Electron impact ionisation and fragmentation of methanol and ethanol

Electron impact ionisation and dissociative ionisation of the two smallest primary alcohols have been investigated. A quadrupole mass spectrometer was used, with an internal ionising electron source resulting in an energy resolution of ∼0.8 eV. The partial ionisation cross sections (PICS) for a number of individual cations have been measured for energies between 10-100 eV. The present data compares well with previous results for methanol. Surprisingly, however, this study is the first to report individual PICS for ethanol. The sum of the present PICS for both methanol and ethanol is also compared to the total ionisation cross sections reported in the literature and are found to be in good accord. Additionally, appearance energies have been derived by fitting the Wannier equation to data measured within ∼2 eV of the cation formation threshold. However, the low energy resolution limits the precision with which these values are able to be determined. This study also establishes the validity of the data collected using the apparatus at Universidade Federal de Juiz de Fora (UFJF) and so provides a solid foundation for further studies of larger alcohols.