Electron ionization method development

This book encompasses 23 contributions on different aspects of CC applied to a large arena of research field. The first contribution by Flores-Moreno and Ortiz deals with the theoretical formulation of electron propagator methods developed to compute accurate ionization potentials and electron affinity of system of different sizes. This review describes recent implementations that can be used for more challenging system without compromising the accuracy of the results. In the next... 

Electron ionization was the first ionization method developed for mass spectrometry, and it remains the most widely used. The most extensive mass spectral libraries assembled are those of electron ionization mass spectra recorded under a standard set of conditions. The ion-... 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Theoretical models of the electron impact ionization process have focused on the calculation of the ionization cross section and its energy dependence they are divided into quantum, semiclassical and semiempirical. Methods for the calculation of the ionization cross section and experimental techniques developed for the measurement of absolute ionization cross sections will be described in more detail below. Cross sections calculated using the semiempirical additivity method developed by Deutsch and Mark (DM) and their coworkers,12-14 the binary-encounter-Bethe (BEB) method of Kim and Rudd,15 16 and the electrostatic model (EM) developed by Vallance, Harland, and Maclagan17,18 are compared to each other and to experimental data. 

A multitude of semiempirical and semiclassical theories have been developed to calculate electron impact ionization cross sections of atoms and atomic ions, with relatively few for the more complicated case of molecular electron impact ionization cross sections. One of the earlier treatments of molecular targets was that of Jain and Khare.38 Two of the more successful recent approaches are the method proposed by Deutsch and Mark and coworkers12-14 and the binary-encounter Bethe method developed by Kim and Rudd.15,16 The observation of a strong correlation between the maximum in the ionization efficiency curve and the polarizability of the target resulted in the semiempirical polarizability model which depends only on the polarizability, ionization potential, and maximum electron impact ionization cross section of the target molecule.39,40 These and other methods will be considered in detail below. 

Electron ionization (earlier called electron impact) (see Chapter 2, Section 2.1.6) occupies a special position among ionization techniques. Historically it was the first method of ionization in mass spectrometry. Moreover it remains the most popular in mass spectrometry of organic compounds (not bioorganic). The main advantages of electron ionization are reliability and versatility. Besides that the existing computer libraries of mass spectra (Wiley/NIST, 2008) consist of electron ionization spectra. The fragmentation mles were also developed for the initial formation of a radical-cation as a result of electron ionization. 

El/MS, formerly electron-impact ionization, methods were among the earliest developed. El/MS is performed in a vacuum and the ions produced are positively charged (M ) due to the loss of an electron from... 

Junk and Meisch [55] developed a low resolution method for PCA analyses based on GC/electron ionization (EI)-MS in the SIM mode. By introducing a commercial formulation directly into the ion source of the MS via a direct insertion probe, and under full scan conditions, they selected the C-TI CR (m/z 105) ion to be the characteristic ion, i.e., the quantitation ion, of the standard. The integrated area of this ion was compared to that of the same ion from an injection of a known amount of an external standard to calculate the amount of PCAs in the sample. 

The key sample set selection for analytical method development has been discussed at length in Chapter 7. There are a great variety of methods used for monitoring impurities.1,2 The primary requirement for such techniques is the capacity to differentiate between the compounds of interest. This requirement frequently necessitates utilization of separation methods (covered in Section V. C) in combination with a variety of detectors (Section V. B). For gas chromatography, flame ionization and electron capture detectors are commonly used. However, these detectors are not suitable for isolation and characterization of impurities, which require... 

The ion source used for the generation of biomolecular parent ions is critical, and only recently have the so-called soft ionization methods been developed.2 Electron-impact ionization sources fall into the category of hard sources, whereby the sample must be in the vapor phase initially, and the ionization process produces a very large number of fragments. Soft methods were introduced to overcome the problems associated with the thermal instability and involatility of macromolecular analytes. Soft ionization produces few fragments under relatively mild conditions. In Table 15.1 a comparison is shown between the three main soft ionization methods some of these values are strongly dependent on individual mass spectrometer configurations and the desired resolution. 

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... 

Surely, this is the result on one hand to the development of ionization methods alternative to electron ionization, able to generate ions from highly polar, high-mass molecules and are easily coupled with chromatographic systems. This aspect has been well recognized by the entire scientific community with the assignment in 2002 of the Nobel... 

Linear enthalpy-entropy compensation is well known to physical organic chemists and has been the subject of controversy since the relationship was first discovered experimentally. We have discussed the complications elsewhere and will only note here that the linearity found by Beetlestone et al. is statistically reliable for most of their examples. The most extensively studied set of small-solute compensation processes in water are the ionizations of weak acids. When acids such as acetic acid or benzoic acid are substituted in their nonpolar parts to form homologous series, the standard enthalpies and entropies of ionization are found to demonstrate compensation behavior with 7], values in the 280-290°K range but only after extraction of all the contributions to these quantities from the electronic rearrangements using methods developed by Hepler and Ives and their coworkers. The obvious conclusion is that this behavior in small-solute processes is due to solvation effects and thus a manifestation of some property of water. As a result of the comparison of their data with these small-solute examples, Beetlestone et al. suggested that bulk water also plays an important role in the protein processes they studied. 

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