Ion-Exclusion Chromatography

Abstract

Ion exclusion is the term that describes the mechanism by which ion-exchange resins are used for the fractionation of neutral and ionic species. Ionic compounds are rejected by the resin, due to Donnan exclusion, and they are eluted in the void volume of the column. Nonionic or weakly ionic substances penetrate into the pores of the packing, they are retained and, thus, separation is achieved, as they partition between the liquid inside and outside the resin particles. Ion-exclusion chromatography is a mode of HPLC and, thus, the same equipment can be used, with the proper eluent, column, and detection technique. The technique is mostly used for the analysis of organic acids, sugars, alcohols, phenols, and organic bases. It provides a convenient way to separate molecular acids from highly ionized substances. Ionized acids pass rapidly through the column while molecular acids are held up to varying degrees. A conductivity detector is commonly used. Carboxylic acids can be separated by using water, a dilute mineral acid, or a dilute benzoic or succinic acid as eluent.