Stoichiometric Theories

In this article IPC means liquid chromatography performed on columns with covalently bonded nonpolar stationary phases, in combination with polar mobile phases containing an ion pair reagent. The earliest retention theories for IPC were stoichiomet-ric, i.e. the retention is assumed to be caused by the formation of a 1 : 1 stoichiometric complex between the ion pair reagent and the oppositely charged analyte ion. Depending on the particular theory, complex formation is considered to take place in the mobile phase or in the bonded phase. Other theories consider the ion pair reagent to be adsorbed to the stationary phase surface and describe retention as a result of a stoichiometric ion exchange process between the analyte ion and the counter ions to the ion pair reagent. In 1981 it was pointed out by Knox and Hartwick that many of the proposed stoichiometric retention theories are thermodyn-amically equivalent and that it is therefore not possible to distinguish them from each other by retention measurements alone.

The physicochemical cause of retention in ion pair chromatography has been a controversial issue. Because of this disagreement many alternative names have been proposed for this technique. These include: dynamic ion exchange chromatography, ion-interaction chromatography, hetaeric chromatography, and soap chromatography. Correspondingly, many names have been used for the ion pair reagent.

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