Excess-Kinetic-Energy Ions in Organic Mass Spectra

Abstract

The characteristics of excess‐kinetic‐energy ions in mass spectra have been studied with a Dempster‐type mass spectrometer. A method was devised whereby the total initial kinetic energies possessed by such ions could be measured more accurately than has been done previously, and the kinetic energies of methyl ions, both near‐thermal and excess‐kinetic‐energy, from a wide variety of different organic compounds, are presented. Appearance potential curves were obtained for several excess‐kinetic‐energy methyl ions; these show a second‐power dependence of cross section on electron energy in the threshold region with threshold values of the order of 30 eV. Finally, a method of estimating the discrimination factor of the mass spectrometer toward ions possessing excess kinetic energy was utilized to obtain values for the actual abundances of excess‐kinetic‐energy methyl ions in the fragmentations of several organic compounds. Actual abundances of from 2% to 10% of the total ions in the mass spectrum were found from many compounds. The values of appearance potentials, kinetic energies, and abundances that have been measured for the complementary excess‐kinetic‐energy ion pairs from methyl amine (masses 15 and 16) and propane (masses 15 and 27) are consistent with a mechanism for formation in which both ions arise from the same initial state in each respective case. The data support the suggestion that excess‐kinetic‐energy fragment ions from the organic compounds are formed by breakdown of doubly charged parent ions.