Electron Impact and Chemical Ionization
Electron impact (EI) ionization is one of the most classic ionization techniques used in mass spectrometry. A glowing filament produces electrons, which are then accelerated to an energy of 70 eV. The sample is vaporized into the vacuum where gas phase molecules are bombarded with electrons. One or more electrons are removed from the molecules to form odd electron ions (M+') or multiply charged ions. Solids, liquids and gases can be analyzed by EI, if they endure vaporization without decomposition. Therefore the range of compounds which can be analyzed by EI is somewhat limited to thermally stable and volatile compounds. The coupling with gas chromatography has been well established for decades. The ionization energy of most organic compounds to form a radical cation is below 15-20 eV. The excess of energy transferred to the molecules causes reproducible fragmentation. Fragmentation of odd electron ions has been extensively studied but remains still a challenging task for non-experts. Under standard conditions at 70 eV, EI spectra are reproducible and instrument independent. Large commercial libraries are available to rapidly identify compounds present in a sample . A limitation of the use of EI is that similar spectra can be obtained for isomers. Most analytical applications use EI in the positive mode but negative mode operation is also possible. EI is mostly combined with single quadrupole mass analyzers because often in the same spectrum, the molecular ions as well as fragment ions are present. Figure 1.6A shows the electron impact spectrum of a compound with a relative molecular mass of 355. The radical cation ion at m/z 355 as well as many fragments can be observed. Chemical ionization would generate the protonated molecule ion at m/z 356 (see Fig. 1.6B). To obtain structural information requires tandem mass spectrometry. Interestingly, odd and even electron ions undergo different fragmentation pathways, as observed in Fig. 1.6. This information is complementary, underlining that electron ionization remains an important technique for structural elucidation.
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Fig. 1.6 (A) Electron impact spectrum obtained on a single quadrupole mass spectrometer of a compound with Mr = 355. (B) Product ion spectrum after atmospheric pressure ionization obtained on a triple quadrupole instrument. Chemical ionization and atmospheric pressure ionization give in both cases protonated precursor ions, which is ideal for tandem mass spectrometry.
Protonated or deprotonated molecules can be generated by chemical ionization (CI) sources with similar design to the classic EI sources . The principal difference between CI and EI mode is the presence of a reagent gas which is typically methane, isobutane or ammonia. The electrons ionize the gas to form the radical cations (in the case of methane, CH4 + e— ! CH4+' + 2e—). In positive chemical ionization (PCI) the radical cations undergo various ion-molecule reactions to form "CH5+" and finally lead to the formation, after proton transfer (CH5+ + M ! [M+H]+), of protonated molecules. Negative chemical ionization (NCI), after proton abstraction, leads to deprotonated molecules [M—H] —. Negative ions can be produced by different processes, such as by capture of low energy electrons present in the chemical ionization plasma. The major advantages of negative CI over positive EI or CI are higher sensitivity, the occurrence of the molecular ion and less fragmentation. Due to its high sensitivity NCI is mainly used in quantitative analysis after derivatization of the analyte .
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