Modulation of the separation selectivity of inorganic anions in electrostatic ion chromatography using acidic eluents

Abstract

The separation selectivity for inorganic anions in electrostatic ion chromatography (EIC) was modulated by control of the concentration of H⁺ in the eluent. For example, in the separation of NO₂⁻, Br⁻ and NO₃ using H₂SO₄ as eluent, the elution order could be NO₂⁻ < Br⁻ < NO₃⁻; Br⁻ < NO₂⁻ < NO₃⁻; or Br⁻ < NO₃⁻ < NO₂⁻, depending on the concentration of H₂SO₄ in the range 1–30 mM. This behaviour is explained by protonation of the sulfonate functional groups of the stationary phase and the resultant changes in the binary electrical double layer (EDL) established at the charge centres of the zwitterionic stationary phase. The elution order for analyte anions was governed by the balance between the cation–EDL and anion–EDL, which is determined by the concentration of H⁺ in the eluent and results in varying degrees of ion-exchange contributing to a mixed-mode retention mechanism. It was also found in this study that addition of a small amount of perchlorate ion into the eluent was an effective means to elute very polarizable anions (e.g., I⁻, and SCN⁻). This was attributed to the high propensity for ClO₄⁻ to enter the anion–EDL on the positively charged quaternary ammonium groups of the zwitterionic stationary phase.