Characterization of a novel trace-level impurity in lisinopril using multi-stage mass spectrometry.

An impurity in the bulk drug lisinopril was detected by simple reversed-phase high-performance liquid chromatography (HPLC). This trace-level impurity was rapidly identified as 2-(2-oxo-azocan-3-ylamino)-4-phenyl- butyric acid on the basis of the on-line multi-stage mass spectrometric evidence, and the proposed structure was further confirmed by multi-stage mass spectrometry of lisinopril and three related compounds.