Application of derivative spectrophotometry for simultaneous determination of quinapril and hydrochlorothiazide in the combination tablets.

A new second-order-derivative spectrophotometric method using zero-crossing technique measures quinapril (QUI) and hydrochlorothiazide (HYD) in 2-component mixtures. The procedure does not require prior separation of components from the sample. QUI was determined at a wavelength of 211.6 nm (zero-crossing wavelength point of HYD). Similarly, HYD was measured at 270.8 nm (zero-crossing wavelength point of QUI ). Calibration graphs were constructed over the concentration range of 4.0 to 24.0 microg/mL for QUI and 2.5 to 15.0 microg/mL for HYD. Detection and quantitation limits were 0.85 and 2.5 microg/mL for QUI and 0.12 and 0.4 microg/mL for HYD, respectively. The accuracy (recovery 100.5-102%), precision (relative standard deviation less than 3.5% for QUI and 1.5% for HYD), selectivity, and sensitivity of the elaborated methods were satisfactory. The proposed method was applied successfully for the determination of both drugs in QUI-HYD tablets.