Electron impact measurements, ionization

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

The molecular formula of a molecule can also be defined by high resolution mass spectrometry (hrms). The observed mass for the molecular ion or pseudo molecular ion must normally be within 5ppm of the calculated mass for El (electron impact) measurements, or within lOppm for Cl (chemical ionization) measurements. It is important to note that high resolution mass spectrometry confirms that some molecules of a particular molecular formula are present in the sample, but does not give any indication of purity. Some other evidence of compound purity will therefore be required. 

Dietrich, A.M., D.S. Millington, and Y.-H. Seo. 1988. Specific identification of synthetic organic chemicals in river water using liquid-liquid extraction and resin adsorption coupled with electron impact, chemical ionization and accurate mass measurement gas chromatography-mass spectrometry analysis. J. Chromatogr. 436 229-241. 

Dissociation energies for C—H bonds have been obtained from electron impact measurements both by the direct and indirect method. The dissociation energy D CH3—H) in methane was deter-niinecD by measuring the appearance potential of the CHI ion when methane was fed into the ionization chamber. This is taken to be the energy of the process... 

Fig. 4.15. The ratio between the K-shell ionization cross sections of Ag for positron and electron impact. Measurements by Ito et al. [3.49] and Ebel et al. [3.17]. The arrow indicates the K-shell ionization potential.

In addition, the enthalpies of atomization were calculated from the appearance energies of R ions measured experimentally (Chervonnyi and Chervonnaya, 2004b,e,f Evdokimov et al., 1984 Sapegin, 1984 Sapegin et al., 1982a, 1984) under the conditions of electron-impact-induced ionization of RCI3 molecules. 

Electron-impact energy-loss spectroscopy (EELS) differs from other electron spectroscopies in that it is possible to observe transitions to states below the first ionization edge electronic transitions to excited states of the neutral, vibrational and even rotational transitions can be observed. This is a consequence of the detected electrons not originating in the sample. Conversely, there is a problem when electron impact induces an ionizing transition. For each such event there are two outgoing electrons. To precisely account for the energy deposited in the target, the two electrons must be measured in coincidence. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

Sputtered Neutral Mass Spectrometry (SNMS) is the mass spectrometric analysis of sputtered atoms ejected from a solid surface by energetic ion bombardment. The sputtered atoms are ionized for mass spectrometric analysis by a mechanism separate from the sputtering atomization. As such, SNMS is complementary to Secondary Ion Mass Spectrometry (SIMS), which is the mass spectrometric analysis of sputtered ions, as distinct from sputtered atoms. The forte of SNMS analysis, compared to SIMS, is the accurate measurement of concentration depth profiles through chemically complex thin-film structures, including interfaces, with excellent depth resolution and to trace concentration levels. Genetically both SALI and GDMS are specific examples of SNMS. In this article we concentrate on post ionization only by electron impact. 

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