Separation and characterization of impurities and isomers in cefpirome sulfate by liquid chromatography/tandem mass spectrometry and a summary of the fragmentation pathways of oxime‐type cephalosporins

Rationale Although the identification of degradation products of cefpirome sulfate has been reported, there has been no report concerning the impurities in bulk samples of this compound. To meet the requirements of the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use, the structures of impurities whose content are over 0.1% need to be confirmed. Thus, characterization of the impurities in cefpirome sulfate bulk samples is critical for controlling the production of this drug. Methods The structures of cefpirome sulfate impurities were investigated using two‐dimensional liquid chromatography (LC) coupled to electrospray ionization tandem mass spectrometry. In the first LC dimension, a Kromasil 100‐5C18 column (4.6 mm × 250 mm, 5 μm) was used, and the mobile phases were 0.03 M ammonium dihydrogen phosphate solution and acetonitrile. In the second dimension, the column was a Shimadzu Shim‐pack GISS C18 column (50 mm × 2.1 mm, 1.9 μm), and the mobile phases were 10 mM ammonium formate solution and methanol. An ion trap time‐of‐flight mass spectrometer operated in both positive and negative ion mode was employed in this study. Results Nine impurities and isomers in cefpirome sulfate, eight of which were previously unknown, were separated and characterized. Structures were proposed for the eight unknown compounds based on the MSn fragmentation data. The degradation behavior of cefpirome sulfate was also studied. Conclusions Based on the characterization of impurities and isomers, this study could be used to improve the quality control of the cefpirome sulfate drug recommended in pharmacopoeias. The degradation behavior of cefpirome sulfate provides a basis for the selection of storage conditions.