Ion Formation from Inorganic Samples

The energy provided by the impacting primary particle causes a collision cascade in the upper atomic or molecular layers of the sample. Within 30-60 ps, a cylindrical expansion is effected in the sample along the path of penetration [22]. Not all of this energy is dissipated and absorbed in deeper sample layers. A portion is directed toward the surface, where it effects ejection of material into the vacuum (Fig. 10.4) [24]. Due to the primary particle flux employed, this mode of operation corresponds to the dynamic SIMS mode as described in Chap. 15.5.1 in more detail. 

Example Bombardment of cesium iodide or gold delivers cluster ion series which are useftil for mass calibration of the instrument over a wide range. Csl works equally well in positive- and negative-ion mode to yield [(CsI) Cs] and [(CsI) I] cluster ions, respectively (Fig. 10.5). Starting from n = 0, [(CsI) Cs] cluster ions have been observed up to m/z 90,000 [39]. Larger [(CsI) Cs] cluster ions dissociate to yield smaller ones [(CsI) Cs] [(CsI) Cs] + (Csl) [40]. 

Gold produces a negative AUn cluster ion series up to about m/z 10,000 [41]. 

Note Csl and Au both bear the advantage of being monoisotopic. This insures the peak top to exactly represent the theoretical isotopic mass of the respective cluster ion, independent of its m/z ratio or actual resolution (Chaps. 3.3.5, 3.4). Csl, KI, and other alkali salts that provide more narrow-spaced cluster ion series can alternatively be enployed as saturated solutions in glycerol [42-44]. 

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