Fast atom bombardment matrix spectra

The low-resolution mass spectrum of Indinavir sulfate was measured using fast atom bombardment mass spectrometry on a JEOL HXl lOA mass spectrometer set at a resolution of 5000. The sample was ionized from a 5 1 dithiothreitol dithioerythritol matrix using xenon as the FAB gas. The low-resolution mass spectrum [12] ofindinavir is shown in Figure 14, while the structures of the structurally significant fragment ions are illustrated in Figure 15. 

Figure F4.5.1 Positive ion fast atom bombardment (FAB) with collision-induced dissociation (CID) tandem mass spectrum of pyropheophytin a isolated from spinach leaves. The FAB matrix was 3-nitrobenzyl alcohol.

Several non-volatile ionic complexes of As and Sb have been characterized by fast atom bombardment mass spectrometry (Table 12). The mass spectrum of 53 (a mixture of 3-nitrobenzyl alcohol with CH2CI2 was used as the matrix) exhibits (M -I- BF2), M, (M - 2CO)- -, CpFe(CO)2 +, Cp2Fe+ and Bi-" ions ... 

Other ionization techniques were also utilized in LC/MS such as FAB, which puts the sample in a liquid matrix such as glycerol and uses fast atom bombardment as the source of energy for the ions. This technique practically does not heat the sample, allowing the formation of molecular ions for certain unstable molecules. An example of a FAB spectrum for syringoresinol detected in lignin pyrolysis is given in Figure 5.7.4. 

The molecules hit by the fast atoms are destroyed in the collision and do not appear in the spectrum, whereas the molecules not directly hit (matrix and analyte) acquire enough energy to ionize and desorb from the target surface. Therefore, fast atom bombardment in a liquid matrix should allow desorption of the intact analyte molecule from the condensed phase. 

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.

By passing a continuous flow of solvent (admixed with a matrix material) from an LC column to a target area on the end of a probe tip and then bombarding the target with fast atoms or ions, secondary positive or negative ions are ejected from the surface of the liquid. These ions are then extracted into the analyzer of a mass spectrometer for measurement of a mass spectrum. As mixture components emerge from the LC column, their mass spectra are obtained. 

Barber et al. introduced FAB in 1981. In this technique, bombardment of a liquid target surface by a beam of fast atoms such as xenon or argon, causes the continuous desorption of ions that are characteristic of the liquid. In a typical FAB spectrum, the analyte ion is usually formed as protonated or cationized ions in positive FAB, and deprotonated ions in negative FAB mode. A few fragmented ions may also be formed. The spectrum usually contains peaks from the matrix, such as protonated matrix clusters of glycerol if it is used as the matrix solvent. FAB utilizes a liquid matrix such as glycerol. The matrix is used to enhance sensitivity and ion current stability. 

In order to obtain the mass spectrum of a compound, it must first be ionized. Early methods involved thermal evaporation of samples into the ion source of the mass spectrometer where they were ionized by electron beam bombardment or chemical reactions. Thermal instability, where present, was frequently overcome by derivatization. For large and involatile molecules, desorption techniques using high electric fields or bombardment with fission fragments, fast atoms, fast ions, or laser beams were later developed. The newest techniques, developed in the late 1980s, involve laser ionization of samples imbedded in a solid matrix and evaporation of solutions by electrospray. Both of these techniques are able to ionize large molecules such as proteins with masses of up to several hundred ki-lodaltons. This article describes these techniques in more detail and indicates the type of mass spectrometer that is suitable for analysis of the types of ion that they produce. 

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