Characterization of an exception to the ‘even‐electron rule’ upon low‐energy collision induced decomposition in negative ion electrospray tandem mass spectrometry

Abstract

Electrospray-generated precursor ions usually follow the 'even-electron rule' and yield 'closed shell' fragment ions. We characterize an exception to the 'even-electron rule.' In negative ion electrospray mass spectrometry (ES-MS), 2-(ethoxymethoxy)-3-hydroxyphenol (2-hydroxyl protected pyrogallol) easily formed a deprotonated molecular ion (M-H)(-) at m/z 183. Upon low-energy collision induced decomposition (CID), the m/z 183 precursor yielded a radical ion at m/z 124 as the base peak. The radical anion at m/z 124 was still the major fragment at all tested collision energies between 0 and 50 eV (E(lab)). Supported by computational studies, the appearance of the radical anion at m/z 124 as the major product ion can be attributed to the combination of a low reverse activation barrier and resonance stabilization of the product ions. Furthermore, our data lead to the proposal of a novel alternative radical formation pathway in the protection group removal of pyrogallol.

Protective Effect of DetoxifiedRhus vernicifluaStokes on Human Keratinocytes and Dermal Fibroblasts against Oxidative Stress and Identification of the Bioactive Phenolics

“…The molecular mass and respective fragment ions of three peaks (2, 5, and 7) obtained by the TOF analysis were compared to the data in published studies. Peak 2 with ½M À H À at m=z 183.0323 and a fragment at m=z 124.0193 was tentatively identified as 2-(ethoxymethoxy)-3-hydroxyphenol, 31) peak 5 with ½M À H À at m=z 787.1223 and a relevant fragment ion pattern, as tetragalloyl glucose, 32) and peak 7 with ½M À H À at m=z 939.1300, a fragment at m=z 769.1070 resulting from the loss of gallic acid (170 Da) and a fragment at m=z 617.0945, as pentagalloyl glucose. 33) Peaks 3 and 4, peaks 5 and 6, and peaks 10 and 11 were identified as isomers based on the product ions.…”

N-Centered Odd-Electron Ions Formation from Collision-Induced Dissociation of Electrospray Ionization Generated Even-Electron Ions: Single Electron Transfer via Ion/Neutral Complex in the Fragmentation of ProtonatedN,N′-Dibenzylpiperazines and ProtonatedN-Benzylpiperazines

“…When these closed-shell ions are subjected to CID, they commonly dissociate to daughter closed-shell ions and neutral fragments based on 'even-electron rule' [1]. It is rare that an electrospray ionization generated even-electron (EE) ion directly dissociates to form an odd-electron (OE) ion, though exceptions are more common in electron ionization (EI) [1][2][3][4] than in ESI mass spectrometry [5][6][7][8][9][10][11][12]. It is significant and desirable to investigate the underlying mechanism of formation of OE ions from EE ions because such considerations are important for a better understanding of ESI mass spectrometry.…”

Fragmentation pathways of oxygenated tetracyclic triterpenoids and their application in the qualitative analysis of Ganoderma lucidum by multistage tandem mass spectrometry

“…[21][22][23] However, in our experiment, we observed the radical ionic fragment formed by the homolytic cleavage in the negative ion mode.…”