Positive desorption chemical ionization/tandem mass spectrometry of natural isomeric morphinans: Regioselectivity of protonation, positional and stereochemical effects upon decomposition

Fragmentations of naturally occurring morphinan isomers have been studied using a triple quadrupole mass spectrometer. Protonated molecules were generated via desorption chemical ionization using ammonia and methane as reagent gases. Mechanistic analysis of conformationally specific and stereospecific decomposition pathways has been performed and corroborated using deuterium labelling. Strong evidence of regioselective protonation has been offered, suggesting significant differences in the distribution of protonation sites as a function of reagent gas proton affinity.