Electron ionization data interpretation

Combined electron ionization mass spectroscopy (EIMS) and matrix isolation FTIR spectroscopic data on vacuum pyrolysis of 1,1-dimethyl-l-germa-3-thietane assisted by theoretical calculations provide a reasonable foundation for mechanistic interpretation of its thermal decomposition (see Section 2.21.6.1, Equation 7) <1998JA5005>. 

The MST/EPA/NIH Mass Spectral Library 1998 database ( www.nist.gov/ srd/analv.htm) is the product of a muftiyear, comprehensive evaluation and expansion of the world s most widely used mass spectral reference library, and is sold in ASCII or Windows versions. It contains 108,000 compounds with electron ionization spectra, chemical structures, and molecular weights. It is available with the NIST MS Search Program for GC/MS deconvolution, MS interpretation, and chemical substructure analysis. The NIST chemistry WebBopk ( http //webbook.nist.gov) is a. free online system that contains the mass spectra of over 12,000 compounds (this Standard Reference Data Program also has IR and UV-Vis spectra). 

After high quality mass spectra are obtained, the next step in data interpretation is to deduce the atomic composition of mass spectral features. Since mass spectra relate ion abundances with their mjz values, in order to find the molecular weight (m) of a compound, it is necessary to predict the charge state (z) of recorded ions. The number of electric charges acquired by ions in the gas phase is influenced by the ionization method. This problem used to be less important because old ionization techniques [e.g., electron ionization (El), chemical ionization (Cl), fast atom bombardment (EAB), secondary ion mass... 

The analytical mass spectrometer was introduced commercially in 1941. For two decades its main application was quantitative analysis of light hydrocarbon mixtures and similar samples, often with accuracies of 1% absolute. Such analyses depend directly on linear superposition of the spectra of the components, in the same way that Beer s law governs spectrophotometric mixture analysis. The low-pressure conditions for electron ionization cause each mixture component to contribute independently to the measured spectrum of the sample, so that the spectrum s abundances can be reflected by a series of simultaneous equations using data from reference spectra of the pure components. Thus, if you can identify correctly one component of an unknown mixture spectrum, you can then subtract out the reference spectrum of that component ( spectrum stripping ), and attempt to interpret the residual spectrum. Note, however, that spectral superposition will not necessarily be linear for Cl and other spectra obtained at pressures high enough for competitive ion-molecule reactions. Today, most MS mixture analysis utilizes combined instrumentation such as GC/MS (Karasek and Clement 1988 Evershed 1989 Catlow and Rose 1989). 

The molecular orbital description of the bonding in NO is similar to that in N2 or CO (p. 927) but with an extra electron in one of the tt antibonding orbitals. This effectively reduces the bond order from 3 to 2.5 and accounts for the fact that the interatomic N 0 distance (115 pm) is intermediate between that in the triple-bonded NO+ (106 pm) and values typical of double-bonded NO species ( 120 pm). It also interprets the very low ionization energy of the molecule (9.25 eV, compared with 15.6 eV for N2, 14.0 eV for CO, and 12.1 eV for O2). Similarly, the notable reluctance of NO to dimerize can be related both to the geometrical distribution of the unpaired electron over the entire molecule and to the fact that dimerization to 0=N—N=0 leaves the total bond order unchanged (2 x 2.5 = 5). When NO condenses to a liquid, partial dimerization occurs, the cis-form being more stable than the trans-. The pure liquid is colourless, not blue as sometimes stated blue samples owe their colour to traces of the intensely coloured N2O3.6O ) Crystalline nitric oxide is also colourless (not blue) when pure, ° and X-ray diffraction data are best interpreted in terms of weak association into... 

The results of the alkylbenzene series may also be readily explained in terms of ir complex adsorption. In this series, the molecular orbital symmetry of individual members remains constant while the ionization potential, electron affinity, and steric factors vary. Increased methyl substitution lowers the ionization potential and consequently favors IT complex adsorption. However, this is opposed by the accompanying increase in steric hindrance as a result of multiple methyl substitution, and decrease in electron affinity (36). From previous data (Tables II and III) it appears that steric hindrance and the decreased electron affinity supersede the advantageous effects of a decreased ionization potential. The results of Rader and Smith, when interpreted in terms of tt complex adsorption, show clearly the effects of steric hindrance, in that relative adsorption strength decreases with increasing size, number, and symmetry of substituents. 

Briefly, XANES is associated with the excitation process of a core electron to bound and quasibound states, where the bound states interacting with the continuum are located below the ionization threshold (vacuum level) and the quasibound states interacting with the continuum are located above or near the threshold. Thus, XANES contains information about the electronic state of the x-ray absorbing atom and the local surrounding structure. However, as stated above, unhke EXAES, since the excitation process essentially involves multielectron and multiple scattering interactions, interpretation of XANES data is substantially more complicated than that of EXAFS data. 

Up to the time that one of the authors first began this investigation, the interpretation of the data on ionization potentials of polyatomic gases ha d been almost wholly a matter of conjecture, since no attempts had been made to resolve the group of ions, produced by impact electrons of various velocities, into their constituent parts. It was with the object of interpreting these data that the present investigation was started, and, while the work was in progress, several annoimcements of experimentations similar to that conducted by the authors have appeared. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

Final confirmation of this interpretation was attempted by studying the electron transfer between fluorene and m-trifluoromethylnitrobenzene in basic solution monitored by ESR spectroscopy in the absence of oxygen. Table VI summarizes data yielding an ionization rate constant of 0.9... 

In many field emission and field ionization experiments, field strength is a basic parameter which has to be known accurately before a lot of experimental data can be interpreted properly. Determination of field strength at the field emitter surface and field distribution above the field emitter surface in field electron and field ion emission, however, is not an easy task because of the complicated geometry of the tip. In field emission, the validity of the Fowler-Norheim theory has been established experimentally to within about 15%, and the current density as a function of the field has been tabulated.26 Thus it is possible to determine the field strength simply from the field emission current density. The field strength so determined cannot of course be more accurate than 15%. 

Different capillary columns are available for organic acid separation and analysis. In our laboratory, the gas chromatography column in all GC-MS applications is crosslinked 5% phenyl (poly)methyl silicone, 25 m internal diameter 0.20 mm stationary phase film thickness 0.33 pm (Agilent HP-5, DB-5, or equivalent). Several instrument configurations are commercially available, which allow for positive identification of compounds by their mass spectra obtained in the electron impact ionization mode. A commercially available bench-top GC-MS system with autosampler (Agilent 6890/5973, or equivalent) is suitable. Software for data analysis is available and recommended. The use of a computer library of mass spectra for comparison and visualization of the printed spectra is required for definitive identification and interpretation of each patient specimen. 

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