Dissociation mechanisms of neutral methyl stearate and its hydrogen atom adduct formed from the respective positive ions by electron transfer

Abstract

The mechanism of dissociation of neutral methyl stearate and its hydrogen atom adduct was investigated by charge inversion mass spectrometry using an alkali metal target. Migrations of functional groups in fatty acid ester ions are often observed during the dissociation of the cations in collisionally activated dissociation (CAD). In the charge inversion spectrum, the main dissociation channels of methyl stearate molecule are the loss of a CH3 radical or a H atom. To identify the source of the CH3 radical and the H atom, the charge inversion spectra of partially deuterated methyl stearate (C17H35COOCD3) were measured. The loss of CH3 occurred through elimination from the methoxy methyl group and that of H occurred through elimination from the hydrocarbon chain of the fatty acid group. In the protonated ester, a simultaneous loss of CH3 (from the methoxy methyl group) and a H atom or a H2 molecule was observed. The charge inversion process gave the dissociation fragments with almost no migration of atoms. Only a few peaks that were structure sensitive were observed in the higher mass region in the charge inversion spectra; these peaks were associated with dissociations of energy-selected neutral species, unlike the case of CAD spectra in which they result from dissociation of ions. Charge inversion mass spectrometry with alkali metal targets provided direct information on the dissociation mechanism of methyl stearate and its hydrogen atom adduct without any migration of functional groups.

Potential crossing position in electron transfer of a doubly charged ion and an alkali metal target measured using thermometer molecule W(CO)6

“…The cross section of the order of 10 −14 cm 2 estimated in the present work is much larger than that for collisional activation, which has been estimated to be in order of 10 −16 cm 2 [27,38]. We believe that the electron transfer processes of multiply charged ions of large biomolecules that follow collision with alkali metal targets will provide information about the dissociation of molecular projectile ions, particularly about amino-acid sequences in peptides and proteins, in the same manner as do the isomeric differentiations in charge-inversion mass spectrometry [27,[38][39][40][41].…”

Study of Ion Dynamics by Electron Transfer Dissociation: Alkali Metals as Targets

“…Charge inversion mass spectrometry has also provided information concerning the dissociation pathways of energy-selected neutral species, such as CH n (n=3-5), 44) C 2 H 2 , 45) C 3 H 4 , 46) chlorophenol, 47) and methyl stearate. 48) e dissociation of energy selected neutral of halogen containing simple molecules, such as CH 2 X 2 49,50) and CH 3 X (X=Cl, Br, I) 51) showed a strong dependence on both the target species and halogen in the molecules in charge inversion spectra using alkali metal targets (Cs, K and Na). While the CHCl 2 − ion was predominant in the spectra of CH 2 Cl 2 + regardless of target species, the most intense peaks were CH 2 Br 2 + and CH 2 I 2 + were ascribed to being derived from either Br − or CH 2 Br − and either I − or I 2 − , respectively, depending on the target metal used.…”