Ionization chamber, electron impact

This instrument [7] measures three types of ions in a sequential mode the positive and negative ions emitted from the surface of the ion emitter, and the neutral species volatilizing from the surface and ionized by electron impact (El). A commercially available quadrupole mass spectrometer equipped with an El source was modified to allot a specially designed thermal emitter to be just barely inserted into the ionization chamber. The chamber is much cooler than the emitter there-... 

The DEMS system requires two turbomolecular pumps. A 200-L/s pump located in the ionization chamber to maintain a vacuum at the 10 " torr level, and a 50-L/s pump in the analyzer section for evacuation to a pressnre below 10 torr. A shatter between the ionization and analyzer compartments preserves the differential pressure. Reaction prodncts that are able to pass throngh the hydrophobic membrane are ionized by electron impact and separated... 

There are two principal components of mass spectrometers the ionization chamber, where ionization of the sample occurs, and the mass analyzer, where ion sorting and detection occur. Mass spectrometer instruments vary in design with regard to both of these components. Thus far we have mentioned only one ionization technique, electron impact (El). In Section 9.18A we discuss El ionization in more detail, as well as discuss two other important ionization methods electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI). 

Chemical ionization. In this case, the sample to be analyzed is ionized by ion-molecnle interactions with reagent ions (RH). Thns, when the sample is introduced in the ionization chamber with a large excess of a reagent gas, the reagent molecules, ionized by electron impact, react with other reagentmolecules to form reactant ions RH+, which protonate the sample [2,5,7, 8] ... 

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-... 

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

There are several other methods which have been used in the experimental determination of electron impact ionization cross sections. Nottingham and Bell76,77 developed a method specifically for the purpose of accurately determining the absolute electron impact ionization cross section of mercury. A semicircular electron velocity analyzer included in their design ensured that very high energy resolution was possible since only electrons of the required velocity emerged from the analyzer into the ionization chamber. Other aspects of the experiment are similar to the condenser plate method. 

The most common conventional gas source is an electron impact (El) source. This consists of a metal chamber with a volume of a few cm3, through which the sample flows in the form of a gas. Electrons produced by thermionic emission from a heated tungsten filament are passed through this gas, and accelerated by a relatively low voltage ( 100eV), causing ionization within the sample gas. A plate inside the chamber carries a low positive potential (the repeller ) which ejects the positive ions into a region which contains a series of plates (called lenses) and slits, which serve to focus, collimate, and accelerate the ion beam into the next part of the system... 

Figure 8.1 Schematic diagram of electron impact (El) source for mass spectrometry. The sample enters the evacuated chamber as a gas and is intersected by a beam of electrons released from the heated cathode and accelerated towards the positive anode at the top. The impact of the electrons atomizes and ionizes the sample, and the resulting positive ions are attracted towards the annular cathode on the right, passing through it and out of the source towards the mass selection device.

In order to measure the absorption of the beam of rays coming from the mercury vapor an ionization chamber with a thin mica window in it and containing methyliodide was set up opposite the window, F, and lead plates with holes in them were placed in the line of the beam so that only the radiation coming from the impacts of the electrons against the mercury entered the chamber with sufficient intensity to be detected. That this was the case in the actual experiments is indicated by the fact that no perceptible ionization current could be observed when the mercury pump was not running. A quadrant electrometer measured the ionization current. 

Figure 18. Illustration of the molecular beam-electron beam crossing region used in the electron impact ionization experiments. The molecular beam source, buffer, and hexapole chambers are similar to those shown in Figure 3.

The first conventional mode of MS involves El ionization, in which the neutral flavonoid is impacted in the gas phase with an electron beam of 70 to 100 eV. Resulting mass spectra of the flavonoid aglycones are characterized by molecular ion peaks (M ), and fragment ions from both the A and B rings. The use of a reactant gas in the ionization chamber. Cl, normally results in the production of a more abundant molecular ion and simpler fragmentation patterns. General information about mass spectra of flavonoids recorded by these methods has been published by several authors. More specific mass spectra analyses... 

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. 

Electron impact ionization is the method of choice for the generation of molecular ions to be analyzed in a mass spectrometer [55, 56]. This method is based on essentially the same principle as radiolytical generation, but it is used at considerably lower pressures, typically 10 6 torr. High energy electrons, tuned to energies between 10 and 70 eV, are ejected from a heated filament and impact on molecules contained in an evacuated ionization chamber. In the collision, energy is transferred to the substrate causing ionization. 

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