Optimization of capillary electrophoretic separation of several inhibitors of the angiotensin-converting enzyme

Capillary electrophoretic separation of eight inhibitors of the angiotensin-converting enzyme, viz., enalapril, lisinopril, quinapril, fosinopril, perindopril, ramipril, benazepril and cilazapril, was investigated with respect to the following parameters: pH of the running buffer, organic modifiers and surfactants. The most critical parameter is the pH of the running buffer. The addition of sodium dodecyl sulfate had a negative influence on the peak symmetry, and selectivity was not improved. The separation of the eight compounds can be performed by means of two phosphate buffers (each 100 mM) at pH 7.0 and pH 6.25, respectively. This combination is necessary for the selective identification of structurally related substances because of their similar pKa values.