Rapid characterization of impurities in the bulk drug of nifedipine by high performance liquid chromatography-quadrupole time of fight mass spectrometry

Four impurities in the bulk drug of nifedipine were detected by means of high performance liquid chromatography coupled with quadrupole time of fight mass spectrometry (HPLC-QTOF MS). A Kromasil C18 HPLC column (250 mm x 4.6 mm, 5 microm) was used with methanol-water (60:40, v/v) as mobile phase. The column temperature was maintained at 35 degrees C with an ultraviolet (UV) detection wavelength of 235 nm for analysis. The eluates were detected with a UV detector and a quadrupole time of fight mass spectrometer in positive mode. The sample was infused into the mass spectrometer from the HPLC system through a T-junction with a splitting ratio of 3:1. The ion source temperature was set at 320 degrees C and the electrospray ionization (ESI) needle voltage was always set at 4 000 V. Nitrogen was used as the drying gas and nebulizer gas. The protonated molecular ions were selected for determining the structures, and their fragmentation pathways were deduced. Based on the fragmentation behavior of nifedipine, the structures of 3 impurities were identified and one of them was identified as 3-ethyl-5-methyl-2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate, which was an unknown impurity to the best of our knowledge. An other impurity was identified by the comparison of the retention time and tandem mass spectra with the standard substance. The four proposed structures were all confirmed by exact mass evidence. The results indicated that the described method can be effectively applied to perform the identifications in nifedipine and its impurities.