Electron-impact source

Using tandem MS (DFS with EI/FPFD source), electron impact and collision activation mass spectra of a THF extract of an orthopaedic polymer bandage identified IV-isopropyl-A/ -phcnyl-p-phenylenediamine (IPPD, m/z 226) as a cause for contact dermatitis [232]. Fl-MS of the extract of surgeons gloves indicated thio-bis (t-butylcresol) (m/z 358 343, after CID). 

C,HJ (Benzene) sf6- >0 Unspecified Photoionization -mass spectrometry Single-source electron-impact ionization-mass spectrometry CeH (QHj —)C,2H SF6 (XFVSFJXFj- X-Se.Te,U SF6-(TeF6 SF6,F)TeF5- The ion SF - produced by thermal electron capture of SF in a vibrational degree of excitation equal to electron affinity of the neutral 116a... 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

FIGURE 20-2 Mass spectrum of 1 -decanol from (a) a hard ionization source (electron impact) and (b) a soft ionization source (chemical ionization),... 

There is no doubt that many devices are superior to the single-source, electron-impact mass spectrometer for physiochemical measurements. In fact, the instrument has been declared obsolete for at least fifteen years. Yet, for all this, it continues to find extensive use and probably will continue to do so for many years. 

Ionization source Electron impact (El) ionization is employed using a standard El filament assembly from a Finnigan ITS-40 ion trap instrument. The entire circuit, including power unit and test points, is enclosed in a 5 x 5 x 1 cm volume. The electron flow into the CIT is gated during ion cooling and mass analysis. 

The number of ions that are formed in the different metastable states depends, of course, on the conditions in the ion source. Comparison of a few results from different labora )ries indicates that if the ions are produced in an ordinary ion source (electron impact with l(X)-eV electrons), the abundances of the metastable states compared with the ground state do not vary much. Therefore, in Table I, the relative abundances are given for production of ions from the indicated compounds. These values must be considered as very uncertain, as indirect methods involving several assumptions have been used to determine the values. The inclusion of these values in Table I implies an attempt to fulfil the need for information on excited states that has been expressed repeatedly during recent years. ... 

Thermal ionization Spark source Electron impact (El) Photoionization (PI)... 

I The ionization sources electron impact (El), chemical ionization (Cl), field desorption (FD), and field ionization (FI). 

Figure Bl.10.7. Electron impact ionization coincidence experiment. The experiment consists of a source of incident electrons, a target gas sample and two electron detectors, one for the scattered electron, the other for the ejected electron. The detectors are coimected tlirough preamplifiers to the inputs (start and stop) of a time-to-amplitiide converter (TAC). The output of the TAC goes to a pulse-height-analyser (PHA) and then to a nuiltichaimel analyser (MCA) or computer.

Figure Bl.10.11. Electron impact double ionization triple coincidence experiment. Shown are the source of electrons, target gas, tluee electron detectors, one for the scattered electron and one for each of the ejected...

The cross-section for electron impact ionization has already been mentioned in Sect. 2.2.2.2 in connection with electron sources, and a variety of experimental and theoretical cross-sections have been shown in Fig. 2.18 for the particular case of the K-shell of nickel. The expression for the cross-section derived by Casnati et al. [2.128] gives reasonably good agreement with experiment the earlier expression of Gry-zinski [2.131] is also useful. 

Several ion sources are particularly suited for SSIMS. The first produces positive noble gas ions (usually argon) either by electron impact (El) or in a plasma created by a discharge (see Fig. 3.18 in Sect. 3.2.2.). The ions are then extracted from the source region, accelerated to the chosen energy, and focused in an electrostatic ion-optical column. More recently it has been shown that the use of primary polyatomic ions, e. g. SF5, created in FI sources, can enhance the molecular secondary ion yield by several magnitudes [3.4, 3.5]. 

Electron impact (El) ion sources are the simplest type. O2, Ar, or another (most often noble) gas flows through an ionization region similar to that depicted in Eig. 3.30. Electrons from an incandescent filament are accelerated to several tens of eV by means of a grid anode. A 20-100 eV electron impact on a gas atom or molecule typically effects its ionization. An extraction cathode accelerates the ions towards electrostatic focusing lenses and scanning electrodes. 

Typical ion sources employ a noble gas (usually Ar). The ionization process works either by electron impact or within a plasma created by a discharge the ions are then extracted from the region in which they are created. The ions are then accelerated and focused with two or more electrostatic lenses. These ion guns are normally operated to produce ions of 0.5-10 keV energy at currents between 1 and 10 pA (or, for a duoplasmatron, up to 20 pA). The chosen spot size varies between 100 pm and 5 mm in diameter. 

Experimental. The mass spectra in Figures 1-8 are positive-ion spectra produced by electron impact and were obtained from a single-focusing, magnetic deflection Atlas CH4 Mass Spectrometer. The ionizing potential was 70 e.v. and the ionizing current 18/a a. An enamel reservoir heated to 120°C. was used from which the sample was leaked into the ion source. 

Combined Electron Impact and Field Emission Source. ... 

Fig 4 is a diagram of an electron impact 1 source. The sample vapor is admitted into the ion source thru the slit in the back of the chamber and it passes thru a collimated electron beam b . On impact of electrons with the neutral molecules, positive ions (to a small extent negative ions) are produced. A small positive potential ( repeller potential ) between the back wall V of the ion source and first accelerator plate d , expels tile positive ions toward the accelerating region and at the same time attracts the negative ions which are then discharged at repeller plate c . The positive ions are accelerated by the potential difference applied to plates d and e , pass thru the exit slit T and continue toward the collector... 

Fig 7 Combined electron impact and field emission 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... 

Some of the problems encountered in the mass spectrometric study of ion-molecule reactions are illustrated in a review of the H2-He system (25). If the spectrometer ion source is used as a reaction chamber, a mixture of H2 and He are subjected to electron impact ionization, and both H2+ and He+ are potential reactant ions. The initial problem is iden-... 

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