Electron ionization applications

Qian et al carried out a rapid identification of polymers using a technique based on ion source direct pyrolysis mass spectrometry and library searching. Polymers were pyrolysed using a coiled filament designed for desorption chemical ionization/desorption electron ionization applications. Pyrolysis products were ionized at 70 ev electron impact. This yielded highly reproducible spectra characteristic of the polymer. Using these techniques and library searching a comprehensive library of 150 polymers was developed. 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

Another application is to the study of the Auger states in which a further electron ionization of attachment may occur, leaving the system with holes in more than one shell. Such states were considered in some detail by Firsht and McWeeny [9] for free atoms here we have made a preliminary applieation to the nitrogen moleeule. The initial aim is simply to identify and assign the principal peaks and satellites in the Auger spectrum of gaseous N2. 

The investigation of electron ionization is clearly in the early stages in comparison with the electron transfer studies, and additional work on the influence of orientation on Augmentation will be required before a coherent pattern emerges and a model for fragmentation can be attempted. However, a simple model that considers ionization in terms of the Coulomb potential developed between the electron and the polar molecule, taking the electron transition probability into account, reproduces the main experimental features. This model accounts qualitatively for the steric effect measured and leads to simple, generally applicable, expressions for the maximum (70 eV) ionization cross section. 

Calibrants are required to calibrate the mass scale of any mass spectrometer, and it is important to find reference compounds that are compatible with a particular ion source. Calibrants commonly used in electron ionization (El) and chemical ionization (Cl), such as perfluorocarbons, are not applicable in the ESI mode. The right calibrants for LC-ESI-MS should (1) not give memory effects (2) not cause source contamination through the introduction of nonvolatile material (3) be applicable in both positive- and negative-ion mode. The main calibrants used or still in use to calibrate ESI-MS can be divided into the following categories polymers, perfluoroalkyl triazines, proteins, alkali metal salt clusters, polyethers, water clusters, and acetate salts. 

A compact ion trap (r0 = 10 mm) mass spectrometer was developed for space-based applications [18]. The trap was made of titanium its hyperboloid surfaces were machined to a tolerance of 0.02 mm. Electron ionization was used by generating electrons from a heated, spiral-wound tungsten wire and accelerated at 75 eV into the trap. The trap operates in an RF only mode without DC. No cooling gas, such as helium, is required. Mass range was 1-300 amu and resolution m/Am = 324. The sensitivity of the trap was determined for N2 and was found to be 2 x 1014 counts/torr.s. 

At that time the mass spectrometric ionization techniques of electron ionization (El) [1] and chemical ionization (Cl) [2] required the analyte molecules to be present in the gas phase and were thus suitable only for volatile compounds or for samples subjected to derivatization to make them volatile. Moreover, the field desorption (FD) ionization method [3], which allows the ionization of non-volatile molecules with masses up to 5000 Da, was a delicate technique that required an experienced operator [4], This limited considerably the field of application of mass spectrometry of large non-volatile biological molecules that are often thermolabile. 

In addition to ESI and APCI, ionization techniques such as electron ionization (El) and chemical ionization (Cl) have also been used in the analysis of flavanones (Weintraub et al., 1995). As the advancement and refinement in analytical techniques continues, it is expected that these techniques will find an application in flavanone analysis. 

Several pieces of information are available which tend to provide support for our estimated EA value. Formation of SF involves the addition of an electron to an antibonding orbital of SF (2) which is located primarily on the sulfur atom. It is reasonable, therefore, to assume that EA(SF) EA(S) = 2.0772 0.005 eV (3). O Hare (2) has pointed out that Koopmans theorem ( ) gives reliable estimates of electron affinities and ionization potentials provided that the parent molecule or ion have a closed-shell electronic configuration. Application of this theorem to SF which has the required closed-shell structure leads to EA = 1.7 eV (2). Other estimates of EA based on MO calculations have included (all in eV) 2.8 (5), 2.5 0.5 (6), and <3.2 (2). 

The rules indicated above seem to not have much connection to the fragmentation results obtained in pyrolysis. For example, Stevenson s rule, the charge site ionization mechanism, and the sigma electron ionization mechanism are not applicable to pyrolysis products, as the molecules in pyrolysis are not ionized. On the other hand, the a cleavage and certain rearrangements may be similar for the two processes. Also the fact that small molecule elimination is favored in mass spectrometry makes possible that, with a certain frequency, pyrolysis products are similar to mass fragments obtained in mass spectrometry. 

To perform mass spectrometry, one must make ions from neutral molecules. Ionization methods have advanced from the classic electron ionization (El), through chemical ionization, field desorption, fast atom bombardment (FAB), h 62 thermospray to the atmospheric pressure ionization (API) techniques currently favored. El is classic, but its is restricted to thermally stable, volatilizable compounds. Field desorption was always a specialized niche technique applicable to some larger compounds. Fast atom bombardment enjoyed a meteoric rise in use when it was first reported in 1981 but it has all but disappeared now, being replaced by the API tech-... 

Tarbah et al. also used a single-step liquid-liquid extraction method with toluene for the determination of 23 OPi in urine, blood, serum and food samples using GC with nitrogen-phosphorus selective detection (GC-NPD) and electron ionization mass spectrometry (GC-EIMS). The recoveries for spiked human plasma ranged between 50% (demetho-ate) and 133% (dialifos). Akgur et al successfully measured OP concentrations in 28 cases of acute OP poisoning using the above application. 

Electron ionization (El) is surely the ionization method most widely employed (Mark and Dunn, 1985). This method was proposed and used from the early days of mass spectrometry (MS) applications in the chemical world and is still of wide interest. This interest is due to the presence of libraries of El mass spectra, which allows easy identification of unknown previously studied analytes. The El method suffers from two limitations It is based on the gas-phase interactions between the neutral molecules of the analyte and an electron beam of mean energy 70 eV. This interaction leads to the deposition of internal energy in the molecules of the analyte, which is reflected in the production of odd-electron molecular ([M]+ ) and fragment ions. These ions are highly diagnostic from a structural point of view. 

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