The main objective of the work discussed in this paper was evaluation of a chromatographic method for simultaneous determination of hydrochlorothiazide (HCTZ), lisinopril (L), and their impurities in pharmaceuticals. Chlorothiazide (CTZ) and disulfonamide (DSA), as potential impurities in hydrochlorothiazide, and diketopiperazine (DKP), as an impurity of lisinopril, were analyzed. The chromatographic behaviour of these substances on different columns was studied using mobile phases of different polarity. The optimum separations were achieved by gradient elution on a 4.6 mm x 20 mm, 3.5 mu m particle size, C-18 column. The mobile phase was a gradient prepared by mixing 7:93 (v/v) acetonitrile-25 mm potassium dihydrogen phosphate, pH 5, and 50:50 (v/v) acetonitrile-25 mm potassium dihydrogen phosphate pH 5 in different ratios. The flow rate was 1.0 mL min(-1). UV detection was performed at 215 nm. Methylparaben was used as internal standard. The method was validated for selectivity, linearity, precision,. and accuracy. The limits of detection, LOD, and quantification, LOQ, were determined experimentally. Because of its speed and accuracy the method can be used for quality-control analysis
