Electrons ionization and

N. Gamier, C. Cren Olive, C. Rolando, M. Regert, Characterization of the archaeological beeswax by electron ionization and electrospray ionization mass spectrometry, Analytical Chemistry, 74, 4868 4877 (2002). 

The most important gas phase ionization techniques are electron ionization and chemical ionization. The latter will be described in Section 2.6.2. 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

Fig. 2.4. Definition of appearance energy and visualization of changes in internal energy distributions, F(E), of relevant species upon electron ionization and subsequent fragmentation. The energy scale is shown compressed for the ions.

The El mass spectrum of acetone is comparatively simple. It basically shows three important peaks at m/z 58, 43, and 15. According to the formula C3H6O, the peak at m/z 58 corresponds to the molecular ion. The base peak at m/z 43 is related to this signal by a difference of 15 u, a neutral loss which can almost always be assigned to loss of a methyl radical, CH3. The m/z 15 peak may then be expected to correspond to the ionic counterpart of the methyl radical, i.e., to the CH3 carbe-nium ion (Fig. 6.3). The question remains, as to whether this mass spectrum can be rationalized in terms of ion chemistry. Let us therefore consider the steps of electron ionization and subsequent fragmentation in greater detail. 

D. One-Electron Ionization and One-Electron Oxidation of Organosilicon Compounds... 

Phosphitenylium cations have been postulated as intermediates in numerous reactions of 1-chlorophosphirenes including phosphirene complexes. Recently, the uncomplexed cation has been generated in the gas phase from 4 upon 70 eV electron ionization and its reactivity toward representative nucleophiles and dienes has been studied by pentaquadrupole mass spectrometry <20030BC395>. The first complex of a phosphirenylium cation 9 was prepared from 8 according to Scheme 5. The phosphirene complex 8 is rather unstable in neat or in solution and decomposes with the formation of the bisphosphirenyl ether 10 <19990M817>. 

Change in p at bond critical point for vertical electron ionization and addition ... 

Revelsky I A, Yashin YS, Sobolevsky TG, Revelsky Al, Miller B, Oriedo V. Electron ionization and atmospheric pressure photochemical ionization in gas chromatography-mass spectrometry analysis of amino acids. Eur J Mass Spectrom 2003 9 497-507. 

Stephanou, E., M. Reinhard, and H.A. Ball. 1988. Identification and quantification of halogenated and non-halogenated octylphenol polyethoxylate residues by gas chromatography/mass spectrometry using electron ionization and chemical ionization. Biomed. Environ. Mass Spectrom. 15 275-282. 

Non-homogeneity of energy deposition in radial direction is of importance when elementary processes are concerned or fluence is low. Upon beam incident to material, energy deposition would cause track structure. Substructure is presumed (physical) core at which an electronic ionization and excitation can... 

Mass Spectrometer Tuning. A three-step process was used to tune the quadrupole mass spectrometer prior to its use as a detector for SFC. In the first step, perfluorotributylamine (Pfaltz Bauer Inc., Stamford, Conn.) was ionized by electron ionization and used to calibrate the mass axis. In the second step, methane was introduced into the Cl source and the reagent ion profiles were optimized. In the third step, the mass resolution was adjusted for improved sensitivity. This was accomplished by introducing a volatile brominated compound, such as 2-bromopentane, into the Cl source. The mass spectrometer s resolving power was reduced such that the peaks... 

H. Siegbahn. R. Medeiros and O. Goscinski Direct Calculation of Core Electron Ionization and Relaxation Energies Using Transition Potentials Applications to Boron Compounds J. Electron Spectroscopy Q8, 149 (1976). 

Electron ionization and chemical ionization of neutral organometallics... 

Because a defined start-stop signal is required for the measurement of time, an essential prerequisite in the TOP operation is that all ions enter the flight tube at exactly the same time. This arrangement would also avoid any artifact left behind from the previous ionization event. For this reason, TOP instruments are optimally combined with pulsed-mode ion sources such as Cf plasma desorption or MALDI. Alternatively, pulsing the accelerating potential can provide a pulsed ion beam from continuous ion beam sources (e.g., electron ionization, and electrospray ionization). 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

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