The fragmentation of dihydropyridine calcium-channel antagonists are compared by electrospray ionization (ESI) and atmospheric pressure photonization

are compared by electrospray ionization (ESI) and atmospheric pressure photonization (APPI). The results demonstrate that in ESI the preferred ionization process is in positive mode, with the mass spectra of [M+H]+ showing base peak ions probably formed by loss of alcohols from carboxyl groups. Conversely, in APPI, a high intense peak is observed in negative mode due to deprotonated molecule [M–H]– after two serial 1, 2-hydride shifts leading to a rearranged deprotonated molecule [M–H]–. These ions undergo another 1,2-hydride shifts to produce a nitro-phenyl product ion of m/z 122. The APPI is also used to develop a method for the quantitation of dihydropyridines (e.g., nifedipine) in human plasma.