Interferences cluster ions

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

A reaction of particular relevance with respect to applied catalysis is the oxidative dehydrogenation (ODH) of hydrocarbon by VmOn ions according to reaction 2, which involves a two-electron reduction of the cluster. By means of a systematic study of the reactions of various YmOn ions as well as the related oxo-vanadium hydroxides VmO H+ ions with a set of C4-hydrocarbons, it was demonstrated recently that the ODH activity of the cluster ions shows a clear correlation with the formal valence of vanadium in the cluster ions with a maximum reactivity for formal vanadium (V) (Fig. 3) [84]. In such a kind of reactivity screening, it is essential to include more than a single reagent as a probe for the reactivity of the different ions in order to reduce interferences by kinetic barriers of one particular combination of neutral and ionic reactants [85]. Accordingly, the sums of the relative rate constants for the ODH reactions of the four different butenes are considered and normalized to the most reactive ion studied, which turns out to be the formally pure vanadium (V) compoimd In addition to isomeric... 

Investigations of cluster formation serve to explain the evaporation and atomization of sample material and ion formation processes. A further aim of cluster research is to find out under what conditions cluster or polyatomic ion formation can be influenced in order to avoid disturbing interferences and decrease the detection limits of elements. On the other hand, polyatomic ions have also been used as analyte ions for analysis, e.g. the application of MCs+ and MCs2+ dimeric and trimeric ions as analyte11 or of cluster primary ion beams (e.g., of bismuth and gold primary clusters)15 16 by the bombardment and sputtering of a solid surface in SIMS.17-21 Especially in SIMS, a multitude of cluster ions with high ion formation rates are observed.18 22 23... 

Table 9.46 Possible interferences of cluster ions in mass spectra and the required mass resolution by the ultratrace analysis of high purity CaF2.

Finally, the calibration curve seldom is linear, due to mutual interference of cluster ions of the analyte and unlabeled molecules in the IS. Although this problem may be circumvented to some extent by calibration over a very narrow concentration range (bracketing) and/or a proper choice of mlz ratios and spiking level (Colby et al., 1981 Yap et al., 1983), a mathematical data reduction as described in Section 3 generally is the best approach. 

In low-resolution ICP-MS the resolution in the spectra at best is 1 dalton and cluster ions are found particularly in the lower mass range signals, which in a number of cases cause spectral interferences with analyte ions. Cluster ions may be formed from different types of compounds present, namely ... 

Furthermore, changes of the nebulizer gas flow also influence the formation and the breakdown of cluster ions, requiring optimization with respect to minimum spectral interferences as well. 

Electrothermal vaporization (ETV) in addition to its features for the analysis of microsamples, in ICP-MS has the additional advantage of introducing a dry analyte vapor into the plasma. Hence, it has been found to be useful for elements for which the detection limits are high as a result of spectral interferences with cluster ions. In the case of 56 Fe, which is subject to interference by 40ArO+, Park et al. [529] showed that the detection limit could be improved considerably by ETV. For similar reasons the direct insertion of samples in ICP-MS leads to the highest absolute power of detection (detection limits in the pg range and lower [530, 531]). 

Deng R. C. and Williams P. (1994) Suppression of cluster ion interferences in glow discharge mass spectrometry by sampling high-energy ions from a reversed hollow cathode ion source, Anal Chem 66 1890-1896. 

Figure 8. Two models describing the spatial organization of the ionic sites, a Two-phase model composed of ionic clusters (ion-rich regions) dispersed in a matrix of the intermediate ionic phase, which is composed of fluorocarbon chains and nonclustered ions. The ionic scattering maximum arises from an interparticle interference effect, reflecting an average intercluster distance S. b Core-shell model in which the ion-rich core is surrounded by an ion-poor shell composed mostly of perfluorocarbon chains. The core-shell particles are dispersed in the intermediate ionic phase. The scattering maximum arises from an interparticle interference effect, reflecting a short-range order distance S of the core-shell particle. Note that the crystalline region was not drawn in the model for the sake of simplification and that the shape of the core-shell particle may not necessarily be spherical.

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