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Why do ions that bind to a resin with different strengths form separate bands in a column?

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chemical structure of cation exchanger — ion-exchange reaction Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked ⁺. The whole structure is permeated by solvent molecules, usually water (not shown).

In ion-exchange chromatography, ions are separated into distinct bands because they possess different affinities for the stationary phase (the resin) compared to the mobile phase (the flowing solution).12

As a mixture of ions moves down the column, the process is governed by the relative strength with which each ion binds to the resin. Ions that are held more strongly by the resin spend more time in the stationary phase and migrate more slowly. Conversely, ions that have a weaker affinity for the resin spend more time in the mobile phase, which transports them down the column at a faster rate.132

Because these species migrate at different speeds based on their specific partition coefficients, the original mixture gradually resolves into separate, individual bands. This differential migration allows for the effective separation of complex mixtures, as each component is transported through the column at a rate determined by its unique interaction with the resin.12