Characterization of a Synthetic Peptide Impurity by Fast‐Atom Bombardment–Tandem Mass Spectrometry and Gas Chromatography–Mass Spectrometry

Abstract Fast‐atom bombardment–tandem mass spectrometry (FAB–MS–MS) was used to characterize the structure of atosiban, a synthetic oxytocin antagonist, and one of its synthesis‐related impurities. The nature of the structural modification in the impurity of interest, replacement of the proline residue by 5‐aminovaleric acid, was determined directly from its product ion MS–MS spectrum. Confirmation of the identity of the impurity was accomplished with GC–MS.