Ionization and fragmentation of valine molecules in the gas phase by electron impact
Résumé
Formation of fragment ions due to single and dissociative ionization of the valine molecule (C5H11NO2) by electrons was studied by mass spectrometry. This is the first experimental mass-spectrometric measurement of total cross-section in arbitrary units of the valine molecule ionization by electron impact. The experiment was carried out using a setup with an MX-7304A monopole mass spectrometer in the range of mass numbers of 0–120 Da. Mass spectra of the molecules were studied at different vapor temperatures. The results obtained were compared with the mass spectra of the D-, L-, and DL-enantiomeric forms of the valine molecule taken from NIST and SDBS databases. The features of the processes of fragment ion formation of valine molecules by electron impact were analyzed in detail. Dynamics of the fragment ion yield in the range of the initial substance evaporation temperatures of 300–440 K was studied. The ionization potential of the valine molecule was measured (8.72±0.22 eV) and calculated ab initio in the adiabatic approximation (9.367 eV). The ionization potential was estimated from the binding energy of the HOMO orbital of the neutral molecule. This is the first calculation of the single ionization cross section of the D-, DL-, and L-forms of this molecule in the Binary-Encounter-Bethe model and using the Gryzinski formula. The experimental cross section values at the threshold are normalized by the theoretical values in absolute units.