Ionization by electrons

We say the molecule AB has been ionized by electron impact The species that results called the molecular ion, is positively charged and has an odd number of electrons—it IS a cation radical The molecular ion has the same mass (less the negligible mass of a single electron) as the molecule from which it is formed... 

Section 13 22 Mass spectrometry exploits the information obtained when a molecule is ionized by electron impact and then dissociates to smaller fragments Pos itive ions are separated and detected according to their mass to charge (m/z) ratio By examining the fragments and by knowing how classes of molecules dissociate on electron impact one can deduce the structure of a compound Mass spectrometry is quite sensitive as little as 10 g of compound is sufficient for analysis... 

The study of metastable ions concerns substances that have been ionized by electrons and have undergone fragmentation. The stable molecular ions that are formed by soft ionization methods (chemical ionization. Cl field ionization, FI) need a boost of extra energy to make them fragment, but in such cases other methods of investigation than linked scanning are generally used. 

Ionization by electron impact has been shown (11) to occur at about 16.1 volts. Condon has given the explanation of the discrepancy between this and the band spectrum value in terms of... 

FIGURE 4.4 Schematic of threshold behavior of ionization processes. Under ideal conditions, one expects a step function for photoionization, a linear variation with energy under electron impact, and a parabolic dependence for double ionization by electron impact. 

Figure 2.18 Schematic diagram showing the hypothetical behaviour of a molecule ionized by electron ionization (a) and its mass spectrum (b)...

Figure 7.10 The principle of field ionization. Left the potential for a helium atom near a metal without field, and (right) in the presence of an electric field of strength F (V/cm). Field ionization by electron tunneling becomes possible when the He Is level (ionization potential /) is above the Fermi level of the metal. Tunneling increases when the He atom is closer to the surface. This, however, requires high local fields, which are present at the edges of crystal facets or at adsorbed atoms.

The particle beam system is a simple transport device, very similar to a two-stage jet separator. The solvent vapour is pumped away, while the analyte particles are concentrated in a beam and allowed to enter the mass spectrometric source. Here they are vapourized and ionized by electron impact. 

Several other types of ion source under development should be watched closely for potential inorganic/organometallic use. Ionization by electrons from a Ni source has been used in an external ion source that is at atmospheric pressure 66,67) giving a reported sensitivity in the subpicogram range. 

Alkanes are among the least reactive classes of compounds they are poor electron acceptors (low electron affinities) as well as donors (high ionization potentials, viz., CH4,12.61 eV C10H22, 9.65 eV). ° The molecular anions of w-alkanes are especially unstable " negative ion yields for simple alkanes are 10" times lower than positive ion yields. Electron attachment results in small fragment ions (CH, CH2, CH3, M-AIkanes can be ionized by electron (MS) or... 

Chemical Ionization (Cl) The vaporized sample is introduced into the mass spectrometer with an excess of a reagent gas (commonly methane) at a pressure of about 1 torr. The excess carrier gas is ionized by electron impact to the primary ions CH4,+ and CH3+. These react with the excess methane to give secondary ions. 

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... 

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