged analytes. Molecules with weaker attraction elute first. Bound molecules are released by raising the salt concentration, because salt ions compete for the resin sites, or by changing the pH so the analyte's net charge shrinks or reverses. The same technique is used for proteins in biochemistry.

On the MCAT: A passage gives a mixture of amino acids or proteins, a resin type and a buffer pH. Compare the buffer pH with each pI. If pH < pI, the molecule has a net positive charge; if pH > pI, a net negative charge. Example: on a cation exchange column at pH 7, glutamic acid (pI about 3.2) is negative and elutes early, while lysine (pI about 9.7) is positive and sticks. Raising the pH or the salt concentration then releases lysine.

Common confusion: The resin's name refers to the ion it binds, not its own charge, so a cation exchange resin is negatively charged. This technique also separates by charge, not by size; size separation is size-exclusion chromatography.