Electron bombardment ion source

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... 

Cameron, A.E. Electron-Bombardment Ion Source for Mass Spectrometry of Solids. Rev. Sci. Instrum. 1954, 25, 1154-1156. 

It is difficult to interpret the experimental findings of Bowie and Stapleton as other than the detection of Z species. However, the mechanism of the formation of the doubly-charged ions requires further study and testing in the opinion of this author. The unsuccessful search for ions reported by Chupka, Spence and Stevens is in disagreement with the observations of the ions with an electron bombardment ion source by Bowie and Stapleton. Since so little experimental information and results were provided by Chupka, Spence and Stevens, the extent of this disagreement is uncertain. Additional experiments should assist in clarifying these points. 

The SFs ion is readily formed from SF in electron bombardment ion sources. Henis and Mabie found the SFs" ion to have an essentially infinite lifetime. 

Sometimes the hot cathode filament of the ion source may disturb measurements where a low light background is needed Dworetsky et al39 have built an electron bombardment ion source which still gives a high ion current even though the cathode temperature is only near 1000°C. 

The overall diagram of a fast atom bombardment ion source is fairly simple. It consists of three main elements i) an atom gun, ii) a sample inlet, and iii) an ion extraction system. The atom gun is made up of an evacuated chamber that encloses a plate to which a high voltage potential (nominally 8 kV) is apphed. The gas to be used for the "bombardment" is allowed into the chamber, through an appropriate inlet, where it is ionised by the high potential plate (see equation (22)). The ion beam so created is repelled by the same plate (at 8 kV) and, to a certain extent, regains its neutral character by electron-capture or by charge-resonance processes such as those exemplified by equations (23) and (24). 

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). 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

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. 

The standard MAT 90 ion source is used for optimized FD/FI mode by means of the newly designed FD/FI probe. Conversion from electron impact (El), chemical ionization (Cl) or fast ion bombardment (FAB) to FD/FI operation does not require the exchange of the ion source. The FD/FI probe accommodates both the field emitter and the extraction electrodes, mounted at the probe tip. Both are introduced as a unit into the ion source through the ionization volume exchange lock without breaking vacuum. The fast and simple changeover illustrates the versatility of the Finnigan MAT 90 with no compromise on the performance. 

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