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Three unknown impurities of Rabeprazole, a proton pump inhibitor, were formed in the formulated drug under the stress conditions, [40 degrees C/75% relative humidity (RH) for 6 months] with relative retention times (RRTs) 0.17, 0.22 and 0.28. The Impurity-I (0.17 RRT) was isolated using preparative HPLC and characterized by NMR and MS. The other two impurities, Impurity-II (RRT 0.22) and Impurity-III (RRT 0.28) could not be isolated, hence they are characterized by HPLC-hyphenated techniques, LC-NMR and high-resolution LC-MS. On the basis of the spectral data, the Impurity-I, Impurity-II and Impurity-III were characterized as 1-(1H-benzo[d]imidazol-2-yl)-3-methyl-4-oxo-1,4-dihydropyridine-2-carboxylic acid, 1H-benzo [d] imidazole-2-sulfonic acid and 4-(3-methoxy propoxy)-3-methyl-2-pyridine carboxylic acid, respectively.