The -1.0 Da mass shift observed in Atmospheric Pressure Chemical Ionization (APCI) but not in Electrospray Ionization (ESI) when analyzing nitrosamines may be due to differences in ionization mechanisms, matrix effects, or solvent interactions that influence the ionization and fragmentation patterns. These factors can lead to varying results between the two ionization techniques.

I agree with Abdelhak Maghchiche. WIth APCI, sometimes negatively charged molecular radical ions [M]• - (e.g. by electron capture ionisation) or radical cations, M•+ are observed, rather then the [M+H]+ or [M-H]- ions. Examples in negative mode are nitroaromatic compounds (e.g. 1-Chloro-2,4-dinitrobenzene), in positive mode PAHs.