A completely sulphonated cation exchanger is often used as the stationary phase. A hydrate shell is formed around the sulphonic acid groups using low-concentration, strong aqueous acids as the mobile phase. This is bounded by an (imaginary) partially negatively charged membrane, which is also referred to in the literature as a Donnan membrane. The stationary phase is therefore only accessible to uncharged, non-dissociated molecules such as water. Analytes with a strongly acidic character are excluded, as these are still partially deprotonated under the conditions of the mobile phase and are therefore repelled by the Donnan membrane.
Excluded ions cannot penetrate the pore system of the stationary phase and are the first to be eluted. Weak acids, on the other hand, are protonated in the solvent, i.e. they are uncharged. They can therefore pass through the Donnan membrane and go to the surface of the stationary phase, where they are separated. Ion-exclusion chromatography is often used for the simultaneous separation of sugars, organic acids and alcohols, e.g. in fermentation analysis.