A mechanism of separation in electrostatic ion chromatography

The retention mechanism of electrostatic ion chromatography (EIC) is curren tly under debate and is the focus of this paper. A comprehensive set of ret ention data has been obtained on a C18 column coated with the zwitterionic surfactant 3-(N,N-dimethylmyristylammonio)propane-sulfonate used with a ran ge of mobile phases in which both the mobile-phase anion and cation have be en varied systematically. Electro-osmotic flow measurements were also obtai ned on fused-silica capillaries coated with the zwitterion (and also some m onofunctional surfactants) and were used to evaluate the nature of the surf ace charge on the layer of adsorbed surfactant in the presence of various b ackground electrolytes. A new retention mechanism for EIC was developed on the basis of these data. This mechanism proposes that equilibration of the bound zwitterions with a mobile phase containing a suitable electrolyte cau ses the establishment of a charged layer created by the terminal sulfonate groups of the zwitterion, which acts as a Donnan membrane. The magnitude an d polarity of the charge on this membrane depends on the nature of the mobi le-phase ions. The Donnan membrane exerts weak electrostatic repulsion or a ttraction effects on analyte anions. A second component of the retention me chanism is chaotropic interaction of the analyte anion with the quaternary ammonium functional group of the zwitterion. This interaction exerts the ma jor effect on the separation selectivity of EIC, such that analyte anions a re eluted in order of increasing chaotropic interactions in accordance with the Hofmeister series.