Properties of the Solvent Matrix Material

Liquids examined by FAB are introduced into the mass spectrometer on the end of a probe inserted through a vacuum lock in such a way that the liquid lies in the target area of the fast atom or ion beam. There is a high vacuum in this region, and there would be little point in attempting to examine a solution of a sample in one of the commoner volatile solvents such as water or dichloromethane because it would evaporate extremely quickly, probably as a burst of vapor when introduced into the vacuum. Therefore it is necessary to use a high-boiling solvent as the matrix material, such as one of those listed in Table 13.1. 

The solvents used for liquid chromatography are the commoner ones such as water, acetonitrile, and methanol. For the reasons just stated, it is not possible to put them straight into the ion source without problems arising. On the other hand, the very viscous solvents that qualify as matrix material are of no use in liquid chromatography. Before the low-boiling-point eluant from the LC column is introduced into the ion source, it must be admixed with a high-boiling-point matrix 

A typical cascade process. A fast atom or ion collides with surface molecules, sharing its momentum and causing the struck molecules to move faster. The resulting fast-moving particles then strike others, setting up a cascade of collisions until all the initial momentum has been dissipated. The dots ( ) indicate collision points. Ions or atoms (O) leave the surface. 

Having considered the various parts of a dynamic-FAB system (atom gun, ionization, and matrix), it is now necessary to see how these are put together in a working inlet/ion source interface. 

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. 

Wide mass range depending on which glycol is used. 

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