Identification and structural elucidation of an unknown impurity in carbamazepine active pharmaceutical ingredient by liquid chromatography–tandem mass spectrometry and semi-preparative chromatographic isolation

Two impurities were detected in the HPLC analysis of crude carbamazepine active pharmaceutical ingredient. One of the impurities of the order of 0.5% was found to be unknown and has not been reported previously. A LC–MS compatible reverse phase isocratic method was developed and tandem mass spectrometry was performed using electrospray ionization source and ion trap mass analyzer. Isolation of unknown impurity was performed by semi-preparative HPLC followed by characterization using nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (FT-IR) and elemental analysis (CHNS) confirmed its structure as tetrabenzo[b,f,b′f′]azepino[4′,5′:4,5]thieno[2,3-d]azepine-3,9-dicarboxamide. A plausible mechanism for the formation of this impurity is proposed.