Electron ionization category

Calibrants are required to calibrate the mass scale of any mass spectrometer, and it is important to find reference compounds that are compatible with a particular ion source. Calibrants commonly used in electron ionization (El) and chemical ionization (Cl), such as perfluorocarbons, are not applicable in the ESI mode. The right calibrants for LC-ESI-MS should (1) not give memory effects (2) not cause source contamination through the introduction of nonvolatile material (3) be applicable in both positive- and negative-ion mode. The main calibrants used or still in use to calibrate ESI-MS can be divided into the following categories polymers, perfluoroalkyl triazines, proteins, alkali metal salt clusters, polyethers, water clusters, and acetate salts. 

We saw that numerous ionization techniques exist that yield radical cations or radical anions, protonated or deprotonated molecules, and various adducts. These ions yield fragments with an even number of electrons (closed shell) or with an odd number of electrons (open shell). Even though the radical cations derived from electron ionization sources retain a privileged status in common mass spectrometry, the other ionization methods become increasingly common. Electron ionization is not possible for many categories of molecules. Therefore, we will not limit the discussion to radical cations. 

Not much is known about these processes, but they must be included to give a total picture. Emissions of Lyman and Balmer spectra of the H atom upon e-impact on hydrocarbons, H2, and H20, discussed in Sect. 4.3.2, fall in this category. Similarly, many of the excited states observed in dissociated radicals via electron impact on stable molecules (Polak and Slovetsky, 1976) also belong to this category. It is known from the dipole oscillator spectrum of H20 (Platzman, 1967) that most ionizations are accompanied by considerable excitation. Excitation transfer to the neighboring neutral molecule followed by fast dissociation cannot be ruled out. 

Ionization methods such as electron impact, chemical ionization, desorption chemical ionization, and negative-ion chemical ionization are all based on ionization of gas-phase samples and, thus, fall within the first category of gas-phase ionization. 

Every organic reaction can be classified into one of three, more or less exclusive categories—ionic, radical and pericyclic. Ionic reactions involve pairs of electrons moving in one direction. In a unimolecular reaction, like the ionization of a tertiary alkyl halide, the carbon-halogen bond cleaves with... 

The ion source used for the generation of biomolecular parent ions is critical, and only recently have the so-called soft ionization methods been developed.2 Electron-impact ionization sources fall into the category of hard sources, whereby the sample must be in the vapor phase initially, and the ionization process produces a very large number of fragments. Soft methods were introduced to overcome the problems associated with the thermal instability and involatility of macromolecular analytes. Soft ionization produces few fragments under relatively mild conditions. In Table 15.1 a comparison is shown between the three main soft ionization methods some of these values are strongly dependent on individual mass spectrometer configurations and the desired resolution. 

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