Fast atom bombardment mass background

A study of the fast atom bombardment mass spectra and low-energy collision activation tandem mass spectra of a series of monoquatemary and bisquatemary bisbenzylisoquinoline Thalictrum alkaloids was undertaken [143]. It was determined that the relative molecular mass of the free base alkaloid and its bisquatemary salt can be obtained from FAB data, but that the monoquatemary ammonium salt derivatives produce only a (M-X)+ ion, thus precluding the determination of the relative molecular mass. For example, the bisquatemary iodide ammonium salts produced ions (M-I)+, (M-I-HI)+, and (M-I-CH3I)+. The relative molecular mass of the counter anion may be determined with facility, thereby affording the relative molecular mass of the alkaloid. However, with a monoquatemary ammonium salt such as a monochloride, only a (M-X)+ ion is produced, and there is insufficient mass spectral data to determine its counter anion from positive-ion FAB. Negative-ion FAB was unable to be employed to determine absolutely the counter anion of the alkaloid, since chloride anions occur as common background anions. 

Figure F2.4.2 Positive ion fast atom bombardment (FAB-MS) mass spectrum of phytofluene isolated from blueberries. The base peak of mlz (mass-to-charge ratio) 542 corresponds to the molecular ion. Characteristic of FAB-MS, background signals are observed at every mlz value. The mass spectrum was obtained during continuous-flow FAB-MS LC/MS using a magnetic sector mass spectrometer. Although the 16-c/s isomer of phytofluene is shown, the FAB mass spectra of the all- trans and other cis isomers are indistinguishable.

See also Fast Atom Bombardment Ionization in Mass Spectrometry Proton Microprobe (Method and Background) Time of Flight Mass Spectrometers. 

Fast-atom bombardment (FAB). Introduced by Barber et al. (1982), FAB is particularly suitable for polar molecules, providing ionized molecular species for molecular weights up to 20 kDa (Williams et al. 1987 Tower 1989). A solution of the sample in a low-volatility matrix such as glycerol is bombarded with fast, heavy atoms (e.g., Xe ions such as Cs" give similar results) in the ion source, producing the continuous ion beam necessary for scanning instruments. The method is very convenient for appropriate samples, but matrix reactions produce background ions at nearly every mass value. 

In FAB, the sample is usually dispersed in a non-volatile liquid matrix, such as glycerol or diethanolamine, and deposited at the end of a sample probe that can be inserted into the ion source. The sample on the probe is ionised when bombarded by the fast atom beam. However, ionisation of the matrix also occurs, leading to a very large background signal. The technique is thus limited for the analysis of small molecules. Fast-moving ions (Cs+ or Ar+) can be used instead of fast-moving atoms, which is the basis of a technique called liquid secondary ion mass spectrometry (LSIMS). 

A modification of the FAB technique is continuous flow FAB (CFFAB). In this approach, the sample in solution is introduced into the mass spectrometer through a fused silica capillary. The tip of the capillary is the target. The solution is bombarded by fast atoms produced as described earlier. Solvent is flowing continuously and the liquid sample is introduced by continuous flow injection (Fig. 9.13). The mass spectrum produced has the same characteristics as that from conventional FAB, but with low background. Typically, the solvent used is 95% water and 5% glycerol. The ability to inject aqueous samples is an enormous advantage in biological and environmental studies. 

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