FAB Ion Sources

A FAB ion source basically is an El ion source (Chap. 5.2.1) modified to give free access to the fast atom beam. The electron-emitting filament and the ion source heaters are switched off during FAB operation (Fig. 10.1). The FAB gas is introduced via a needle valve into the lower part of the FAB gun mounted above the ion source. From there, it effuses into the ionization chamber of the FAB gun and into the ion source housing. The saddle field gun [27] is the most common type of 

---

On leaving the chromatographic column, the liquid flow passes along a narrow tube, into the FAB ion source, and then into the target zone of the fast atoms. 

FD-MS by itself provides only limited chemical information. Lattimer et al. [92] have also compared the analysis of extracted rubber vulcanisates by means of FD-MS and FAB-MS, using the aforementioned EI/FD/FT/FAB ion source. The systems investigated were neoprene/DOPPD, EPDM/(DOP, PBNA, paraffin wax), neoprene-SBR blend/(DOP, DOPPD, TDBHI). Certain compounds were observed by FD but not by FAB (wax, oil, isocyanurate antioxidant TDBHI). In FAB conditions some polymer additives suppress... 

The analyte, either solid or admixed to some liquid matrix, is introduced into the FAB ion source by means of a probe bearing a sample holder or FAB target. The... 

In standard FAB, the surface of the matrix solution is depleted of analyte and suffers from radiational damage during elongated measurements. Refreshment of the surface proceeds by diffusion (limited by the viscosity of the matrix) or evaporation. Continuous-flow fast atom bombardment (CF-FAB) continuously refreshes the surface exposed to the atom beam. [107,108] The same effect is obtained in slightly different way by the frit-fast atom bombardment (frit-FAB) technique. [109,110] In addition, both CF-FAB and frit-FAB can be used for online-coupling of liquid chromatography (LC, Chap. 12) [111] or capillary electrophoresis (CE) to a FAB ion source. [112]... 

MAT 731 double-focussing mass spectrometer, a combined EI/FI/FD/FAB ion source and an AMD Intectra direct introduction system, has been previously described in detail (12). The samples were heated linearly from 50 C to 750 C at a rate of 100 K/min. In general, 35 FI mass spectra were recorded in the m/z 50-900 mass range. 

Since a monolayer of material is completely sputtered in a matter of seconds in a typical FAB ion source, it is essential that the sample surface be continuously regenerated during prolonged examination. This is done naturally by diffusion of the sample to the surface of the solution. It is therefore essential that the sample have some solubility in the low-volatility solvent, to provide the diffusion mechanism and also to act as a reservoir of material. Ionic groups that render compounds involatile, thus ruling out conventional methods of ionization, are also those groups that frequently lead to solubility in polar solvents, and to the associated surfactant properties that facilitate good sample preparation for FAB ionization. It follows that the detection of solvent substrate peaks in a FAB mass spectrum implies that optimal sample preparation has not been achieved. 

The complex solution and surface chemistry involved in a typical sample can lead to large apparent differential sensitivities between quite similar compounds. Nevertheless, the FAB ion source has proven to be a valuable method for the recording of mass spectra of compounds that were previously considered to be intractable by mass spectrometry because they were too ionic and too involatile. Without doubt the californium-252 source is the leader in terms of bombardment technique with respect to high molecular weight determination and this situation is likely to persist if we take into account the large amounts of energy available in this sputter process. 

The main advantage of the FAB technique compared to ESI and MALDI is, in fact, not the ionization method itself but the type of mass analyzer it works with. FAB ion sources are usually attached to magnetic sector, double-focusing mass spectrometers, enabling accurate mass measurements and, which is even more important, allowing the study of high-energy collision-induced dissociation (CID) reactions which provide information about the structure of the studied ion (see next section). 

Figure 5.13 Sketch of a fast atom bombardment (FAB) ion source. The potential difference between the probe tip and extraction grid (not shown) accelerates the ions (from both the Uquid matrix and analyte) into the mass spectrometer. Since a large quantity of neutral matrix is also sputtered by the fast primary beam, the source region must be provided with adequate pumping and the m/z analyzer region differentially pumped. The fast atom beam can be replaced by a beam of fast primary ions (often Cs ) and the technique is then sometimes referred to as Uquid assisted secondary ion mass spectrometry (LSIMS).

In the 1980s, attempts were made to enable continuous introduction of liquid samples (especially aqueous buffer solutions) to ion sources of mass spectrometers. An early con-tiuous flow interface was based on the fast atom bombardment (FAB) ion source [70]. However, it was the ESI interface that greatly facilitated temporal profiling of dynamic... 

Detector MS, JMS DX303 double focusing, xenon FAB ion source, gun current 10 mA, voltage 3 kV, positive ion mode... 

---