Investigation of collision-induced dissociations involving odd-electron ion formation under positive electrospray ionization conditions using accurate mass.

Collision induced dissociation (CID) has been extensively used for structure elucidation. CID in the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes has been found to generate mostly even-electron fragment ions while it has been occasionally reported to form odd-electron free radical ions. However, the structural requirements and the fragmentation mechanisms for free-radical CIDs have not been well characterized in the literature. For this purpose, we studied a series of aromatic and non-aromatic compounds such as sulfonamides, N-aryl amides, tert-butyl-substituted aromatic compounds, aryl alkyl ethers, and O-alkyl aryl oximes using the LTQ and LTQ Orbitrap linear ion trap mass spectrometers. The accurate measurement of the fragment ion masses established the unambiguous assignment of the fragment structures resulting from the test compounds. Our results showed that free radical fragmentation is structure dependent and is to a large extent correlated with the neighboring groups in the structures that stabilize the newly formed free radical ions.