Electrostatic and metal interaction chromatography of sulfonamides

A diethylenetriamine functional stationary phase, capable of complexing transition metal ions, was developed and evaluated for use in HPLC separations. Synthesis involved evaporation of a trimethoxysilylpropyl-monomer. Surface coverages of 3.0 $\mu$moles/m$\sp2$ were obtained on 100 angstrom, 550 m$\sp2$/g silica. Analytical columns were loaded with Cu(II), Ni(II) and Zn(II) by the frontal elution process; breakthrough volumes corresponded well with metal uptake determined by atomic absorption spectrophotometry. Buffered aqueous-organic mobile phases were used for separations of sulfonamide antimicrobials that included sulfa-diazine, sulfa-thiazole, sulfa-merazine and sulfa-pyridine. An empirical retention expression was developed that related chromatographic retention to (a) electrostatic interaction of the acidic sulfonamide function with the cationic stationary phase, (b) secondary metal-ligand complexations involving the heterocyclic aromatic substituents of the solutes and (c) solubilization of the relatively hydrophobic solutes in the mobile phase. Analyte retention was inversely proportional to concentrations of organic modifier and buffer. Eluent pH was generally directly proportional to solute retention. An eluent of pH 7.2 phosphate buffer and 50% methanol was effective in minimizing electrostatic interactions while allowing retention based on metal-ligand interactions.