Ion-cluster positive

In the y-radiolysis of water-isopropyl alcohol mixtures in the vapor phase, the total yields of hydrogen, methane, and carbon monoxide are linear over the whole concentration range, in the presence and absence of electron scavengers. The majority of the hydrogen produced from energy absorption by both water and isopropyl alcohol is formed by H-atom abstraction from isopropyl alcohol. Part of the H-atom yield is formed by electron-positive ion neutralization and part by processes not involving electrons. As far as the formation of H atoms is concerned, both electron-positive ion and ion-ion neutralizations appear to be independent of the composition of the positive ion cluster. A yield of molecular hydrogen is also present in both water and isopropyl alcohol. 

Despite the changing composition of this positive ion cluster with changes in /w and /p, G(H2)t is linear with isopropyl alcohol concentration both in the presence and absence of electron scavengers (Figure 1). Thus, there is no evidence that the composition of the positive ion affects either the conversion of electrons to H atoms via Reaction 9, or ion-ion neutralization in the presence of electron scavengers. This, however, is not completely unambiguous, particularly in view of the numerical similarities between G(H2)W° and G(H2)P° in the presence and absence of electron scavengers. 

Singly Charged Positive Ion Clusters SiOJ and GeOj... 

More complex ions are created lower in the atmosphere. Almost all ions below 70-80 km are cluster ions. Below this altitude range free electrons disappear and negative ions fonn. Tln-ee-body reactions become important. Even though the complexity of the ions increases, the detemiination of the final species follows a rather simple scheme. For positive ions, fomiation of H (H20) is rapid, occurring in times of the order of milliseconds or shorter in the stratosphere and troposphere. After fomiation of H (H20), the chemistry involves reaction with species that have a higher proton affinity than that of H2O. The resulting species can be... 

Schlager H and Arnold F 1985 Balloon-borne fragment ion mass spectrometry studies of stratospheric positive ions unambiguous detection of H (CH3CN), (H20)-clusters Pianet. Space Sc/. 33 1363-6... 

Mostly, positive-ion FAB yields protonated quasi-molecular ions [M -i- H]+, and the negative-ion mode yields [M - H]. In the presence of metal salts (e.g., KCl) that are sometimes added to improve efficiency in the LC column, ions of the type [M -i- X]+are common, where X is the metal. Another type of ion that is observed is the so-called cluster, a complex of several molecules with one proton, [M -i- H]+ with n = 1, 2, 3,. .., etc. Few fragment ions are produced. 

The reagent species in the positive-ion mode may be considered to be proto-nated solvent ions, and in the negative ion mode 02, its hydrates and clusters. 

The ion clusters (Si2H+, Si.sH, ,...) are also present, and larger clusters (positive and neutral) can be formed through reactions with silane molecules. Negative ions have been detected [321], which are thought responsible for the powders in the discharge [322]. 

Subsequently 36 strains of aerobic endospore-forming bacteria, consisting of six Bacillus species and one Brevibacillus species could be discriminated using cluster analysis of ESMS spectra acquired in the positive ion mode (m/z 200-2000).57 The analysis was carried out on harvested, washed bacterial cells suspended in aqueous acidic acetonitrile. The cell suspensions were infused directly into the ionization chamber of the mass spectrometer (LCT, Micromass) using a syringe pump. Replicates of the experiment were performed over a period of six months to randomize variations in the measurements due to possible confounding factors such as instrumental drift. Principal components analysis (PCA) was used to reduce the dimensionality of the data, fol-... 

Figure 1. Fast-flow reaction apparatus. Ions or ion clusters are introduced into the flow tube from various sources and reactions proceed after they encounter the reactants added through a ring injector located at a selected position in the flow tube. The disappearance of the reactant ions and formation of products is monitored with the quadrupole mass spectrometer/electron multiplier shown. Taken with permission from ref. 19.

Ion-molecule reactions involve a positive ion and a neutral molecule, frequently the parent molecule. Historically, there has been a dichotomy in the interpretation of the radiation-chemical yields in hydrocarbon gases. Early work by Lind (1961) and by Mund (1956) indicated the involvement of ion clustering, exemplified in the radiation-induced polymerization of acetylene as follows ... 

Adduct ions are quite frequent in the mass spectra. In positive ion mode, sodium or potassium cluster ions are commonly found. Mineral compounds often lead to multiple cluster ions. For example, spectra of FeCl3 in negative ion mode lead to several peaks from m/z 35 (Cl ) to m/z 487 ([(FeCl3)2FeCl3] ) [Van Ham et al. 2004],... 

Matrix assisted laser desorption ionisation (MALDI) and ESI-MS spectra of non-ionic surfactant blends of AE obtained after positive ionisation were compared [28]. Both the ionisation procedures, which produce [M + Na]+ ion clusters, were very useful for this purpose, but the ESI spectra generated were more complex, whereas MALDI ionisation led to simpler spectra that can be interpreted more easily [28]. 

Example Cesium iodide is frequently used for mass calibration in fast atom bombardment (FAB) mass spectrometry (Chap. 9) because it yields cluster ions of the general formula [Cs(CsI)n] in positive-ion and [I(CsI)J in negative-ion mode. For the [Cs(CsI)io] cluster ion, m/z 2730.9 is calculated instead of the correct value m/z 2731.00405 by using only one decimal place instead of the exact values Mi33Cs = 132.905447 and M1271 = 126.904468. T e error of 0.104 u is acceptable for LR work, but definitely not acceptable if accurate mass measurements have to be performed. 

Fig. 7.6. Standard El versus positive-ion Cl spectra of isobutane (upper) and ammonia (lower part). Ammonia forms abundant cluster ions upon d.

FAB matrix spectra are generally characterized by a series of matrix (Ma) cluster ions accompanied by some more abundant fragment ions in the lower m/z range. In positive-ion FAB, [Ma +H] cluster ions predominate, while [Ma -H] cluster ions are preferably formed in negative-ion FAB (Fig. 9.7). The principal ion series... 

FAB is well suited for the analysis of ionic analytes. In positive-ion mode, the spectrum is usually dominated by the cationic species, C which is accompanied by cluster ions of the general composition [CnH-An-i]". Thus, the distance between these signals corresponds to the complete salt CA, i.e., yields its molecular weight. This behavior is perfectly analogous to FD (Chap. 8.5.3). In negative-ion FAB, the anion A" will cause the base peak of the spectrum, and accordingly, cluster ions of the type [Cn.iH-AJ" are formed in addition. Consequently, both cation and anion are usually identified from the same FAB spectrum, irrespective of the chosen polarity. Nonetheless, it is common practice to select the polarity of the more interesting ion for the measurement. 

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