Experimental design method was used for HPLC determination of irbesartan and hydrochlorothiazide in combined dosage forms. The traditional approach for optimization of experiments is time-consuming, involves a large number of runs and does not allow establishing the multiple interacting parameters. The main advantages of the experimental design method include the simultaneous screening of a larger number of factors affecting response and the estimation of possible interactions. On the basis of preliminary experiments, three factors-independent variables were selected as inputs (methanol content, pH of the mobile phase and temperature) and as dependent variables, five responses (resolution, symmetry of irbesartan peak, symmetry of hydrochlorothiazide peak, retention factor of irbesartan and retention factor of hydrochlorothiazide) were chosen. A full 2³ factorial design, where factors were examined at two different levels (“low” and “high”) was used to determine which factors had an effect on the studied response. Afterwards, experimental design was used to optimize these influent parameters in the previously selected experimental domain. The novelty of our method lies in the optimization step accomplished by Derringer¢s desirability function. After optimizing the experimental conditions a separation was conducted on a Supelcosil C₁₈ (150 mm × 4.6 mm, 5 mm particle size) column with a mobile phase consisting of methanol-tetrahydrofuran-acetate buffer 47:10:43 v/v/v, pH 6.5 and a column temperature of 25 °C. The developed method was successfully applied to the simultaneous separation of these drug-active compounds in their commercial pharmaceutical dosage forms.

Central composite experimental design was used for fast, simple, and accurate high-performance liquid chromatography (HPLC) determination of hydrochlorothiazide, amlodipine and valsartan in combined dosage forms. This method avoids the disadvantages of the traditional analytical approach, which is time-consuming, involves a large number of runs, and does not allow establishing the multiple interacting parameters. On the basis of preliminary experiments and physicochemical characteristic of analyzed substances, three independent variables (methanol content, pH of the mobile phase, and column temperature) were selected as input, while as dependent variables, six responses (retention time of hydrochlorothiazide, retention time of amlodipine, retention time of valsartan, asymmetry of hydrochlorothiazide peak, asymmetry of amlodipine peak, and asymmetry of valsartan peak) were chosen. Face centered central composite design enables an estimation of factors which have the most importance. After optimizing experimental conditions, a separation was conducted on a Zorbax C₈ (150 mm×4.6 mm, 5 μm) column with a mobile phase consisting of methanol-acetonitrile-acetate buffer 40:20:40 ( v/v/v), pH adjusted to 3.5 with acetic acid, flow rate of 1 mL/min and column temperature of 40 ^degC. The method was successfully applied to the simultaneous separations of these active drug compounds in their commercial dosage forms.