Ion‐Exclusion Chromatography

Abstract

Ion-exclusion chromatography (IEC) has developed into a very useful technique for separating relatively small weak acids (carbonic acid, carboxylic acids, hydrocarboxylic acids, etc.), weak bases (ammonia, amines) and hydrophilic molecular species such as carbohydrates and the lower alcohols. The analytical method actually involves the separation of molecular species rather than ions. Of course, ions can often be readily converted into molecular species as when anions of weak acids are acidified. The rationale for including IEC in a book on ion chromatography seems to be that a cation-exchange resin, or occasionally an anion-exchange resin, has generally been used for IEC separations. Also, it has become customary to include IEC in symposia and books devoted to ion chromatography.Ion-exclusion chromatography is a comparatively old technique, attributed primarily to Wheaton and Bauman [1]. Ionic material is rejected by cation-or anionexchange resin and passes through quickly, but non-ionic substances are held up and come through more slowly. Substances that can be separated include weak organic and inorganic acids, weak organic and inorganic bases, and hydrophilic neutral compounds such as sugars.The resin bed consists of three parts: 1. A solid resin network 2. Occluded liquid within the resin beads 3. The mobile liquid between the resin beads.The ion-exchange resin acts as a semipermeable membrane between the two aqueous phases, (2) and (3). Ionized sample solutes are excluded from the interior water (2) and pass quickly through the column. Nonionic materials are not excluded and they partition between the two water phases, (2) and (3). Thus, they pass more slowly through the column. Nonionic solutes differ in their degree of retardation by the resin phase because of: (i) differing polar attraction between the solute and resin functional groups, (ii) differing van der Waals forces between the solutes and the hydrocarbon portion of the resin.Ion Chromatography, 4th Ed. James S. Fritz and Douglas T. Gjerde