Source electron ionization

Besides the ionization of gases an induced fragmentation of molecules by electron impact (El fragmentation) also occurs (Equations 2.21 and 2.22). The fragmentation of organic molecules can be used for structural analysis of unknown compounds. 

Electron ionization sources produce constant ion beams of about 10 8 A with low initial energy spread. The ion current measured depends strongly on the ionization degree of the gas analyzed (type of atoms and molecules). Positive ions and electrons are formed by the interaction of electrons of sufficient energy with gas atoms or molecules. The ion current /+ is proportional to the pressure (p) of the gaseous sample, to the electron current /e, the length (/) of the collision chamber and the differential ionization (s) of elements as a function of the ionization energy  

The ions formed are extracted by the extraction lens, EL, perpendicular to the electron beam. Due to the low energy spread of ions formed in an electron ionization source ( 1 eV) one single-focusing magnetic sector field mass spectrometer or quadrupole analyzer is sufficient for 

Knudsen effusion mass spectrometry is used to measure the composition of the effusing beam (including the composition of the sample investigated, fragments or newly formed species) and in particular to determine the thermochemical data. The instrumentation is briefly described in Section 5.2.10.137 

8 Matrix Assisted Laser Desorption/Ionizadon Source 

---

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

Figure 2.2 Schematic view of the electron ionization source, a, Ion repeller h, anode c, acceleration and focalization plates N, S, magnet poles...

Electron interference devices, 22 169 Electron ionization source, 15 652-653 Electron lenses, 24 78 Electron microprobe analysis (EMA), 24 78, 109... 

The mass spectrometer, when combined with a computer data system, precisely identifies and quantifies each sample component as it elutes from the gas chromatography. The model 1020 uses an electron ionization source... 

Figure 2.32 Principles of electron ionization source. Cathode (W, Re) e - emission e - energy is a function of accelerating voltage pressure (p) of gaseous sample <10 Pa. (H. Kienitz (ed.), Massenspek-trometrie (1968), Verlag Chemie, Weinheim. Reproduced by permisssion of Wiley-VCH.)...

Figure 2.34 Schematic of an electron ionization source with Knudsen cell.

Electron ionization source by e" ionization ne 1015 cm-3 M+ high < 1( 4 Pa single-focusing MS stable isotope ratios thermodynamic data... 

The experiments of Kistiakowsky and Kydd [1] were done by single-pulse photolysis with a 500-J flashlamp, the reaction vessel contents being sampled via a pinhole leak into the electron ionization source of a Bendix time-of-flight (TOF) mass spectrometer. Mass spectra were obtained by pulsed extraction of ions from the ion source at 50-fis intervals after the flash. The signal from the electron multiplier detector was displayed on a cathode ray tube, which was photographed with a rotating drum camera. 

A J. DEMPSTER develops the electron ionization source and the first spectrometer with a sector-shaped magnet (180°) with direction focusing [17]. 

We saw that numerous ionization techniques exist that yield radical cations or radical anions, protonated or deprotonated molecules, and various adducts. These ions yield fragments with an even number of electrons (closed shell) or with an odd number of electrons (open shell). Even though the radical cations derived from electron ionization sources retain a privileged status in common mass spectrometry, the other ionization methods become increasingly common. Electron ionization is not possible for many categories of molecules. Therefore, we will not limit the discussion to radical cations. 

Consider an ion with z = 1. Calculate the kinetic energy added to the ion by acceleration through a potential of 1000 V in an electron ionization source. Does the kinetic energy acquired depend upon the mass of the ion Explain. In an electron ionization source, does the mass of the ion affect its velocity Explain. 

Figure 10.33 Determination of less than 10 ppb of a mixture of explosives spiked into a dirty matrix by negative ion MS using a JEOL TEEMmate with a highly selective tunable energy electron ionization source. [Courtesy of JEOL USA, Inc., Peabody, MA (www.jeol.com).]...

Temperature control factored into a low-pressure, variable-temperature (80 to 400 K) IM-MS with an electron ionization source. This instrument with an... 

---