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

The separation selectivity for inorganic anions in electrostatic ion chroma tography (EIC) was modulated by control of the concentration of H+ in the e luent. For example, in the separation of NO2-, Br- and NO3 using H2SO4 as e luent, the elution order could be NO2- < Br- < NO3-; Br- < NO2- < NO3-; or Br- < NO3- < NO2-, depending on the concentration of H2SO4 in the range 1-3 0 mM. This behaviour is explained by protonation of the sulfonate functiona l groups of the stationary phase and the resultant changes in the binary el ectrical double layer (EDL) established at the charge centres of the zwitte rionic 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 io n-exchange contributing to a mixed-mode retention mechanism. It was also fo und in this study that addition of a small amount of perchlorate ion into t he 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 z witterionic stationary phase.