Tandem mass spectrometry of organic nitro and halogen compounds: Competition between losses of molecules and of radicals

Abstract

Fragmentation of protonated and/or deprotonated small-molecule compounds in tandem mass spectrometry after electrospray ionization involves complex processes and competitive pathways. In order to increase the understanding, the fragmentation observed for large collections of organic molecules, mainly drugs and pesticides, has been studied and reviewed. In the current study, attention is focused at nitro-group related and halogen-related fragmentation of organic compounds with nitro and halogen groups. The fragmentation of 140 nitro-group and 300 halogen-containing compounds has been studied. Both nitro and halogen moieties can show losses of a radical (•OH, •NO, •NO2, •Cl, •Br, or •I), which may or may not be in competition with losses of the corresponding molecules (HNO, HNO2, HCl, HBr, or HI). For compounds with a nitro group, the chemical surroundings may influence the fragmentation, as illustrated by the ortho-effect, but also by examples of concerted losses. The competitive nature of the various fragmentation pathways is important in understanding the fragmentation, but the complexity of the processes hinders the ability to predict whether or not a particular fragmentation route is taken. While nitro-group and/or halogen-related fragmentation is observed for many compounds, there are also many compounds with nitro-group or halogen-substituents that follow different fragmentation pathways.