a) How does decreasing primary electron energy affect electron ionization (EI) mass spectra?
The excess energy deposited onto a molecular ion can obviously be decreased at low electron energy and therefore results in reduced fragmentation and higher relative intensity of the molecular ion peak. Thus, the overall appearance of the spectrum is simpler and the fragmentation pattern is dominated by a few characteristic primary fragmentations carrying the largest portion of structural information.
b) Why has low-energy low-temperature EI not become the standard method in EI?
There is no atom or molecule that cannot be ionized at 70 eV, whereas at 15 eV gases such as He, Ne, Ar, H2, and N2 would not.
The plateau of the ionization efficiency curve around 70 eV makes small variations in electron energy negligible.
This assures better reproducibility of spectra, and therefore allows comparison of spectra obtained from different mass spectrometers or from mass spectral databases.
Low ion source temperatures cause long-lasting „memory“ of previous samples.