Retention models for ions in chromatography

Since chromatography of ions is a widely used technique in analytical chemi stry a basic understanding of the retention mechanism is important. The pri nciples of the different retention models that have been proposed are exami ned in this paper. The focus is on those models that are derived from the p hysical chemistry of charged surfaces immersed in an electrolyte solution. In the first two sections the theory for the electrical double layer and th e Donnan potential are presented together with experimental results from su rface and colloid chemistry. In Section 3 a comparison between stoichiometr ic and non-stoichiometric models is made. In this section the physical mean ing of the retention factor is also examined. The Donnan model and the diff erent double layer models developed for ion exchange chromatography of smal l ions are discussed in Section 4. The next section presents the correspond ing models that have been developed for ion pair chromatography and compare s them with the experimental findings. The theoretical modifications needed when going from small ions to ionic macromolecules are discussed in the la st section and the developed models are compared with the experimental resu lts. (C) 1999 Elsevier Science B.V. All rights reserved.