HPLC/ESI-MS identification of diastereomers of Impurity E – Degradation products of cefaclor

•Solid sample of cefaclor stored in room conditions, was analyzed by HPLC/ESI-MS.•As a result of the degradation process, two diastereomers of Impurity E were formed.•The diastereomers are formed due to the isomerization of C6 carbon atom.•The diastereomers have different efficiencies of formation of dimer ions. A seven years old sample of cefaclor stored in room conditions, as a solid, has been analyzed by using high performence liquid chromatography/electrospray ionization-mass spectrometry. As a result of the sample degradation process, two diastereomers of Impurity E have been found to have formed. It is reasonable to assume that the diastereomers are formed due to the isomerization of C6 carbon atom. They have almost identical fragmentation patterns in both positive and negative ion mode. On the other hand, the diastereomers have different efficiencies of formation of dimer ions, under ESI conditions, especially in negative ion mode.