Ionizer, electron bombardment

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

An ion/neutral mass spectrometer [7] that allows the ions and the neutrals (via electron bombardment ionization) volatilizing from the emitter to be measured in sequence... 

In 1967, Yuan T. Lee (1936- ) joined Herschhach as a researcher after completing his Ph.D. at Berkeley. Lee designed and built an apparatus with supersonic beam nozzles and an electron bombardment ionizer that functioned as a universal detector. Supersonic beams propel species in the same direction at nearly the same speed and allow very few collisions. For this reason, chlorine atoms, much more reactive than potassium atoms, could be employed in the new and even more sensitive apparatus ... 

The resulting spectrum [35] (Figure 3) is dramatically simplified by comparison to that obtained following electron bombardment ionization [36]. The vibrational excitation in the ion can be varied by changing the wavelength of the first, ionization laser to be resonant with a intermediate Rydberg level of differing v. This results in spectra with different vibrational activity and these results combined with the same spectra obtained for isotopically labeled molecules can be used to provide a detailed vibrational analyis [37] for this band system which was previously only subject to qualitative interpretation. 

The electron bombardment ionization of a molecule as described above produces an excited molecular ion, which, in attempting to gain stability, may decompose in a number of different ways by unimo-lecular reaction to fragment ions or neutrals. The relative abundance of the molecular ion is determined by its ability to resist decomposition, but the stability of fragment ions is dependent on the relative rates of reaction that form and destroy the ion. 

A composite plot of the mass spectra of six of the explosives that we have studied is shown in Fig. 9. For each of the explosives, the plots were obtained by summing up all of the counts in the channels of the MCS, up to channel 40,400 ps after detonation of the pellet. The sum for each mass was then compared to that at a standard mass, several shots of which were always recorded for each run of from 17 to 26 shots. Thus provided with an intensity ratio, several instrumental variables were eliminated between runs made on different days, it is these ratios that we show in the plot, and, as we mentioned earlier each mass point is actually an average of several of these ratios. An adjustment is included in the plot to account for the mass transmission of the quadrupole mass filter, but no correction is made to the intensity for the ionization sensitivity of the electron bombardment ionizer. Thus the relative ion mass ratios of Fig. 9 are almost raw data of the mass spectrum. Making the more difficult corrections would necessitate having the identity of the parent molecule that gives ions at each mass. We have not progressed to that point yet. Truncating the sum at 400 m.s is arbitrary, but our intention is to eliminate from our analysis molecules that may appear at late times because they were directed into the detector by collisions with parts of the apparatus. 

The detector, which usually can be rotated to various scattering angles, must be able to differentiate among the species present. Surface-ionization detectors with efficiencies of approximately 10% are employed for reactions of metal atoms yielding metal halide products. Laser-induced fluorescence has proved to be an excellent detector for group Ilb halides and a few other systems. In general, products can be detected by electron-bombardment ionization with subsequent analysis by a mass spectrometer. Unfortunately, these mass spectrometer or universal detectors have a low detection efficiency ( 0.1%,) thus, beam machines with such detectors require several differentially pumped chambers to reduce the background pressure (of the mass of interest) in the detector to torr. Modulation tech-... 

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

Yes caution—in certain cases electron bombardment may ionize or create defects... 

The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electron-gun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 80—... 

Unfortunately, not every compound shows a molecular ion in its mass spectrum. Although M+ is usually easy to identify if it s abundant, some compounds, such as 2,2-dimelhylpropane, fragment so easily that no molecular ion is observed (Figure 12.3). In such cases, alternative "soft" ionization methods that do not use electron bombardment can prevent or minimize fragmentation. 

Inghram and Corner showed that the mass spectra of molecules were much simpler using a field ionization source than with an electron bombardment ion source. Mainly parent ions are formed, unlike under electron impact which gives rise to considerable fragmentation. The simplicity of the mass spectra offers obvious applications in analysis of complex organic mixtures and their use is likely to become widespread... 

The full potential of LC-MS could not be exploited until it was possible to study involatile and thermally labile compounds for which electron and chemical ionization are not appropriate. A relatively small number of reports of the use of the moving-belt interface with fast-atom bombardment ionization for the study of these types of compound have appeared. 

A mass spectrometer is often indispensable for a complete analysis of low-pressure gases, but a description of the various types of spectrometers is beyond the purpose of this book, but see, for example, ref. [18]. We simply remind that a mass spectrometer consists of three parts an ion source where the neutral gas is ionized (usually by electron bombardment) an analyser where ions are selected according to their mass to charge ratio and a collector with an amplifier to measure the weak ion current. 

Ionization and fragmentation of materials by a variety of means, principally by electron bombardment, or the softer techniques of chemical ionization, field ionization or fast atom bombardment. Analysis of the range of mass fragments produced. Elemental composition of non-volatile materials by application of an RF spark. 

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