Gas Chromatography - Tandem Techniques - The Basic Principles of Mass Spectroscopy > The Sector Mass Spectrometer > Chemical Ionization > Page 67
At high electron energies (ca 60 eV) strong molecular disruption takes place, with a particularly high yield of low molecular weight fragments, however, the parent ion is now hardly discernible. Thus although parts of the secondary structure of the molecule are disclosed, the lack of a parent ion would again make structure elucidation difficult. Mid–electron energies (ca 20 eV) still provides numerous low molecular weight fragments but together with a clear, unambiguous, parent ion thus providing ample information for structural identification. It is clear that electron energy is an important variable and must be optimized, to obtain suitable structure elucidation.
Chemical ionization is a far more gentle form of ionization and is frequently used to accurately determine molecular weights. Firstly a reagent gas such as methane is ionized in a simple electron impact ion source. The partial pressure of the reagent gas is adjusted so that it is about two orders of magnitude greater than that of the sample. The reagent ions collide with the sample molecules and produce ions. As the energy of the reagent ions never exceeds 5 eV, the ionization process is a very gentle, and this is true even for those reagent ions that are considered to have relatively high energies. As the process is gentle, there is little fragmentation, and the major ion produced has a m/z value close to that of the singly charged molecular ion. The type of spectrum depends on the nature of the reagent ion and thus different structural information is obtained from different reagent gases. Employing methane the following reagent ions can be produced