Liquid target surface, bombardment

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. 

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. 

The molecules hit by the fast atoms are destroyed in the collision and do not appear in the spectrum, whereas the molecules not directly hit (matrix and analyte) acquire enough energy to ionize and desorb from the target surface. Therefore, fast atom bombardment in a liquid matrix should allow desorption of the intact analyte molecule from the condensed phase. 

Unlike FAB/LSIMS, matrix-assisted laser desorption ionization (MALDI) uses a crystalline, rather than liquid, matrix, and a beam of photons, rather than atoms or ions. The net result is a dramatic increase in both sensitivity and mass range of compounds that may be analyzed. The sample is dissolved in a matrix and is allowed to crystallize on a stainless-steel target. The target is then inserted into the mass spectrometer and the surface bombarded with a pulsed laser beam. Molecules are desorbed from the surface and ionize, usually by protonation or deprotonation. Any fragment or multiply charged ions are generally of low abundance in this ionization mode. The pulsed nature of the laser excitation renders this technique compatible with TOF, and the combined technique enjoys an almost limitless mass range. 

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. 

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. 

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