Elution mechanism in electrostatic ion chromatography with histidine as anisoelectric ampholytic mobile phase

Electrostatic ion chromatography (EIC) using a zwitterionic stationary phas e (formed by coating a C-18 material with a hydrophobic zwitterionic surfac tant) was studied with a mobile phase comprising an aqueous solution of his tidine at the pH of its isoelectric point, together with non-suppressed con ductometric detection. Anions and cations were found to be eluted as separa te peaks, unlike the elution behaviour observed on the same system when pur e water was used as mobile phase. An explanation was suggested in terms of protonation equilibria of the overall uncharged histidine to form small amo unts of histidine cations and anions in the mobile phase which could act as counterions for analyte anions and cations. This suggestion was supported by measured pH changes occurring in the bands of eluted analyte anions (a d ecreased pH compared to the mobile phase) and cations tan increased pH comp ared to the mobile phase). The analytical potential of this type of EIC is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.