Study of the electrospray ionization mass spectrometry of the proton pump inhibiting drug Omeprazole

A detailed analysis of the product ion spectrum generated from the protonated molecule under ESI-MS/MS conditions using a triple quadrupole mass spectrometer is reported for the gastrointestinal proton pump inhibitor Omeprazole. Unambiguous molecular composition data of the fragment ions were obtained with the aid of regioselectively 14C-, 34S- and 18O-labeled analogs. Attempts have been made to provide rational pathways for the formation of the fragment ions from four protonated omeprazole species. These results will facilitate the characterization of the complex metabolic fate of Omeprazole in humans, which involves the excretion of at least 50 metabolites.