Chelation ion chromatography of alkaline earth and transition metals using a monolithic silica column with bonded N-hydroxyethyliminodiacetic acid functional groups

A commercially available porous silica monolithic column (Onyx Monolithic Si, 100mm × 4.6mm I.D.) was ‘in-column’ covalently functionalised with 2-hydroxyethyliminodiacetic acid (HEIDA) groups, and applied to the simultaneous and rapid separation of alkaline earth and transition metal ions, using high-performance chelation ion chromatography (HPCIC). With a 0.3 mM dipicolinic acid (DPA) containing eluent, the baseline separation of various common transition and heavy metal ions and the four alkaline earth metal ions could be achieved in under 14 min with a flow rate of just 0.8 mL/min. Detection was achieved using spectrophotometric detection at 540 nm after post-column reaction (PCR) with 4-(2-pyridylazo)-resorcinol (PAR). Significant effects from variation of eluent nature, concentration and temperature upon selectivity and retention were demonstrated with the new monolithic silica chelating phase. Under optimised conditions (0.165 M LiNO₃ eluent, pH 2.5), peak efficiencies of 54,000, 60,000 and 64,000 N/m, for Zn²⁺, Mn²⁺ and Cd²⁺, respectively, were recorded, far exceeding that previously reported for IDA based chelation ion exchange columns.