Formation of the [M+H]^+ and Abundant Fragment Ions of Methyl Stearate under Low Energy Electron Ionization Conditions

概要

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Mass spectra of methyl stearate (1) obtained under low energy (8-13 eV) electron ionization (EI) conditions showed intense [M+H]+ ions and abundant fragment ions in the positive-ion mode, and intense [M-H]- ions and characteristic fragment ions in the negative-ion mode. Positive-ion mass spectra of 1 under 8-13 eV/EI conditions showed an interesting ion at m/z 75 which may originate from a McLafferty rearrangement reaction of the [M+H]+ ions, an even electron species. The detailed experimental conditions to reproduce these spectra have been explored. It was found that the most important condition is the introduction into the ionizing cell of a relatively high-pressure sample vapor from a liquid state, similar to a spray process, and that the abundant fragment ions come from the [M+H]+ ions. Possible mechanisms are discussed to explain the formation of [M+H]+, [M-H]-, and abundant fragment ions under the 8-13 eV/EI conditions. Furthermore, unusual ion species such as [M+K]+, [M+Na]+, [M+H]+, [M-H]+, K+, and Na+ were observed in the mass spectrum of 1 obtained at the lowest energy (2 eV) EI conditions, suggesting that thermal surface ionization occurred on the Re filament.
日本質量分析学会の論文
1996-10-01