Electron Bombardment Plasma Sources

Electron Bombardment Plasma Sources. These gas-feed sources generally employ Ar or Xe at relatively low vapour pressures (10 3mbar). A heated cathode is a common electron source, and these are accelerated towards an anode to give them 

Inert gas beams allow the chemistry of a surface to be studied by SIMS without modification by the bombarding species. The achievable values of source brightness allow pA currents into spot diameters of approximately 50 pm for dynamic SIMS, or nA currents into spot diameters 5 pm for imaging SIMS. For greater spatial resolution a different, higher-brightness source must be used. 

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More recently, ion sources using desorption ionization have been developed where vaporization and ionization occur essentially at the same time. This approach is required for nonvolatile, polar, thermally labile, and higher molecular weight molecules. These methods include bombardment, rapid heating, spray and electron tunneling. Plasma desorption (PD), static secondary ion (SSIMS) and fast atom bombardment (FAB) are the bombardment methods. Laser desorption (LD) and matrix-assisted LD/I (MALDl) methods are used for rapid heating. There are two spray techniques thermospray (TSP) and electrospray (ESP). The electron tunneling method involves field desorption (FD). 

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. 

There are several types of ionization sources [MALDI, ESI, FAB (fast atom bombardment), PD (Cf-252 plasma desorption), El (electron ionization), Cl (chemical ionization) etc.], different types of mass analyzers [combinations of magnetic and electric sectors, quadrupolar filters (Q) and ion traps (IT), time-of-flight (TOF) and FT-ICR] and different detectors, each with its own advantages and drawbacks. We describe herein only the systems that presently have widespread use for the study of biomolecules ESI coupled to a quadrupole (or triple quadrupole, QqQ) mass analyzer or an ion trap, the MALDI source with the linear or reflectron TOF analyzer, and the FT-ICR system which can be equipped with both ESI and MALDI sources. 

The principles of ion sources which use a primary ion beam for sputtering of solid material on sample surface in a high vacuum ion source of a secondary ion mass spectrometer or a sputtered neutral mass spectrometer are shown in Figure 2.30a and Figure 2.30b, respectively. Whereas in SIMS the positive or negative secondary ions formed after primary ion bombardment are analyzed, in SNMS the secondary sputtered ions are suppressed by a repeUer voltage and the sputtered neutrals which are post-ionized either in an argon plasma ( plasma SNMS ), by electron impact ionization ( e-beam SNMS ) or laser post-ionization are nsed for the surface analysis (for details of the ionization mechanisms see references 122-124). 

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