This document specifies a method for the determination of the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) and its oxidate form N-(n-butyl)phosphoric triamide (NBPTO) in urea based organo-mineral fertilizers, using the liquid chromatography coupled with triple quadrupole mass spectrometry (LC-MSQQQ). This document is applicable to organo-mineral fertilizers. NOTE It is possible to apply this method to inorganic fertilizers; in this case a validation is carried out by the laboratory for the procedure and data generated.