FAB Matrices

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Why might the addition of the FAB matrix to the HPLC mobiie phase have an effect on the separation obtained ... 

Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

FAB-MS has been used for the analysis of lubricant additives, thermally labile or involatile organic compounds, such as macromolecules and dyes, and inorganic compounds. Cationic dyes and dye intermediates, which are typically acid salts, readily yield preformed ions in the FAB matrix solution. They are also very difficult to address by other MS ionisation methods due to their involatility. Lay and Chang [85] used positive ion FAB to characterise a mixture of amine and ketimine cross-linking agents for polymer coatings. Bentz et al. 

Whereas the use of conventional fast atom bombardment (FAB) in the analysis of polymer/additive extracts has been reported (see Section 6.2.4), the need for a glycerol (or other polar) matrix might render FAB-MS analysis of a dissolved polymer/additive system rather unattractive (high chemical background, high level of matrix-, solvent- and polymer-related ions, complicated spectra). Yet, in selected cases the method has proved quite successful. Lay and Miller [53] have developed an alternative method to the use of sample extraction, cleanup, followed by GC in the quantitative analysis of PVC/DEHP with plasticiser levels as typically found in consumer products (ca. 30 %). The method relied on addition of the internal standard didecylphthalate (DDP) to a THF solution of the PVC sample with FAB-MS quantitation based on the relative signal levels of the [MH]+ ions of DEHP and DDP obtained from full-scan spectra, and on the use of a calibration curve (intensity ratio m/z 391/447 vs. mg DEHP/mg DDP). No FAB-matrix was added. No ions associated with the bulk of the PVC polymer were observed. It was... 

Fig. 11.6. Diagram depicting desorption ionization (MALDI, FAB or SIMS). The operating principles of the three techniques are similar. The initiating event is exposure of the analyte to a beam of photons, atoms or ions. In order to prevent damage to the fragile analyte molecules and enhance the conversion of the involatile molecules into gas-phase ions, a matrix is employed. For MALDI, the matrix compounds are UV absorbing compounds such as hydroxycinnamic acid. The most commonly used FAB matrix was glycerol and ammonium chloride was employed by some investigators in SIMS experiments (although at low ion beam fluxes molecular species could be effectively ionized for many analytes with minimal evidence of damage by the primary ion beam).

FAB matrix spectra are generally characterized by a series of matrix (Ma) cluster ions accompanied by some more abundant fragment ions in the lower m/z range. In positive-ion FAB, [Ma +H] cluster ions predominate, while [Ma -H] cluster ions are preferably formed in negative-ion FAB (Fig. 9.7). The principal ion series... 

The role of the matrix in MALDI is analogous to that in FAB (Chap. 9.3.1). Different from FAB, MALDI matrices are generally crystalline solids of low vapor pressure in order not to be volatalized in the ion source vacuum. While basically any liquid can serve as a FAB matrix, the matrix in MALDI has to absorb light of the wavelength which is intended to be used for the experiment. [63] In UV-... 

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. ... 

General. NMR spectra were obtained on a Bruker WM-250 instrument with tetramethylsilane (TMS) as internal standard. UV-visible spectra were taken on a Varian Cary 219. Fast-atom bombardment (FAB) mass spectrometry (MS) analysis were completed on a Vacuum Generators ZAB-2F double-focusing or a Varian MAT CH5 mass spectrometer equipped with an Ion Tech FAB Gun. Solvents for FAB matrix were made up of thioglycerol, dithiothreitol, and dithioerythretol (2 1 1) addition of 0.1M trifluoroacetic acid to the matrix facilitated the ionization of the porphyrins during FAB analysis.(13)... 

The FAB matrix is essentially a nonvolatile liquid material, such as those illustrated in Scheme 1, that serves to constantly replenish the surface with new sample as the incident ion beam bombards the surface. The matrix also serves to minimize sample damage from the high-energy particle beam by absorbing most of the incident energy and is believed to facilitate the ionization process. The spectrum produced often includes matrix peaks along with some fragments and a peak for the protonated or cationized (i.e., M + Na+) molecular ion. 

Ion genera lion can be achieved in a number of ways electron impact (Eh ionization, chemical ionization (CI). fas I atom bombardment (FAB), matrix assisted taser desorption ionization (MAI.DI), eleclrospray ionization (ESI) and atmospheric pressure chemical ionization (APC I are the most common methods,... 

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.

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. 

Strege summarized the technique of high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS) in dereplication of natural products. In contrast to earlier electron impact ionization (El), ESI technique is applicable to virtually any ion in solution with a soft ionization process. A comparison of ESI with fast atom bombardment (FAB), matrix assisted laser desorption ionization (MALDI), atmospheric pressure chemical ionization (APCI) and other techniques demonstrates its superior sensitivity, compatibility and reliability when coupled with HPLC [51]. 

The study of various doping materials in a FAB matrix can, in fact, lead to the determination of chemical equilibrium constants as demonstrated by Caprioli (31) who was able to measure the pKa values of a series of acids. 

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. 

Two approaches based on fast-atom bombardment (FAB) and introduced almost simultaneously were soon after their first description in 1985 and 1986 commercialized, i.e., the frit-FAB [80] and the continuous-flow Cf-FAB [81]. Both systems are used to introduce part of the column effluent (typically 1-10 pl/min) into a FAB somce. In the frit-FAB system, a capillary transfers the effluent to a stainless-steel or PFTE frit used as a FAB target, while in Cf-FAB system the effluent flows in a thin uniform film over the FAB target. A suitable FAB matrix, e.g., glycerol, should be added to the mobile phase. Analyte molecules are directly desorbed and ionized from the hqtrid film by FAB. These approaches are discussed in Ch. 4.6. 

In a continuous-flow fast-atom bombardment (Cf-FAB) interface, typically a 5-15 pl/min liquid stream, mixed with 5% glycerol as FAB matrix, flows through a narrow-bore fused-sihca capillary towards either a stainless-steel frit or a (gold-plated) FAB target. At the target or frit, a uniform liquid film is formed due to a subtle balance between solvent evaporation and solvent dehvery. Ions are generated by bombardment of the liquid film by fast atoms or ions, common to FAB. The Cf-FAB interface for LC-MS have been reviewed [37-38]. 

In a comparison of thermospray with FAB (14), ammonium acetate was used in the FAB matrix as well as tlie thermospray buffer. Its presence significantly alters the cation FAB spectra... 

Mass spectroscopy [electron ionisation (El), chemical ionisation (Cl), electrospray ionisation (ESI), fast atom bombardment (FAB), matrix-associated laser desorption ionisation (MALDI), inductively coupled plasma-mass spectrmetry (ICP-MS, cf and ), etc]... 

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