Gas Chromatography - Tandem Techniques - Gas Chromatography Mass Spectroscopy (GC-MS) Systems > Ion Generation > Electron Impact Ionization > Page 66

 

 

Figure 54. The Approximate Relationship Between Ion Current and Electron Energy

 

The electron energy can be simply controlled by adjusting the accelerating potential of the electrons in the ion source. Thus, by adjusting the accelerating voltage optimum fragmentation can be achieved that will provide the maximum information with respect to the structure of the compound being examined.

 

The effect of electron energy on the fragmentation pattern of a compound is depicted in figure 55. At low energies (ca 14 eV) a large proportion of the parent ion is produced but relatively few fragments. This would allow the molecular weight of the material to be easily identified but its structure difficult to elucidate.

Figure 55. Fragmentation Patterns for a Molecule Ionized with Electrons Having Different Energies