Surface ionization and electron-impact-induced fragmentation of the [ M + H] + ions of methyl stearate

Abstract

Surface ionization (SI) characteristic ions such as [ M + K] +, [ M + Na] +, [ M + H] +, [ M + H] +, K +, and Na + appeared in th mass spectrum of methyl stearate ( 1), obtained under the lowest impact energy conditions of 2eV/electron impact ionization (E1). Mass spectra of 1 obtained under 8–13eV/E1 conditions showed intense peaks of [ M + K] +, and abundant fragment ions, while the spectra did not show the peaks of [ M + K] +, [ M + Na] +, [ M + H] +, K +, and Na + ions. The 8–13 eV/EI mass spectra of 1 showed an interesting ion at m/ z 75 ion, while the collision-induced dissociation (CID) spectrum of the [ M + H] + ions lacked the m/ z 75 ion. It has been assumed that the m/ z 75 ion is formed through the McLafferty rearrangement reaction for the [ M + H] + ions of 1. The mechanism and conditions for the formation of [ M + H] + and abundant fragment ions under 8–13 eV/EI conditions are discussed, and it is presumed on the basis of the CID spectrum that both SI and EI-induced fragmentation processes occur. The EI-induced ion-pair formation as a possible process is also discussed.