Retention mechanism for ion-pair chromatography with chaotropic reagents

During the last decade, the extensive use of ion-pair chromatography (IPC) in protein, peptides and basic drugs applications prompted chromatographers to evaluate new additives, since traditional ion-pairing reagents (IPRs) are not usually compatible with LC–MS hyphenation and tend to stick very strongly to the stationary phase, thereby impairing the initial column properties. Chaotropic salts received a great share of credit as tentative IPRs, since they proved to be able to mimic the role of classical IPRs, thereby increasing the retention of oppositely charged analytes. Very few quantitative theoretical studies faced the retention modelling when chaotropic additives are made use of in a chromatographic system and, unfortunately, they used a stoichiometric approach. We hereby debate the present state of the theory and illustrate the first attempt to explain the retention mechanism in the presence of chaotropic reagents in RP-HPLC at a thermodynamic level. We quantitatively validate this model for typical positively and negatively charged analytes as well as for ionic liquid, zwitterionic and neutral analytes.