Electron ionization separation techniques

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

Interfacing of solution-based separation techniques with mass spectrometry has historically been a challenge because of the incompatibility of the used solvent with the vacuum system. Standard electron impact (El) ionization with techniques such as particle beam require samples to be vaporized under high vacuum for ion formation to occur. 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

GC/MS analyses most often employ one of two complementary ionization processes, electron ionization (El) or chemical ionization (Cl). This is because both El and Cl are gas phase ionization phenomena and are therefore well suited to interface with a separation technique (GC) that is also accomplished in the gas phase. The extractables profiles shown in Figs. 3-5 along with the Abietic Acid GC/MS analysis shown in Fig. 1, were acquired using GC/MS with El. The El ionization process is based on the interaction of an energetic electron beam (70 eV) with neutral analyte molecules in the gas phase, producing a radical cation, or molecular ion (M+ ) that can undergo fragmentation in the gas phase after redistribution of excess... 

Advances in GC separation and ionization detection techniques during the 1960 s generated a great deal of interest and justifiable excitement for endocrinologists, due to the new possibilities of following hormones and their metabolites in body circulation. In particular, the development of electron-absorbing derivatives... 

In conjunction with these efforts, calculations were made to predict the chemical properties of the Superheavies so that likely ores could be chosen for investigation and separation schemes devised. Separation techniques were developed to purify and identify elements with lifetimes as short as a thousandth of a second. Models were developed to predict such aggregate properties as entropies from samples as small as 500 atoms. Ground-state electron configurations, oxidation states, ionization energies, metallic radii, ionic radii, densities, melting points. 

An alternative to PFI-ZEKE would be mass-analyzed threshold ionization (MATI), where cations rather than ZEKE electrons are detected.In principle, MATI is attractive because of the inherent mass selection. However, MATI experiments are difficult to implement because of ion separation fields affecting the high Rydberg levels required for delayed pulsed field ionizations. The technique has so far only been used for the smallest metal molecules. ... 

Two general types of the application of mass spectrometry should be emphasized. One is the detection and characterization of compounds introduced into the mass spectrometer. In this case the mass spectrometer can be considered as a powerful detector. It is able to characterize individual compounds, or work online with (chromatographic) separation techniques. Many applications of mass spectrometry, including analytical aspects, are based on the fact that this method involves physical or chemical transformations of the compounds being studied. These transformations may include evaporation, desolvation, electron transfer to or from the analyte, and chemical transformations before or after ionization. Chemical transformation can involve the formation of new bonds, or bond cleavage in ions of the compound under study. 

---