Low-Energy Electron Ionization Mass Spectra

Nonetheless, during the first decades of analytical mass spectrometry, low-energy El spectra were the only way to minimize fragmentation, and thereby to increase the relative intensity of a weak molecular ion peak. Nowadays, El mass spectra are preferably complemented with spectra obtained from so-called soft ionization methods (Chaps. 7-13). 

Note Signals from previous samples in the mass spectrum of the actual analyte are usually termed memory. Memory is caused by contamination of ion source or sample introduction system. To reduce memory one should use the lowest amount of saiiple necessary to produce good spectra, keep the ion source around 200°C, and allow some pumping between measurements. 

The excess energy deposited onto a molecular ion can obviously be decreased at low electron energy. The use of 12-15 eV electrons instead of the routinely employed 70 eV electrons still allows to ionize most analytes while reducing disadvantageous fragmentation, e.g., the El mass spectra of large hydrocarbons benefit from such measures. [12] Especially in conjunction with low ion source tempera- 

Unfortunately, there are also some disadvantages i) the decreased ionization efficiency at 12-15 eV also means a significant loss of sensitivity (Fig. 5.4), ii) low ion source temperatures cause long-lasting memory of previous samples due to slow desorption from the surfaces that have been in contact with the sample vapor, and iii) a weak molecular ion peak may well be enhanced, however, a spectrum showing no molecular ion peak at 70 eV will not turn into a spectrum exhibiting a strong molecular ion peak at 12 eV. 

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