Influence of Different Ionization Conditions On the Mass-spectrometric Behavior of Some Fluorinated Acyclic Nucleoside Analogs

The behaviour of three fluorinated acyclic nucleoside analogs has been examined under different ionization conditions, i.e. by electron ionization, fast-atom bombardment and positive-ion chemical ionization using methane, isobutane and ammonia as reactant gases. The protonated molecules have been studied by collisional spectroscopy. In general, protonation takes place on the functional groups exhibiting a proton affinity higher than that of the reactant species but some discrepancies are present. They have been explained by the presence of quasi S-H-F structures implicating proton bridging between sulphur and fluorine atoms.