Influence of Capacity on the Retention and Selectivity of Inorganic Ions Separation Over a Homologous Series of Sulfobetaine Based Stationary Phases in Zwitterionic Ion Chromatography

Background: Sulfobetaine zwitterionic stationary phases are offering various retention modes, but the influence of the capacity has never been investigated systematically. Objective: The capacity dependence of the retention and selectivity in zwitterionic ion chromatography mode has been investigated for common UV absorbing anions. Methods: Stationary phases with variable distances between the charged groups (one to four methylen groups) were prepared with capacities from 296 to 790 (one and two methylene groups as charge separator) or from 212 to 298 μeq g-1 (three and four methylene groups) by grafting polymerization on highly porous divinylbenzene polymers. The different spacer lengths and capacities are used as an investigative tool for the retention behavior of the anions nitrate, nitrite, bromide, iodide and thiocyanate. The separation mechanism is explored by varying eluent ionic strength and pH. Results: Remarkably, an increasing capacity leads depending on the charge distance to an increase for two and three methylen groups, decrease for four or no significant change in the retention times of inorganic anions for one methylene group charge distance. No significant change of selectivity occurred in investigated range of capacities. It should be noted that the higher capacity of the exchanger with two methylene groups between the oppositely charged groups exhibit a different separation behavior for inorganic anions in comparison to a previous study. Conclusions: The capacity plays in zwitterionic ion chromatography an important role for the retention behavior. The changes in retention behavior are depending on the spacer length between the charged groups.