Atom or Ion Beams

Instead of the fast-atom beam, a primary ion-beam gun can be used in just the same way. Generally, such an ion gun emits a stream of cesium ions (Cs ), which are cheaper to use than xenon but still have large mass (atomic masses Cs, 139 Xe, 131). Although ion guns produce no fragment ions in the primary beam, they can contaminate the mass spectrometer by deposition with continued use. 

---

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. 

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. 

Fast-atom bombardment An ionization method used for involatile and thermally labile materials. In this technique, the sample is dissolved in a matrix material and bombarded with a high-energy atom or ion beam. 

The earliest applications of MS/MS that utilized LSI methods (FAB or FIB) required placing a few microliters of a viscous glycerol/methanol matrix on a stainless steel probe, insertion into the fast-atom bombardment/ fast-ion bombardment (FAB/FIB) source housing, removal of air from the source, then further insertion of the probe to expose the sample and matrix to the atom or ion beam. Although this procedure was tedious, a... 

Both FAB and LSIMS can be used in conjunction with continuous-flow inlet systems to optimise sensitivity, to improve reproducibility and to allow the use of in-line liquid chromatography for LC-MS. In the flow technique an LC pump delivers a continuous flow of solvent (containing a low concentration of the FAB/LSIMS matrix) to the point where the atom or ion beam is applied. The sample is introduced into this solvent flow by an LC injection valve and is delivered to the point of ionisation in a sharp concentrated slug. A good account of the method of continuous flow FAB is given in reference [5]. 

Note The interaction time is controlled through both the incidence/exit angle of the atom or ion beam with respect to the surface and the velocity, which itself is... 

Fig.11.17. Schematic experimental arrangement for lifetime measurements in fast atomic or ion beams...

---