Electron impact ionization of alanine: Appearance energies of the ions

The mass spectrum and the appearance energies of the positive ions formed via electron impact ionization of l-alanine have been measured for the first time in a crossed electron-molecule beam experiment with a high-resolution electron beam. The ionization energy of the molecule and the appearance energies of the fragment ions were obtained from ion efficiency curves. Additionally, high-level quantum chemical calculations of the reaction enthalpies for dissociative ionization into positive ions and neutral radicals were performed and the results were compared with the experiment. On the basis of the calculations possible ionic structures for the measured ions were assigned. The value of the ionization energy for the parent ion with m/z = 89 was determined experimentally to be 9.12 ± 0.1 eV. For the principal positive fragment ions with m/z of 74, 44 and 28 the experimental values of the appearance energies are 10.74 ± 0.1, 9.10 ± 0.05 and 10.85 ± 0.1 eV, respectively.