Cluster ion formation

Two scientific aspects in cluster science have been investigated using pulsed-laser field desorption technique with a considerable degree of success.85 One is the critical numbers, which are the smallest numbers of atoms in multiply charged cluster ions when the ions can still resist being 

Cluster ions are often observed in field evaporation of compound semiconductors such as GaAs and GaP etc. In high voltage pulse field evaporation of GaAs and GaP, only a few per cent of cluster ions are 

Mo+ and Mc 2+ one may resort to a multinomial expansion coefficient analysis. Let us represent the fractional abundances of the 7 Mo isotopes by a, b, c, d, e, f and g. The calculated and experimental fractional abundances of the 15 mass lines in Mo + are listed in Table 2.4. When the calculated abundances are compared with the experimental abundances, one reaches the conclusion that the spectrum shown in Fig. 2.17 contains few if any Mo+ ions. If on the other hand, a fraction of the ions are Mo+, say p, then the relative abundances of each mass line can also be calculated. For example, that of Min = 92 should be [pa + (1 — p)a2], and that of M/n = 94 should be [pb + (1 — p)(b2 + 2ad), etc. Thus the fraction p can be obtained by best fit of theoretical abundances and experimental abundances of different Min mass lines. 

An attempt has also been made to derive the binding energy of atoms in clusters from a measurement of the critical energy deficit of cluster ions. For n+ cluster ions of m atom size, from consideration of Born-Haber energy cycle, the critical ion energy deficit can be easily shown to be given by100 

At the present time, such calculations are impossible because of the lack of data for the needed parameters such as / and b(m), etc. 

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Mass spectral analysis of quaternary ammonium compounds can be achieved by fast-atom bombardment (fab) ms (189,190). This technique rehes on bombarding a solution of the molecule, usually in glycerol [56-81-5] or y -nitroben2yl alcohol [619-25-0], with argon and detecting the parent cation plus a proton (MH ). A more recent technique has been reported (191), in which information on the stmcture of the quaternary compounds is obtained indirectly through cluster-ion formation detected via Hquid secondary ion mass spectrometry (Isims) experiments. 

Veith, H.J. Field Desorption Mass Spectrometry of Quaternary Ammonium Salts Cluster Ion Formation. Org. Mass Spectrom. 1976,77,629-633. 

Fig. 9.6. In LSIMS and FAB, sample-matrix cluster ion formation and desolvation processes occur on a longer time scale. Adapted from Ref. [24] by permission. John Wiley Sons, 1995.

More recently, studies concerning cluster ion formation from solid or deeply cooled liquid alcohols [120-122] have gained new interest. [123,124] Low-temperature fast atom bombardment (LT-FAB) of frozen aqueous solutions of metal salts provides a source of abundant hydrated metal ions. [125-127] Organic molecules can also be detected from their frozen solutions. [128] Such LT-FAB applications are particularly interesting when enabling the detection of species that would otherwise not be accessible by mass spectrometry, because they are either extremely air- and/or water-sensitive [129,130] as the phosphaoxetane intermediate of the Wittig reaction [131] or insoluble in standard FAB matrices. [106,132]... 

Part of a mass spectrum for the determination of Fe and Cr contamination in boron nitride contaminated with carbon measured by LIMS is shown in Figure 6.9. The analyte ions 53Cr+ and 54Fe+ due to different masses of isobaric atomic and cluster ions are clearly separated from boron and boron carbide cluster ions as demonstrated in Figure 6.9. Cluster ion formation has been studied by laser ionization mass spectrometry (LIMS) on a boron nitride target.10... 

Figure 9.54 Schematic of cluster (ion) formation in inorganic mass spectrometry.

Figure 9.56 Cluster ion formation in laser plasma in dependence of laser power density . (C. Seifert, j. S. Becker and H. J. Dietze, Int.J. Mass Spectrom. Ion Proc., 184, 121(1988). Reproduced by permission of Elsevier)...

Figure 9.59 Cluster ion formation of a boron nitride/graphite mixture in a laser plasma. (J. S. Becker and ff. J. Dietze, Fresenius ). Anal. Chem., 359, 338(1997). Reproduced by permission of Springer Science and Business Media.)...

C-C bonds in the metal-containing cluster ion. Formation of these... 

The fullerenes were first discovered by laser ablation cluster ion formation in a mass spectrometer.89 Soon after the identification of C60 as a magic number cluster, [LaC60]+ was also identified.90 Later, macroscopic quantities of fullerenes were produced by the carbon arc method91 soon followed by the macroscopic production of the lanthanum endohedral fullerenes.92 Fullerene chemistry has developed rapidly since the preparation of macroscopic quantities of these compounds. The development of the metallofullerenes has been hampered by the low yield... 

P. Pradel, P. Monchicourt, J.J. Laucagne, M. Perdrix, and G. Watel, Carbon cluster ion formation in a direct laser vaporization source, Chem. Phys. Lett. 1989, 158, 412-416. 

These days, remarkably high-resolution spectra are obtained for positive and negative ions using coaxial-beam spectrometers and various microwave and IR absorption techniques as described earlier. Information on molecular bond strengths, isomeric forms and energetics may also be obtained from the techniques discussed earlier. The kinetics of cluster-ion formation, as studied in a selected-ion flow tube (SIFT) or by high-pressure... 

Creasy, W. R. and Breima, J. T., "Large Carbon Cluster Ion Formation by Laser Ablation of Polyimide and Graphite," Chem. Phys., 126, 453-468,1988. 

Hortal et al. [52,53] and Martinez-Haya et al. [54] showed that the LD-MS spectrum of an asphaltene depended on the concentration of species in the sample and on the laser fluence apphed.The thickness of sample applied to the target plate was a factor in the formation of aggregates or clusters of molecules in the reactive gas plume formed by the laser shot thin layers were less likely to form cluster ions than thick layers. High laser fluences were more hkely to lead to cluster ion formation than fluences slightly above ionization threshold. However, these works did not examine fractions of asphaltenes where low- and high-mass molecules were separated from each other. 

This sample has been discussed elsewhere [78,79,92]. Table 33.1 shows the elemental analyses of the sample and the fractions, while Table 33.2 shows the proportions of the maltene and asphaltene fractions. Figure 33.7 shows LD-MS of a distillation residue of Syncrude sweet blend from the Athabasca tar sands, at different LPs and different HMA voltages (9 and 10 kV). At 30% LP, the spectra (1 and 2) were very weak and close to the ionization threshold at 40% LP, the maximum ion intensity shifted slightly to higher mass (m/z 300-400) and the upper mass limit moved to about m/z 5000 at 50% LP, the maximum ion intensity remained between 300 and 400 though off-scale, while the upper mass limit moved to m/z 20,000-30,000 with evidence of cluster ion formation. Figure 33.8 shows the Syncrude mass... 

However, it is also clear that mass spectra can be generated by LD-MS for maltenes and asphaltenes (without further fractionation by TLC) that reflect relative differences in their mass ranges by variation of LP and ion intensity to define conditions when fragmentation and cluster ion formation begin. 

With the formation of volatile hydrides, the detection limits for elements such as As. Se, Sb, and Pb can be improved [310]. The improvement is due to enhanced analyte sampling efficiency and to the decrease of cluster ion formation. 

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