Experimental Design Optimization of a Capillary Zone Electrophoresis Method for the Screening of Several Diuretics and ACE Inhibitors

Experimental design methodologies are applied to the development of a capillary zone electrophoretic method for the separation of the angiotensin-converting enzyme inhibitor enalapril and its derivative enalaprilat and the diuretics xipamide and hydrochlorothiazide. The effects of pH, buffer concentration, proportion of boric acid in the mixed boric acid‐potassium dihydrogen phosphate background electrolyte, temperature, applied voltage, and percentage of organic modifier are studied. Critical factors are identified in a screening design (a 2 6 ‐ 2 fractional factorial design), and afterwards, optimal conditions for the separation are reached by means of an optimization design (a 22 + 2 × 2 + k central composite design). The studied response is the resolution between peaks. The four studied compounds can be separated in less than 3.5 min using an electrolyte of 20mM boric acid‐potassium dihydrogen phosphate (75:25, v/v) with 5% MeOH adjusted to pH 8.0 with KOH, at a potential of 30 kV. The detection wavelength and temperature are 206 nm and 35°C, respectively.