Retention models for ions in chromatography

Since chromatography of ions is a widely used technique in analytical chemistry a basic understanding of the retention mechanism is important. The principles of the different retention models that have been proposed are examined in this paper. The focus is on those models that are derived from the physical chemistry of charged surfaces immersed in an electrolyte solution. In the first two sections the theory for the electrical double layer and the Donnan potential are presented together with experimental results from surface and colloid chemistry. In Section 3 a comparison between stoichiometric and non-stoichiometric models is made. In this section the physical meaning of the retention factor is also examined. The Donnan model and the different double layer models developed for ion exchange chromatography of small ions are discussed in Section 4. The next section presents the corresponding models that have been developed for ion pair chromatography and compares them with the experimental findings. The theoretical modifications needed when going from small ions to ionic macromolecules are discussed in the last section and the developed models are compared with the experimental results.