Clusters SIMS studies

The secondary ion yield depends both on the energy and the nature of the primary ions. It has been demonstrated that the use of clusters instead of monoatomic ions improves the secondary yield. Classical primary ions, like Ga+ and Cs+, are increasingly replaced by Au3+, Bi3+ or C60+ which has permitted great progress in organic ToF-SIMS studies [Kollmer 2004, Touboul et al. 2004, Winograd 2005]. 

Most analytical studies using FT-ICR mass spectrometry, where ions have been produced inside (or just outside) the analyzer cell, have used lasers as ionization sources. Other than some very limited Cs secondary ion mass spectrometry (SIMS) studies [77], most research utilized direct laser desorption to form various organic [78] and inorganic [79] ions, including metal [80] and semiconductor [81] (including carbon) clusters. More recently matrix assisted laser desorption ionization (MALDI) has been used to form ions of high molecular weight from polymers [82] and many classes of biomolecules [83]. 

Schulten (110, 111) has used laser-assisted field desorption mass spectrometry to study some inorganic and organometallic systems. This method is intermediate between LAMMA and simple FD. Metal cations predominate from inorganic salts. The technique also showed clusters of the type reported from both FAB and SIMS studies. By carefully controlling the laser, a chlorophyll molecular ion could be obtained as well as fragments relating to its structure. 

The impact of an ion beam on the electrode surface can result in the transfer of the kinetic energy of the ions to the surface atoms and their release into the vacuum as a wide range of species—atoms, molecules, ions, atomic aggregates (clusters), and molecular fragments. This is the effect of ion sputtering. The SIMS secondary ion mass spectrometry) method deals with the mass spectrometry of sputtered ions. The SIMS method has high analytical sensitivity and, in contrast to other methods of surface analysis, permits a study of isotopes. In materials science, the SIMS method is the third most often used method of surface analysis (after AES and XPS) it has so far been used only rarely in electrochemistry. 

SIMS (61,64,86), microscopy (65), XPS (56), electron microprobe techniques (14,66), electron paramagnetic resonance (EPR) (67) and luminescence experiments (68) have been successfully employed to probe and study V mobility and reactivity on a catalyst surface. TEM, STEM and energy dispersive X-ray emission (EDX) measurements have indicated that V interaction with REY-crystals induced vanadate clusters formation (65). Vanadium was also found capable of reacting with rare-earths outside the zeolite cavities to form LaVQ4... 

In using a similar approach, Thiine et al. [29] applied static SIMS to show that Cr/SiC>2 model catalysts which are active for ethylene polymerization contain only monochromates. Secondary ions with more than one Cr ion in the cluster, such as Cr2C>4 and Cr203, disappeared from the spectra after the catalysts had been calcined only CrSiOx ions remained. Aubriet et al. [30] studied the anchoring of chromium acetyl acetonate, Cr(acac)3 to a planar SiO2/Si(100) model support with static SIMS. Chromium polymerization catalysts are discussed further in Chapter 9. 

Campana s group has pioneered the development of high-performance SIMS instrumentation (93), in particular for studies at very high masses. They have investigated metal salt clusters in a manner analogous to the FAB results described in the previous section (94-100). For cubic-like structures, the Csl spectrum shows stable structures for species of the type [Cs(CsI) ]+ where n = 13, 22, 31, 37, 52, end 62 (94) with results now surpassing n = 100 (95). 

There the intensity ratios between ion clusters were taken from experimental measurement of low resolution MS [89], but intensity ratios within each isotope cluster was calculated as the sum of binary distributions of the natural abundance of each isotope involved. High resolution m/z values of major peaks (for HRMS) were calculated as sums of exact masses of isotopes. This simulation method (ISOCLUST) developed by the present author (J. Paasivirta) is operable with a desk computer. Low resolution electron impact MS is suitable for TCBT determinations, especially by selected ion monitoring (SIM). In this mode, focusing to four ions, m/z 213.0,283.0,285.0, and 320.0, which are not interfered with by PCBs is recommended. In a TCBT study, quantitative results were based on their sum intensity for better signal to noise ratio [91]. 

This system is included primarily for historical purposes. Nylons were the first series of polymers studied using TOF-SIMS in the high mass range. The specific nylons studied were nylon-6 (N-6), N-8, N-66, N-69, and N-66(a6). The major series of clusters observed for all nylons were R fragments, presumably cyclic peaks were observed to 2500-3000 Da. Weaker peaks were seen at every ca. 14 mass units. Spacing between major peaks corresponded to the repeat unit, and side chains on the nylons remained intact. No studies were done to compare different molecular weights this system is worth revisiting. 

There are only a relatively small number of articles reporting on the use of specific sample preparation techniques in combination with cluster beams for sensitivity improvement, and even fewer with pronaising results. Several studies demonstrated that MetA-SIMS with cluster projectiles at the usual impact energies often lead to lower, instead of enhanced, molecular ion yields [333,341]. In particular, MD simulations showed that 10-15 keV Cw projectiles were inefficient at generating bond scissions and energetic subcascades in the organic sample when impinging on metallic clusters [336]. 

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