Solvent enhanced ion chromatography of alkaline earth and transition metal ions on porous monolithic silica

The high-performance separation of alkaline earth and transition metal ions on a bare porous silica 10.0 cm × 0.46 cm monolith (Merck Chromolith Si) was achieved using solvent enhanced ion chromatography. Using acetonitrile or methanol based ammonium and sodium acetate buffered mobile phases, the effect upon cation retention of mobile phase ionic strength, organic content and pH were evaluated. Increasing mobile phase acetonitrile concentration from 60 to 80% acetonitrile resulted in 10–15-fold increases in cation retention due to reduced hydration of the metal ions and hence enhanced ion-exchange interactions with surface silanol groups. The optimised conditions for the separation of Cu(II), Cd(II), Ni(II), Co(II) and Mn(II) were found to be 80% MeCN with 10.3 mM ammonium acetate (pH 4.6). Detection of transition metal ions was obtained using post-column reaction with 4-(2-pyridylazo) resorcinol and absorbance at 510 nm. Under elevated flow conditions (up to 5 mL/min), total runs times could be reduced to under 4 min. Optimum peak efficiencies were seen at flow rates of between 1.5 and 2 mL/min and were equivalent to an average efficiency of 25,000 N/m