Fast atom bombardment mechanism

The treatment of sucrose with anhydrous HF89 results in the formation of a complex mixture of pseudooligo- and poly-saccharides up to dp 14, which were detected by fast-atom-bombardment mass spectrometry (FABMS). Some of the smaller products were isolated and identified by comparison with the known compounds prepared86 88 a-D-Fru/-1,2 2,1 -p-D-Fru/j (1), either free or variously glucosylated, was a major product, and this is in accord with the known stability of the compound. The mechanism of formation of the products in the case of sucrose involves preliminary condensation of two fructose residues. The resultant dianhydride is then glucosylated by glucopyranosyl cation.89 The characterization of this type of compound was an important step because it has permitted an increased understanding of the chemical nature of caramels. 

Electron ionization (El) was the primary ionization source for mass analysis until the 1980s, limiting the chemist to the analysis of small molecules well below the mass range of common bioorganic compounds. This limitation motivated the development of the techniques commonly known as ESI, 1 MALDI, 2 and fast atom bombardment (FAB) 3,4 (Table 1). These ion sources allow for rapid and easy peptide analyses that previously required laborious sample preparation or were not possible with electron ionization. The mechanism of ionization these ion sources employ, which is somewhat responsible for their ability to generate stable molecular ions, is protonation and/or deprotonation. 

Figure 16.18—FAB and MALDI techniques, a) Principle of fast-atom generation using xenon b) formation of a fast-atom beam of argon in a collision chamber and bombardment of the sample (using a FAB gun) c) MALDI. The impact of a photon leads to a similar result as with a fast atom. The mechanism for desorption ionisation is not entirely known. These ionisation modes are particularly well suited for the study of medium to high molecular weight species. They are mostly used in biomedical sciences but not for routine determinations.

Thus as a starting point for understanding the bombardment process we have developed a classical dynamics procedure to model the motion of atomic nuclei. The predictions of the classical model for the observables can be compared to the data from sputtering, spectrometry (SIMS), fast atom bombardment mass spectrometry (FABMS), and plasma desorption mass spectrometry (PDMS) experiments. In the circumstances where there is favorable agreement between the results from the classical model and experimental data It can be concluded that collision cascades are Important. The classical model then can be used to look at the microscopic processes which are not accessible from experiments In order to give us further insight into the ejection mechanisms. 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

Gross and coworkers129 also studied the unimolecular dissociation of protonated acy-lanilines, viz. A-[2-(benzoyloxy)phenyl]benzamides formed via both fast-atom bombardment (FAB) and electrospray ionization (ESI). They found that cyclization occurs upon the loss of a molecule of benzoic acid, and that a similar process occurs for the molecular ion under El. This gas-phase reaction is analogous to a solution reaction leading to phenyl-benzoxazoles. The proposed cyclization process, for which concurrent mechanisms were proposed (Scheme 38 depicts only the displacement reaction route), was corroborated by accurate mass measurements, tandem mass spectrometric experiments with comparison with reference ions, isotopic labeling and theoretical calculations. 

Ganong, B. R., Loomis, C. R., Hannun, Y. A., Bell, R. M. (1986). Specificity and mechanism of protein kinase C activation by sn-l,2-diacylglycerols. Proc. Natl. Acad. Sci. USA 83,1184-1188. Gross, R. W. (1984). High plasmalogen and arachidonic acid content of canine myocardial sarcolemma a fast atom bombardment mass spectroscc ic and gas chromatography-mass spectroscopic characterization. Biochemistry 23,158-165. 

Wong, S.S., Rollgen, F.W, and Przybylski, M., Evidence for a Surface Self-cleaning Sputtering Mechanism in Fast Atom Bombardment Mass Spectrometry, Biomed. Mass Spectrom., 12, 43 1985. 

Figure 3 Collision-induced negative-ion fast-atom bombardment tandem mass spectrum (MS/MS) of GLLEGLLGTLGL(NH2). ZAB 2HF instrument. Glycerol was used as matrix. Other details as for Figure 1. For the mechanisms of the backbone cleavages, see Scheme 3.

Clayton, E. Wakefield, A. J. C. Fast atom bombardment (FAB) mass spectrometry mechanism of ionization. 7. Chem. Soc., Chem. Commun. 1984,15, 969-970. 

The mass spectrometry section (Chapter 8) has been completely revised and expanded in this edition, starting with more detailed discussion of a mass spectrometer s components. All of the common ionization methods are covered, including chemical ionization (Cl), fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDl), and electrospray techniques. Different types of mass analyzers are desalbed as well. Fragmentation in mass spectrometry is discussed in greater detail, and several additional fiagmentation mechanisms for common functional groups are illustrated. Numerous new mass spectra examples are also included. 

Figure 16.20 FAB and MALDI techniques, (a) The principle of fast-atom beam formation with xenon (b) The formation of fast atoms of argon in a collision chamber and subsequent bombardment of the sample by this atom beam, usually of 5-10 kV kinetic energy (c) MALDI or ionization by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. The impact of the photon is comparable with that of a heavy atom. Through a mechanism, as yet not fuUy elucidated, desorption and photoionization of the molecules is produced. These modes of ionization by laser firing are particularly useful for the study of high molecular weight compounds, especially in biochemistry, though not for routine measurements.

It thus appears that a possible and fast mechanism for the production of ozone is by way of oxygen atoms which act as catalysts for the conversion of 02 O3. Because oxygen atoms are essentially slow in destruction of ozone, the limiting stationary process must be the destruction of ozone via the same type of process which is responsible for oxygen destruction—e.g., electron bombardment—or else the increase in temperature of the discharge which would finally provoke the thermal decomposition of ozone and make Reaction 3 a limiting process. 

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