2639162 EI-MS electron ionization mass spectrometry 121 2639165 EI-MS electron impact mass spectrometry 52 2639164 EIMS electron ionization MS 3 2639156 EI-MS electron impact-mass 2 2639158 EI-MS Electron Bombardment Ion Source and Mass Spectrometry 1 2639157 EI-MS Electron impact-mass spectrometric analysis 1 2639163 EI-MS electron ionization high-resolution mass spectrometry 1 2639161 EI-MS electron ionization mass spectrometry of collected HPLC fractions 1 2639160 EI-MS electron ionization quadrupole mass spectrometer 1 2639159 EI-MS electron ionization-mass 1