Retention trends for inorganic ions on aminated silica and poly(styrene-divinylbenzene)-based stationary phases in acetonitrile-rich mobile phas-es

Abstract

In this work, we studied the retention trends for seven inorganic ions (NO₃^-, Cl^-, K⁺, Na⁺, Li⁺, Mg²⁺, Ca²⁺) on four polar stationary phases based on silica and poly(styrene-divinylbenzene) containing amino groups of various substitution degree. Mobile phases contained more than 80 vol.% of acetonitrile. A volatile ammonium acetate buffer solution pH 4.7 was added in the eluent to ensure a constant ionization state of the adsorbent groups and allowed us to use an evaporative light scattering detector. Columns possessed an anion exchange capacity from 16 to 90 μmol per 1 cm³ of the column. The elution order of both anions and cations was the same for all the stationary phases and opposite to the one typical for ion chromatography. As expected, an increase in the buffer solution concentration led to a decreased retention of anions and increased one for cations. Increasing acetonitrile content in the eluent resulted in increased retention factors for cations, but decreased one for nitrate and a U-shaped retention curve for chloride. These patterns indicated that ion exchange is realized for nitrate and chloride along with adsorption and partitioning to the surface water layer of adsorbent. Significant retention factors of cations resulted from partitioning and complexation of alkaline earth cations with nitrogen- and oxygen-containing groups of adsorbents despite electrostatic repulsion from protonated amino groups. Guided by the established patterns, it is possible to select the conditions for simultaneous separation of inorganic ions even on anion exchangers with a capacity of about 90 μmol/1 cm³ of the column accounting for the influence of hydrophilicity, anion-exchange capacity, and complexing properties of stationary phases on ion’s retention.