Matrix in FAB

Barber, M. Bell, D. Eckersley, M. Morris, M. Teller, L. The Use of M-Nitrobenzyl Alcohol As a Matrix in FAB-MS. Rapid Commun. Mass Spectrom. 

Another way to use this kinetic energy addition mode is to selectively reject unwanted ions from the ion trap. These could be ions derived from solvent or from the matrix in FAB or LSIMS experiments. A constant frequency field at high voltage during the ionization period will selectively reject a single ion. Multiple ions can also be selected in this mode. 

An ideal FAB matrix should fulfill the following criteria [22,23,33,56,57] 0 The analyte should be soluble in the matrix. Otherwise, addition of co-solvents, e.g., dimethylformamide (DMF), dimethylsulfoxide (DMSO), or other additives [71,72] can become necessary, ii) Only low-vapor pressure solvents can be easily used as a matrix in FAB. In principle, volatile solvents can be employed, provided a stable surface can be obtained on the time scale of recording a mass spectrum Hi) The viscosity of the solvent must be low enough to ensure the diffusion of the solutes to the surface [73]. iv) Ions from the matrix itself should be as unobtrusive as possible in the resulting FAB spectmm. v) The matrix itself has to be chemically inert. However, specific ion formation reactions promoting secondary ion yield are advantageous. 

Fast-atom bombardment (FAB) is one of a number of ionization techniques which utilize a matrix material, in which the analyte is dissolved, to transfer sufficient energy to the analyte to facilitate ionization. In FAB, the matrix material is a liquid, such as glycerol, and the energy for ionization is provided by a high-energy atom (usually xenon) or, more recently, an ion (Cs+) beam. In conventional FAB, the solution of analyte in the matrix material is applied to the end of a probe which is placed in the source of the mass spectrometer where it is bombarded with the atom/ion beam. 

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. 

Sample preparation for the common desorption/ionisation (DI) methods varies greatly. Films of solid inorganic or organic samples may be analysed with DI mass spectrometry, but sample preparation as a solution for LSIMS and FAB is far more common. The sample molecules are dissolved in a low-vapour-pressure liquid solvent - usually glycerol or nitrobenzyl alcohol. Other solvents have also been used for more specialised applications. Key requirements for the solvent matrix are sample solubility, low solvent volatility and muted acid - base or redox reactivity. In FAB and LSIMS, the special art of sample preparation in the selection of a solvent matrix, and then manipulation of the mass spectral data afterwards to minimise its contribution, still predominates. Incident particles in FAB and LSIMS are generated in filament ionisation sources or plasma discharge sources. 

C. Dass. The Role of a Liquid Matrix in Controlling FAB-Induced Fragmentation. J. Mass Spectrom., 31(1996) 77-82. 

Because of their asymmetry, CDs exhibit chiral effects towards chiral molecules under FAB" and MALDl conditions. The main ambiguity of these studies remains regarding the environment in which chiral recognition occurs, whether in the bulk matrix, in the selvedge vaporization region, or in the gas phase. Besides, neither MALDI nor EAB ensure attainment of purely kinetic or equilibrium conditions so as that quantitative interpretation of the MS patterns in terms of relative stabihty of diastereomeric host/guest intermediates or transition stmctures... 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

The conditions of the FAB process also promote unwanted reactions between analyte and matrix. Even though such processes are not relevant in the majority of FAB measurements, one should be aware of them Besides addition or condensation reactions with matrix fragment ions, [81,82] reduction [83-86] and dehalo-genation [87,88] of the analyte represent the more prominent side-reactions in FAB. Electron transfer to cause the reduction of otherwise doubly charged ions have also been observed. [47]... 

LT-FAB mass spectra are obtained during thawing of the frozen solution in the ion source of the mass spectrometer, thereby allowing to employ almost any solvent as matrix in LT-FAB-MS. Consequently, neither volatility nor unwanted chemical reactions with the matrix restrict the choice of a matrix. Instead, the solvent matrix may be tailored to the analyte s requirements. 

Musselman, B. Watson, J.T. Chang, C.K. Direct Evidence for Preformed Ions of Porphyrins in die Solvent Matrix for FAB-MS. Org. Mass Spectrom. 1986, 27, 215-219. 

Gross, J.H. Use of Protic and Aprotic Solvents of High Volatility As Matrixes in Analytical Low-Temperature FAB-MS. Rapid Commun. Mass Spectrom. 1998,12, 1833-1838. 

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-... 

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. 

FAB peptide preparation is accomplished by dissolving a relatively pure (>70%) peptide sample into a 3-nitrobenzyl alcohol (NBA) matrix. There are many matrices used in FAB analyses, however, 3-nitrobenzyl alcohol has become a standard for FAB peptide analysis (Figure 12). The peaks associated with the NBA matrix can make the spectra more cumbersome to interpret. However, to the experienced mass spectroscopist the matrix ions can act as reference ions and are a useful indicator of accuracy. Rarely are the NBA matrix ions observed above m/z 1000. 

FAB MS has been applied in a number of studies to characterize ILs. Since most ILs are viscous liquids with negligible vapor pressure, the measurement of FAB MS is possible without the addition of a liquid matrix. In principle, ILs can therefore also be used as matrix substances for the FAB analysis of other analytes dissolved therein [12]. Spectra could be measured both in the positive and in the negative ion modes as has been demonstrated, for example, for butylpyridinium- chloroaluminates and gallates [12,13]. Beside the molecular ions, fragments mainly formed by the loss of the substituents of the central core of the cations, for example, butyl groups, were observed. Together with the isotope patterns, these fragments provide valuable information about the structure of newly composed compounds and help also to identify unexpected by-products like oxidized or hydrolyzed compounds in the ILs (see section 14.3.2). 

Mass spectrometric studies were carried out as a first qualitative means of checking for dicarboxylate anion binding (see also Section 3). Here, mixtures of sapphyrin dimer 15 and several representative dicarboxylate anions, such as oxalate, 4-nitrophthalate, 5-nitroisophthalate and nitroterephthalate in methanol, were subjected to high resolution FAB mass spectrometric (HR FAB MS) analysis using FAB positive NBA matrix. In general, peaks for the putative complexes were seen, lending credence to the hypothesis that the dicarboxylate substrates in question were, in fact, being bound by 15 under the matrix desorption/gas phase conditions used to effect these mass spectrometric analyses. 

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). 

The fragmentation patterns and characteristic fragment ions for the carotenoids observed in FAB-MS and LSIMS tandem mass spectra are also observed in the tandem mass spectra obtained following ESI (see Basic Protocol 4), APCI (see Basic Protocol 5), and other methods. A detailed account of structure determination of carotenoids using FAB ionization with CID and MS/MS is presented in van Bree-men el al. (1995). Finally, another advantage of MS/MS is that matrix ions formed during FAB-MS or LSIMS, and any other contaminating ions, are eliminated, which simplifies interpretation of the mass spectrum. 

The choice of matrix for FAB-MS, LSIMS, and MALDI is essential for efficient sample ionization. For example, the use of 3-nitroben-zyl alcohol instead of glycerol, thioglycerol, or most other more common matrices is essential for the formation of abundant carotenoid ions during FAB and LSIMS. Nonpolar carotenoids (e.g., the carotenes) are insoluble in polar ma-... 

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