Electron Ionization EI

The most important method for the production of ions in GC-MS instruments uses the electron ioniza-

Figure 1 Schematic of an electron ionization (EI) ion source. Ions formed by interaction of the sample molecules with electrons emitted from the filament are extracted and focused into the mass analyser by the action of the repelller and the focusing lens.

Both types of initial fragment ions may also further fragment:

and/or:

Fragmentation will continue until the excess internal energy is dissipated. The appearance of EI mass spectra is a function of the compound under investigation, the electron energy used and the ion source temperature. For this reason it is usual to record EI mass spectra at an electron energy of 70 eV which gives good sensitivity, interpretable fragmentation and allows comparison to be made between spectra recorded on different instruments and with standard spectra stored in computerized libraries.

An EI mass spectrum of 2-nitrofluorene, a nitrated polycyclic aromatic hydrocarbon, is shown in Figure 2A. This mass spectrum illustrates some of the key features of EI spectra. A small molecular ion can be seen at m/z 211 along with fragment ions corresponding to the loss of 'OH and NO2 groups. The pattern of fragment ions, i.e. their intensity and distribution is characteristic of 2-nitrofluorene and library search, used where possible in combination with GC retention time (obtained from a standard sample), allows the sample to be easily identified.

Figure 2 Comparison of (A) electron ionization, (B) positive chemical ionization and (C) negative chemical ionization mass spectra of 2-nitrofluorene. Note the higher degree of fragmentation in the EI mass spectrum.

Electron ionization is the most widely used ionization technique for GC-MS. However, it has a number of limitations. The most important of these is caused by the excess internal energy of the initially formed molecular ions. For certain classes of compounds, they all fragment in the ion source and hence a molecular ion is not observed in the recorded mass spectrum. This removes one of the key pieces of information from the mass spectrum, i.e. the relative molecular mass of the compound under investigation. In order to overcome this, other ionization techniques are available to the mass spectroscopist, the most important of these being chemical ionization (CI).

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