Electron impact technique

Traditionally, conventional electron impact techniques have been used to effect sample ionization. The often complex fragmentation pattern which results can provide invaluable information for elucidating the structure of unknown compounds. Vast computer libraries of electron impact spectra are available to assist the analyst with identifications. However, electron impact is an ionization technique that is somewhat limited in scope. By contrast, sample ionization by chemical ionization techniques offers the potential for selectivity, increased sensitivity, complimentary fragmentation, and confirmation or determination of molecular weight. (7). In this paper we will describe several examples in which the versatility of the chemical ionization technique has simplified or improved an analysis. 

Mass spectrometry has been an extremely useful tool for the characterization of neutral organometallic clusters, except for those few cases which have extremely high molecular weights or possess such ligands as PPh3 that reduce the volatility of the compound. Mass spectrometry has not been a useful characterization technique for ionic clusters because these compounds are insufficiently volatile to permit study by conventional electron-impact techniques. However, there is some hope that with the development of field-desorption techniques ionic clusters as well as neutrals will be capable of being analyzed by mass spectrometry (146). 

Electron impact technique (Linda et at182). 6 Data from Linda and Marino.84... 

Mass spectrometry must be used with caution, as the weakness of the E-M bonding framework in some cases leads to fragmentation of the parent ion, which is thus not observed. For the smaller, more volatile molecules, conventional electron-impact techniques can be valuable,42 but softer ionization techniques may be required for these as well. Chemical ionization (Cl) has been successfully employed for the characterization of Fe3 (CO)10(PR).192 Electrospray techniques have proven useful for characterizing even nonvolatile, ionic complexes.231 Positive ion-laser desorption methods were employed successfully to characterize (CO)4W PW(CO)5 4.461... 

Interestingly, it has been observed that the ionization potentials (determined by the electron-impact technique) of several substituted five-membered heterocycles correlate with the substituent effects is larger than for benzene (p = —14.7) and larger than that measured for the most sensitive electrophilic substitution (bromination in acetic acid, which gives p = —10.0 for thiophene). As pointed out by Linda et al. [12], the sensitivity to substituent effects... 

Blais and Cottin (1 ) have measured the appearance potential (AP) of SCl (g) from SClgCE) as 12.5 0.2 eV using the electron impact technique. Subsequent work by Hartmann et al. (2) using the same method led to AP(SC1 /SClg) = 12.2+0.1 eV which confirms the earlier results. We assume that the fragment ions produced in the dissociative ionization process e" + SClgCg) = SCI (g) +... 

Although these techniques were developed partially as a result of improvements of the electron impact techniques, new ionization methods were developed simultaneously. These new methods appeared when the discipline of mass spectrometry... 

The electron impact technique has provided valuable information regarding core electron excitation spectra. A laboratory-based apparatus can provide high-quality spectra with a resolution comparable if not superior to that obtained from monochromators used with synchrotron sources. The... 

Mass spectrometry has proven to be an important tool for quickly and reliably identifying the number of monomeric units of 1 that are incorporated into an oligomer. A variety of ionization techniques have been used on resveratrol oligomers with widely varying results. Standard electron impact techniques have occasionally resulted in failure to observe a molecular ion [100] however, the use of other soft ionization methods has helped circumvent this problem. Some of the more commonly employed techniques that have repeatedly been used with a high degree of success include electrospray [106], field desorption [47,65], and fast atom bombardment [39,42,53,67,68,92,99,103] ionization. 

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

The fragmentation patterns of relatively volatile derivatives of penicillins (e.g. benzyl-penicillin methyl ester) under electron impact (B-72MI51101) and chemical ionization (75MI51100) conditions have been described. For both techniques the primary fragmentation is that shown in Scheme 1. 

In this chapter, three methods for measuring the frequencies of the vibrations of chemical bonds between atoms in solids are discussed. Two of them, Fourier Transform Infrared Spectroscopy, FTIR, and Raman Spectroscopy, use infrared (IR) radiation as the probe. The third, High-Resolution Electron Enetgy-Loss Spectroscopy, HREELS, uses electron impact. The fourth technique. Nuclear Magnetic Resonance, NMR, is physically unrelated to the other three, involving transitions between different spin states of the atomic nucleus instead of bond vibrational states, but is included here because it provides somewhat similar information on the local bonding arrangement around an atom. 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

There are numerous ionization techniques available to the mass spectrome-trist, but for GC/MS almost all analyses are performed using either electron impact ionization or chemical ionization. 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

Mass spectroscopy is a useful technique for the characterization of dendrimers because it can be used to determine relative molar mass. Also, from the fragmentation pattern, the details of the monomer assembly in the branches can be confirmed. A variety of mass spectroscopic techniques have been used for this, including electron impact, fast atom bombardment and matrix-assisted laser desorption ionization (MALDI) mass spectroscopy. 

Similarly, the thermal sensitivity of sulfur allotropes makes mass spectrometry of elemental sulfur and sulfur-rich compounds difficult especially with the conventional electron impact ionization. Nevertheless, valuable information has been obtained by this technique also. 

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

Due to the high mass, low volatility, and thermal instability of chlorophylls and derivatives, molecular weight determination by electron impact (El) MS is not recommended. Desorption-ionization MS techniques such as chemical ionization, secondary ion MS, fast-atom bombardment (FAB), field, plasma- and matrix-assisted laser desorption have been very effective for molecular ion detection in the characterization of tetrapyrroles. These techniques do not require sample vaporization prior to ionization and they are effective tools for allomerization studies. 

Panesh et al. [157] were the first to make an attempt to detect rare gas metastable atoms (RGMAs) with the aid of semiconductor sensors. The sensing element (a sensor) was represented by a sintered polycrystalline film of ZnO metastable atoms were obtained in a neon ambient by electron impact. It was shown that electrical conductivity of ZnO film irreversibly increases under the action of RGMAs. However, the signals obtained were too small and that did not allow one to utilize the sensing technique to survey the processes with participation of metastable atoms. 

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