Electron ionization primary electrons

Generally, the evaporated mobile phase acts as the ionizing gas and reactant ions are produced from the effect of a corona discharge on the nebulized solvent. Typically, the corona discharge forms by electron ionization primary ions such as N2 + or 02 +. Then, these ions collide with vaporized solvent molecules to form secondary reactant gas ions. 

Fig. 2.18. Electron-impact ionization cross-section for the Ni K shell, as a function of reduced electron energy U [2.128] U = Ep/Ek, where Ep is the primary electron energy and E

If ionization of a core level X in an atom A in a solid matrix M by a primary electron of energy Ep gives rise to the current Ij (XYZ) of electrons produced by the Auger transition XYZ, then the Auger current from A is... 

Appearance potential methods all depend on detecting the threshold of ionization of a shallow core level and the fine structure near the threshold they differ only in the way in which detection is performed. In all of these methods the primary electron energy is ramped upward from near zero to whatever is appropriate for the sample material, while the primary current to the sample is kept constant. As the incident energy is increased, it passes through successive thresholds for ionization of core levels of atoms in the surface. An ionized core level, as discussed earlier, can recombine by emission either of a characteristic X-ray photon or of an Auger electron. 

Negative Ion Chemical Ionization Negative ions are produced under ci conditions by electron capture. Under the higher pressure conditions of the ci ion source, electrons, both primary (those produced by the filament) and secondary (produced during an ionization event), undergo collisions until they reach near-thermal energies. Under these conditions, molecules... 

General. The use of alpha particles instead of electrons as in conventional ion-molecule reaction mass spectrometers introduces a difference in primary ionization conditions which is not as great as might be supposed. Thus, the primary ion mass spectra produced by alpha particles are very similar to these produced by say 70-e.v. electrons (30). Secondary electrons produced by the alpha particles are responsible for more than 50% of the total ionization. The energies of these electrons peak in the range 20-100 e.v. so that again the primary ions will be similar to those produced by 70-e.v. electrons. 

Another procedure for calculating the W value has been developed by La Verne and Mozumder (1992) and applied to electron and proton irradiation of gaseous water. Considering a small section Ax of an electron track, the energy loss of the primary electron is S(E) Ax, where S(E) is the stopping power at electron energy E. The average number of primary ionizations produced over Ax is No. Ax where o. is the total ionization cross section and N is the number density of molecules. Thus, the W value for primary ionization is 0)p = S(E)/No.(E). If the differential ionization cross section for the production... 

Unfortunately many compounds do not have a peak for their M+ in electron ionization mass spectra, as it is unstable. That is why it is of primary importance to be able to identify M+ correctly. There are four necessary but not sufficient conditions to name an ion peak molecular. If at least one of these conditions fails the ion is not molecular. If all four statements are fulfilled, the ion can be molecular. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Ionization in Cl is the result of one or several competing chemical reactions. Therefore, the sensitivity in Cl strongly depends on the conditions of the experiment. In addition to primary electron energy and electron current, the reagent gas, the reagent gas pressure, and the ion source temperature have to be stated with the sensitivity data to make a comparison. Modem magnetic sector instmments are specified to have a sensitivity of about 4 x 10" C pg for the [Mh-H] quasi-molecular ion of methylstearate, m/z 299, at / = 1000 in positive-ion Cl mode. This is approximately one order of magnitude less than for El. 

More advanced techniques are also available. For instance, mass spectrometry (MS) can be used by converting neutral radicals to ions, via chemical ionization and electron impact these ions are then separated and detected according to their mass-to-charge ratios (m/z). The primary drawback of this method is that it cannot directly... 

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