A Closer Look at FAB-MS

The overall diagram of a fast-atom bombardment ion source is depicted in Fig. 5. This is fairly simple, consisting of three main elements (1) an atom gun, (2) a sample inlet, and (3) an ion-extraction system. 

The atom gun is made up of an evacuated chamber that encloses a plate to which a high voltage potential (nominally 8 kV) is applied. The gas to be used as the bombardment gas is allowed in the chamber, through an appropriate 

Saddle field ion sources such as those by Ion Tech Ltd. (74MI3) are very compact, simple, and efficient they have been widely adopted for this purpose. Their characteristics are well documented and they have been used by Barber et al. during the course of their development of FAB (81MI7 82AC645 A). 

A typical sample inlet is shown schematically in Fig. 6. The sample is loaded onto the tip of a probe along with a support matrix which is inserted into the 

Since a monolayer of material is completely sputtered in a matter of seconds in a typical FAB ion source, it is essential that the sample surface be continuously regenerated during prolonged examination. This is done naturally by diffusion of the sample to the surface of the solution. It is therefore essential that the sample have some solubility in the low-volatility solvent, to provide the diffusion mechanism and also to act as a reservoir of material. Ionic groups that render compounds involatile, thus ruling out conventional methods of ionization, are also those groups that frequently lead to solubility in polar solvents, and to the associated surfactant properties that facilitate good sample preparation for FAB ionization. It follows that the detection of solvent substrate peaks in a FAB mass spectrum implies that optimal sample preparation has not been achieved. 

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