Xenon atoms, ionization

Ionized xenon atoms are accelerated to the required potential (2 to 10 kV). Next, the fast-moving Xe+ ions are neutralized in a dense cloud of excess neutral gas atoms to generate a continuous stream of high-translational-energy xenon atoms. Any residual undischarged ions are deflected with a positive potential onto a deflector plate. 

The compound of interest is dissolved in a high-boiling viscous solvent such as glycerol a drop is placed on a thin metal sheet, and the compound is ionized by the high-energy beam of xenon atoms (Xe). Ionization by translational energy minimizes the amount of vibrational excitation, and this results in less destruction of the ionized molecules. The polar solvent promotes ionization and allows diffusion of fresh sample to the surface. Thus ions are produced over a period of 20-30 min, in contrast to a few seconds for ions produced from solid samples. 

In fast-atom bombardment (FAB), first developed by Barber et al. [46], the analyte is dissolved in a liquid, viscous matrix, typically mixtures of water with 5-90% glycerol, 3-nitrobenzylalcohol, thioglycerol, dithiothreitol/dithioerythritol, or triethanolamine. The analyte-matrix mixture is bombarded at the probe tip with a beam of fast atoms or ions, such as 5-8 kV xenon atoms or 10-40 kV cesium ions formed in a gun. The beam hits the analyte-matrix mixture, inducing a shock wave which desorbs ions and molecules from the uppermost layer of the solution. Ionization of the analytes occurs either in the matrix or in the vapor phase above the matrix, usually by proton transfer reactions between analyte and matrix ions. Subsequently, the gas... 

This procedure provides a model of the xenon atom which accounts only for the manifold of singly excited states based on the lowest ionic core, P3/2- For all rare gases, a second manifold of states converges to the next spin-orbit component of the ion, the Pi/2 state. For example, these two ionization limits in xenon are separated by 1.3 eV corresponding to different total angular momenta, J, of the 5p configuration. The lower ionization potential is 12.15 eV. We assume that multiphoton excitations into these two manifolds are very weakly coupled so they can be treated separately. This assumption is reasonable because once one of the electrons is excited outside a particular core configuration, transitions... 

Other Methods of Ionization. There are several other methods for ionization in addition to ESI and MALDI. However, most of them are not commonly used in proteomics. Some of these include chemical ionization, electron ionization, fast atom bombardment (FAB), and many others. Most of these lead to disintegration or fragmentation of analyte molecules and are not commonly used in proteomics. However, FAB has some application in the analysis of proteins and peptides, because this is a soft ionization procedure and does not cause the fragmentation of molecules under analysis. In the FAB method, a nonvolatile matrix such as m-nitrobenzyle alcohol is used to hold the analyte molecules. Analyte molecules are vaporized and ionized by bombardment with the high-energy beam of xenon or cesium from a probe inserted directly into the device containing the sample. Ionized molecules thus obtained are then subjected to separation by the mass... 

The classicaf FAB ionization technique consists in the bombardment of the sample dissolved in a nonvolatile, viscous, and polar liquid (the matrix) with neutral xenon atoms. However, a much more popular version is Liquid-matrix Secondary-ion Mass Spectrometry (LSIMS) in which the sample solution is bombarded with heavy ions, usually Cs+ or Xe+. Basic information about FAB and LSIMS can be found in mass spectrometry monographs [3, 4]. Comprehensive reviews on the application of FAB for analyzing biomolecules [5-7] and, specifically, saccharides and glycoconjugates [8] are also available. 

Here the source of primary beam is xenon atoms, which because of higher mass and momentum provides better sensitivity than that of the beam of argon atoms used in the original discovery. The xenon atoms are first ionized in the FAB gun by collisions with electrons that are moving in a saddle-field configuration. 

P, Agostini, F. Fabre, G. Mainfray, G. Petite, N. Rahman Free-free transitions following she-photon ionization of xenon atoms. Phys, Rev. Lett. 42, 1127 (1979)... 

The data for liquid xenon were obtained in a coaxial test cell consisting of a thin wire as anode (2.0 to 5.0 pm diameter) and an outer cylinder (8 mm diameter) as cathode. For the production of the initial electrons, a radioactive ° Hg source was used. It emits quanta of 85 and 279 keV energy. When a 279-keV quant interacts with a xenon atom it produces an energetic photoelectron which dissipates its energy in approximately 12,500 ionization events. These ionization events are distributed around the point of interaction in a radius of 240 pm. It can be assumed that each photoelectron produces a sphere filled with electrons (and positive ions) at some distance r from the wire. Neglecting for the moment recombination and attachment, these electrons drift toward the anode wire and at some distance their mean energy is sufficient for collisional ionization. At that point they drift with the saturation velocity of 3 x lO cm/s. The number of electrons formed up to a distance r is obtained from integration of Equation 1 as... 

It occurred to Bartlett that the ionization energies of the oxygen molecule (1180 kj/mol) and the xenon atom (1167 kj/mol) were remarkably similar. He decided to try the same reaction as above with xenon replacing the diatomic oxygen. He prepared known volumes of xenon (in slight excess) and platinum hexafluoride and carefully noted the pressure of each. When he allowed the two gases to mix. 

---