Fast Atom-Ion Bombardment

Fast atom/ion bombardment (FAB/FtB/LSIMS) MH+, (M + H)+,M + (M - H) , M-Intact molecular ions M, Q Polar organics, proteins, organometalhcs, nonvolatiles <20000 ... 

Derivatization for Fast Atom/Ion Bombardment Mass Spectrometry... 

Schematic diagram of the fast atom/ion bombardment process. 

It may at first appear at variance with the rest of the handbook to discuss derivatization for fast atom/ ion bombardment mass spectrometry, where chromatography may be only incidentally rather than directly involved. Nevertheless, it seems appropriate to include this aspect because these techniques are complementary to the electron impact and chemical ionization methods used with gas chromatography—mass spectrometry (GC-MS) as described in Chapter 14, and thus extend the coverage to include derivatization methods for the majority of mass spectrometric techniques. 

Derivatization for Fast Atom/lon Bombardment Mass Spectrometry Table 1 Applications of derivatives for fast atom/ion bombardment mass spectrometry... 

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). 

Dodonoy A I, Mashkova E S and Molchanov V A 1989 Medium-energy ion scattering by solid surfaces. Ill ejection of fast recoil atoms from solids under ion bombardment Rad. Eff. Def Sol. 110 227-341... 

Fast-Atom Bombardment (FAB) and Liquid-Phase Secondary Ion Mass Spectrometry (LSIMS) Ionization... 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

The basic principles of fast-atom bombardment (FAB) and liquid-phase secondary ion mass spectrometry (LSIMS) are discussed only briefly here because a fuller description appears in Chapter 4. This chapter focuses on the use of FAB/LSIMS as part of an interface between a liquid chromatograph (LC) and a mass spectrometer (MS), although some theory is presented. 

Bombardment of a liquid surface by a beam of fast atoms (or fast ions) causes continuous desorption of ions that are characteristic of the liquid. Where the liquid is a solution of a sample substance dissolved in a solvent of low volatility (often referred to as a matrix), both positive and negative ions characteristic of the solvent and the sample itself leave the surface. The choice of whether to examine the positive or the negative ions is effected simply by the sign of an electrical potential applied to an extraction plate held above the surface being bombarded. Usually, few fragment ions are observed, and a sample of mass M in a solvent of mass S will give mostly [M + H] (or [M - H] ) and [S -I- H]+ (or [S - H] ) ions. Therefore, the technique is particularly good for measurement of relative molecular mass. 

By passing a continuous flow of solvent (admixed with a matrix material) from an LC column to a target area on the end of a probe tip and then bombarding the target with fast atoms or ions, secondary positive or negative ions are ejected from the surface of the liquid. These ions are then extracted into the analyzer of a mass spectrometer for measurement of a mass spectrum. As mixture components emerge from the LC column, their mass spectra are obtained. 

The LC/TOF instmment was designed specifically for use with the effluent flowing from LC columns, but it can be used also with static solutions. The initial problem with either of these inlets revolves around how to remove the solvent without affecting the substrate (solute) dissolved in it. Without this step, upon ionization, the large excess of ionized solvent molecules would make it difficult if not impossible to observe ions due only to the substrate. Combined inlet/ionization systems are ideal for this purpose. For example, dynamic fast-atom bombardment (FAB), plas-maspray, thermospray, atmospheric-pressure chemical ionization (APCI), and electrospray (ES)... 

Some mild methods of ionization (e.g., chemical ionization. Cl fast-atom bombardment, FAB electrospray, ES) provide molecular or quasi-molecular ions with so little excess of energy that little or no fragmentation takes place. Thus, there are few, if any, normal fragment ions, and metastable ions are virtually nonexistent. Although these mild ionization techniques are ideal for yielding molecular mass information, they are almost useless for providing details of molecular structure, a decided disadvantage. 

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

FAB has evolved, and fast atoms are being replaced by fast ions, such as cesium (Cs+). This variation is called liquid secondary ion mass spectrometry (LSIMS) because the sample solution affords the secondary ion beam while the bombarding ions constitute the primary beam. Spectra... 

Instead of bombarding the matrix surface with fast atoms, fast ions can be used. Often these are cesium (Cs+) ions. 

As with fast atoms, bombardment of the matrix with fast ions causes very similar desorption of ions and neutrals. 

The impact of a primary beam of fast atoms or ions on a target matrix (substrate and solvent) causes desorption of molecular or quasi-molecular ions characteristic of the substrate. The process is called FAB for atom bombardment or LSIMS for ion bombardment. 

Instead of bombarding the matrix with fast atoms, fast ions (FIB) can be used. Often these are cesium ions (Cs+ ). As with fast atoms, fast ions cause desorption of ions and neutrals from the surface of a bombarded matrix. 

By allowing any solution, but particularly the eluant from a liquid chromatographic column, to flow continuously (dynamically) across a target area under bombardment from fast atoms or ions (FAB or FIB), any eluted components of a mixture are ionized and ejected from the surface. The resulting ions are detected and recorded by a mass spectrometer. The technique is called dynamic FAB or dynamic LSIMS. 

Fast-atom bombardment (FAB) is an ionization technique that produces a protonated or deprotonated molecular ion, hence a molecular mass for the sample. It can be used for analysis of peptides up to m/z about 5000. 

Desorption ionization (DI). General term to encompass the various procedures (e.g., secondary ion mass spectrometry, fast-atom bombardment, californium fission fragment desorption, thermal desorption) in which ions are generated directly from a solid or liquid sample by energy input. Experimental conditions must be clearly stated. 

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