FAB Probes

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 

FAB target usually is a stainless steel or copper tip that exposes the analyte at some angle (30-60°) to the fast atom beam The target can have a plane or more specially cup-shaped surface to hold a 1-3 pi drop of matrix/analyte mixture (Fig. 9.3). Normally, the target is maintained at ion source temperature, i.e., only slightly above ambient temperature. Heating or - more important - cooling can be provided with special FAB probes only (Chap. 9.4.6). 

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A dynamic-FAB probe having a simple copper target. The narrow fused-silica tube passes through the shaft, its end lying flush with the target surface. 

A dynamic-FAB probe tip incorporating a screen and wick assembly at the target surface. 

It has been previously noted (see Section 4.2 above) that use of the moving-belt interface allows El spectra to be obtained from compounds that do not yield spectra when analysis is attempted using a conventional El probe. The same is true when the dynamic-EAB probe is used in that spectra may be obtained from compounds that do not yield spectra when a static-FAB probe is used. This has been attributed to the presence of the mobile phase. 

The pressure difference between the source of the mass spectrometer and the laboratory environment may be used to draw a solution, containing analyte and matrix material, through the probe via a piece of capillary tubing. When an adequate spectrum of the first analyte has been obtained, the capillary is simply placed in a reservoir containing another analyte (and matrix material) and the process repeated. This may therefore be used as a more convenient alternative to the conventional static FAB probe and this mode of operation may also benefit from the reduction in suppression effects if the analyte is one component of a mixture. 

Since only a small amount of matrix is required in dynamic FAB, when compared to conventional FAB, less chemical background is observed and, because of the stability of the signal, it is convenient to use computer enhancement to generate good quality mass spectra. For this reason, the dynamic-FAB probe is often used as a convenient alternative to the conventional FAB probe. 

The use of the dynamic-FAB probe (see Section 4.4 above) has allowed the successful coupling of HPLC to this ionization technique but there is an upper limit, of around 5000 Da, to the mass of molecules which may be successfully ionized. Problem solving, therefore, often involves the use of chemical methods, such as enzymatic hydrolysis, to produce molecules of a size more appropriate for ionization, before applying techniques such as peptide mapping (see Section 5.3 below). 

Fig. 9.3. FAB probe of a JEOL JMS-700 magnetic sector instrument (left). The probe tip with a drop of glycerol placed onto the exchangeable stainless steel FAB target (right).

Cooled FAB probes have been designed to prolong the acquisition time for FAB measurements with more volatile matrices. [117] Research on sputtering processes from solid gases has contributed to FAB at cryogenic temperatures. [118,119]... 

The most commonly used FAB interface in LC/MS is known as continuous-flow fast-atom bombardment (CF-FAB) ionization, in which the fast atoms or ions are directed at a target along which the LC eluent flowsd In a CF-FAB, the LC eluent, mixed with a FAB matrix such as 5% aqueous glycerol, is continuously introduced and deposited on the tip of a FAB probe. The maximum flow rate is in the range of 5 to 15 pL/min. A comprehensive review of the principles and application of CF-FAB for LC/MS has been written by Caprioli. ... 

A coaxial CF-FAB interface was applied to the coupling of CE with tandem MS. A pair of coaxial fused silica capillary columns were used to deliver, independently, the microcolumn effluent and the FAB matrix directly to the FAB probe tip face. The advantages of the system are that the composition and flow rates of the two liquid streams can be independently optimized, the FAB matrix does not affect the microcolumn separation process, and peak broadening is minimized because the two streams do not mix until they reach the tip of the FAB probe, where ion desorption occurs. 

Diagram of a continuous flow FAB coupling system. The HPLC capillary column made of fused silica enters through the nozzle of an introduction FAB probe. The solvent evaporates and the glycerol remains as a FAB matrix. 

A flat copper FAB target was used in these first experiments, while later a variety of FAB probe tips were described, differing in design, e.g., flat, hemispherical, or conical, or flat with a drain channel, and/or material, e.g., brass, copper, stainless-steel, gold, stainless-steel with a gold-plated channel. 

Application of FAB-MS to the identification and quantification of additives contained in polymer samples has been reported. In fact, the amount of phthalate contained in plasticized PVC could be determined by analyzing directly on the FAB probe the solid pieces of PVC suspended in the glycerol liquid matrix. ... 

The continuous-flow (CF)-FAB probe, discussed in Section 2.11, has also achieved some success as an LC/MS interface for the analysis of nonvolatile and... 

Continuous-Flow Fast Atom Bombardment Interface Although currently not a popular approach, the coupling of CE mass spectrometry was once achieved via a CF-FAB probe. Makeup flow is required in this coupling because of the mismatch of the low flow rates of the CE solution with the liquid flow rates of stable CF-FAB operation. The sheath-flow and liquid-junction designs discussed above have been used successfully for this purpose [69-71], The sheath-flow design has the advantages that the composition and the flow rates of the CE effluents and of the FAB matrix solution can be optimized independently, and that the separation efficiency is higher. 

On-Probe Oxidation In this procedure, thiol- and disulfide-containing peptides are treated with performic acid on the FAB probe to convert each cysteine residue to cysteic acid a concomitant increase of 98 Da in the mass of a peptide that contains an intramolecular disulfide bond results [9] ... 

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