High-mass clusters

In the lower>mass distribution, cluster ions are observed with an interval of An-1, while the high>mass distribution Is characterized by An-2, with only even n values being observed. The low mass distribution is essentially the same in each method, and observation of high-mass clusters has not been reported with direct formation methods. 

The observation of high mass clusters is familiar in SIMS, and... 

It must be the most abundant ion in the high-mass cluster. In most cases this statement is true, but there are a few exceptions, especially when the compound contains more than two Cl atoms or one Br atom. For example, in the spectrum of CH2Br2, the most abundant ion in the molecular ion region is at m/z 174, although its molecular mass is 172 u. Also, for a few compounds, the (M — 1)+ ion is more abundant than the M+ ion. 

Solution From the high-mass cluster it is obvious that the unknown contains a bromine atom. The difference in mass between m/z 122 and 43 supports this observation. The elemental composition of m/z 43 is C3H7+, which could be an isopropyl or propyl cation. The unknown is either 1- or 2-bromopropane, both of which give nearly identical mass spectra. 

Chan, T.-W. D. Colburn,A.W. Derrick, P.J.Matrix-assisted laser desorption/ionization using a liquid matrix Formation of high-mass cluster ions... 

Morand, K L. Cox, K. A. Cooks, R. G. Efficient trapping and coUision-induced dissociation of high-mass cluster ions using mixed target gases in the quadrupole ion trap. Rapid Commun. Mass Spectrom 1992, 6, 520-523. 

In FD, the sample is deposited directly onto carbon dendrites serving on the anode as activated emitters. For hydrocarbon-type anionic, cationic, and nonionic surfactants, FD usually produces molecular or quasimolecular ions free of fragmentation. For amphoteric nonfluorinated surfactants, molecular ions have been obtained together with fragment ions providing structural information [95-97], which showed that perfluoroalkanesulfonates are desorbed as high-mass clusters under FD conditions. 

There are three main reasons for this choice. Firstly, it becomes more and more difficult to obtain recordable, molecular-ion signals from un-derivatized carbohydrates as their M, increases significantly above 3000. Secondly, the mass spectrometers that have been used in all high-mass-carbofiydrate studies published at the time of writing this article are not capable of very sensitive analysis above —3800 mass units (see later). Thirdly, at masses >4000, it is usually not practicable to work at the resolution necessary for adjacent peaks to appear as separate signals in the spectrum. To do so would require that the source and collector slits be narrowed to such a degree that there would be an unacceptable loss in sensitivity. Thus, spectra acquired at mass >4000 are usually composed of unresolved clusters. 

Fig. 6.—One of the Molecular-ion Clusters Obtained from a Sample of Deuteropermethy-lated. Cyclic /3(l- 2)-Glucans (see Section VI,S). [All high-mass samples give unresolved clusters of this type if the mass spectrometer is operated at low resolution. The peak is 6 mass units wide at half height. The mass is assigned by using the mass marker, which gives marks every 4 mass units, as shown. The center of the peak corresponds to the chemical molecular weight of an [M + NKJ species.]...

This Study has shown that reasonably uniform platinum crystallites can be made on y-alumlna, and that platinum and palladium can be segregated and maintained In that form for the most part even after exposure to high temperature oxidation-reduction conditions. Highly dispersed clusters of palladium, nickel, cobalt, and Iron can be observed. Cluster size determination could not be accurately made because of the lack of contrast between the cluster and the support. The marginal detectability by EDS for these clusters enabled elemental Identification to be made, however, mass uniformity determinations could not be made. 

Figure 16 shows G(D) of a simulated polymer mixture at two scattering angles ("Q", 14° and 17°). The mixture consists of two polystyrene standards having distinctly different weight average molar masses (3.Ox 105 and 5.9x 106 g/mol) and a high mass polystyrene which is used to simulate the polymer cluster [66]. The area ratio Ar of the two peaks is expressed by... 

The high masses of clusters may be explained by the high masses of the dominant cD galaxies in other words - there is no extra mass in clusters ... 

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