First-order UV-derivative spectrophotometry in the analysis of omeprazole and pantoprazole sodium salt and corresponding impurities

The first-order UV-derivative spectrophotometry, applying zero-crossing method was developed for the determination of omeprazole (OM), omeprazole sulphone (OMS), pantoprazole sodium salt (PANa), and N-methylpantoprazole (NPA) in methanol-ammonia 4.0% v/v, where the sufficient spectra resolutions of drug and corresponding impurity were obtained, using the amplitudes D-1(304), D-1(307), D-1(291.5) and D-1(296.5), respectively. Method showed good linearity in the ranges ( mug ml(-1)): 1.61-17.2 for OM; 2.15-21.50 for OMS; 2.13-21.30 for PANa and 2.0-20.0 for NPA, accuracy and precision (repeatability and reproducibility). The experimentally determined values of LOD (mug ml(-1)) were 1.126; 0.76; 0.691 and 0.716 for OM, OMS, PANa and NPA, respectively. The obtained values of 2.91% w/w for OMS and 3.58% w/w for NPA in the presence of their parent drug, by applying the method of standard additions, point out the usage of the proposed method in stability studies. Zero-crossing method in the first-order derivative spectrophotometry showed the impurity-drug intermolecular interactions, due to the possible intermolecular hydrogen bonds, confirmed by divergences of experimentally obtained amplitudes for impurities OMS and NPA in comparison to expected values according to regression equations of calibration groups