Simultaneous ion chromatographic separation of anions and cations on poly(aspartic acid) functionalized silica

Abstract

Poly(aspartic acid)–silica (PolyCAT A), originally designed for the cation-exchange chromatography of proteins, is proposed for the simultaneous ion chromatographic separation of inorganic anions and cations. This is possible owing to the zwitterion-exchange properties of this stationary phase, which are attributed to the presence of both protonated aminopropyl and dissociated carboxylic groups in poly(aspartic acid) attached to the silica. The retention of alkali metal (Li⁺, Na⁺, K⁺), alkaline earth metal (Mg²⁺, Ca²⁺), ammonium and inorganic anions (Cl⁻, H₂PO₄⁻, Br⁻, NO₂⁻, I⁻, IO₃⁻, NO₃⁻, ClO₄⁻, SCN⁻) was tested in aqueous solutions of sulfuric, perchloric, sulfosalicylic, citric, oxalic, maleic and aspartic acids with conductimetric detection. The effect of eluent pH, together with the concentration and characteristics of the eluting ions, were studied. Under optimum conditions (0.3 mmol dm⁻³ H₂SO₄–0.2 mmol dm⁻³ Li₂SO₄ eluent), the simultaneous separation of three anions (Cl⁻, H₂PO₄⁻, NO₃⁻) and four cations (Na⁺, K⁺, Mg²⁺, Ca²⁺), on a PolyCAT A column (200 × 4.6 mm id, 5 μm film thickness) was achieved in 9 min.