Ionization, associative electron impact

These Cl [M + H]+ ions (quasimolecular ions) are often prominent. Chemical ionization spectra sometimes have prominent [M — H]+ ions because of hydride ion abstraction from the M,+ ion by CH5+. Since the [M + H]+ ions are chemically produced, they do not have the great excess of energy associated with ionization by electron impact, and they undergo less fragmentation. For example, the El spectrum of 3,4-dimethox-yacetophenone shows, in addition to the molecular ion at m/z 180, 49 fragment peaks in the range of mJz 40-167 these include the base peak at m/z 165 and prominent peaks at m/z 137 and m/z 77. The CH4 induced Cl spectrum shows the quasimolecular ion (M + H+, m/z 181) as the base peak (100%), and virtually the only other peaks, each of just a few percent intensity, are the... 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

Peterkop, R. K. Theory of Ionization of Atoms by Electron Impact, Boulder, Colorado, Colorado Associated University Press Boulder, CO, 1977. 

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

The values obtained for the ionization potentials of organometallic molecules are much lower than the ionization potentials of the ligands (Table XIV) and are much closer to the ionization potentials of the central metal atom (Table XV), indicating that ionization subsequent to electron impact involves an electron associated with the metal atom. 

Various ionization techniques applied in association with Py-GC/MS are reported in literature (see e.g. [12]). However, the most common ionization method is electron impact with the detection of positive ions (EI+). The chemical ionization (Cl) is sometimes used, but Cl spectra interpretation is difficult because of the lack of fragmentation and because the reproducibility in Cl is affected by the experimental conditions in which the spectra are generated. However, Cl spectra provide valuable information regarding the molecular mass of the analyte, and this can be very useful in combination with EI+ spectral information. 

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. 

Further examples are the structural isomers of CjHj and 03 . The linear and cyclic structures of both the C3H2 and 03 ions are formed in electron impact ionization from methylacetylen (CH3CCH) (Smith and Adams, 1987), n-butane, or propane (Hansel et al, 1989). The linear and cyclic isomers of C3H and C3H are distinguished by either their different association rates with CO or then-different reactivity with C2H2. In all cases, the linear (or open-chain) structural isomers 1-C3H, 1-C3H ) are more reactive then the respective cyclic forms (C-C3H+, C-C3H3+). 

Mass Spectrometric Methods for Capillary SFC-MS. A significant advantage associated with capillary SFC-MS methods, and in contrast to all mechanical (e.g., moving ribbon) HPLC-MS interfaces, results from the flexibility in selection of ionization methods. Although initial studies were conducted using chemical ionization, and it remains the method of choice for most applications, the DFI process is also compatible with electron impact ionization (37). 

In this set of elementary reactions, k,-, k, and k represent the specific reaction rates for electron-impact excitation, electron-impact ionization, homonuclear associative ionization, and radiative decay of the reactive state X to an unreactive one X respectively. It is assumed in the treatment that only one excited state is involved. 

For the cases of Ar2, Kr2, and Xe2 formation, Huffman and Katayama have shown that all optically accessible excited states whose energies lie above the threshold energy undergo homonuclear associative ionization. It would seem highly likely that the same statement would apply also to all optically forbidden states above threshold. Therefore, the electron-impact ionization-efficiency curves must be a superposition (weighted with respect to excitation cross sections) of the excitation functions of all excited states above threshold. 

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