The Alkalinity Analyser from ISS based on Ion-exclusion chromatography (IEX) is a specialized instrument designed to detect and quantify carbonate anions in water. IEX is defined as a technique used to separate weak acids, amino acids, sugars, alcohols and other substances on an ion-exchange column. Because of Donnan exclusion (a pseudo-semi-permeable membrane around the ion-exchange resin), ionic material is excluded from the ion-exchange resin and passes quickly through the column. The strongest acids elute at the column void volume because they are highly ionized and are repelled by the immobilized negative charge of the resin. Weaker acids exist largely in the unionized molecular form and are separated by partitioning between the mobile phase and the occluded solvent.