Isotope cluster analysis

Isotopic cluster analysis can be built into the automated processing method and is used to target potential metabolites with the desired isotope ratios (Kind and Fiehn, 2006). For example, chlorine or bromine or radiolabeled drugs/metabolites can be pinpointed, at low levels, within a complex matrix background to dramatically enhance specificity and increase confidence in metabolite identification (Jindal and Lutz, 1986 Shirley et al., 1997). 

Least-squares-fitted monoisotopic mass spectra have been tabulated for B4H10, B5H8Br, B5H8I, B5H , EtB10H13, and B10H16. Isotope cluster analysis of the spectrum said to be due to B20H26 shows that it is actually a mixture... 

Of course nowadays exact mass measurement could also distinguish these two molecules, as could a variety of other instrumental techniques. The analysis of isotopic clusters is most useful for detecting the presence of halogens, sulfur, and silicon, all of which have abundant isotopes of two atomic weight units higher, thus leading to relatively large M + 2 peaks. 

McGibbon, G. A., Bayliss, M. A., Antler, M., Lashin, V. (2008). Automated software analysis of isotope cluster mass differences for components in LC-MS datasets. In Proceedings of the 56th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO. 

These isotopes are responsible for the peaks in the mass spectrum appearing as isotopic clusters that are characteristic of the elemental composition. They provide important analytical data. Indeed, even without exact mass measurement, the possibilities for elemental composition determination can often be restricted by using isotopic abundance data. For example, the fragments CioH2o and C x 11 i 2O2, both with a nominal mass of 140 u, produce peaks at mass 141 with 11 and 8.8 %, respectively, of the abundance at mass 140 u. This is the result of a different statistical probability of having 13C isotopes. These two elemental compositions can thus be distinguished in a mass analysis. 

This can be used to automatically analyse single beads with no knowledge of M.Wt Fig. 5.26 shows the cluster analysis result on four single beads that were analysed in triplicate. The TIC traces only show responses that fitted with the isotopic difference criteria and by selecting these peaks , mass spectra are generated depicting the doublets only with the M.Wt values. 

The ehallenge in this analysis lies in the efficient extraction and analysis of the complex matrices in which PBDEs are found. The compounds are used in a variety of plastic materials found in consumer products, and samples are prepared typically using either Soxhlet extraction or microwave digestion, followed by few or no clean-up steps. MS/MS can be used to isolate M + and [M-2Br] + isotopic cluster ions for accurate identification and quantitation. Three stages of the MS/ MS analysis of a PBDE are shown in Eigure 15.43. Eigure 15.44 shows a TIC of the common PBDE isomers that are tested for under the RoHS regulations these common PBDE isomers are listed in Table 15.2. Excellent quantitative results... 

Changes in structure of nuclei between magic numbers 50 and 82 as indicated by a rotating cluster analysis of the energy values of the first 2+ excited states of isotopes of cadmium, tin, and tellurium. Proc. Natl. Acad. Sci. 78 (1981) 5296-5298. 

Resonance Raman studies of the recombinant proteins showed vibrational bands at the 200-430 cm region characteristic of iron-sulfur clusters (124). Most interestingly, on Fe and O isotope sensitive band was detected at 801 cm which could be attributed to either a Fe(IV)=0 species or a monobridged Fe-O-Fe structure. This observation, together with Mossbauer analysis, which indicated a mixed N, 0, and S ligand environment for cluster 2, suggests a Fe-O-Fe or Fe=0 unit as part of the structure for cluster 2. 

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. 

Microwave spectroscopy is probably the ultimate tool to study small alcohol clusters in vacuum isolation. With the help of isotope substitution and auxiliary quantum chemical calculations, it provides structural insights and quantitative bond parameters for alcohol clusters [117, 143], The methyl rotors that are omnipresent in organic alcohols complicate the analysis, so that not many alcohol clusters have been studied with this technique and its higher-frequency variants. The studied systems include methanol dimer [143], ethanol dimer [91], butan-2-ol dimer [117], and mixed dimers such as propylene oxide with ethanol [144]. The study of alcohol monomers with intramolecular hydrogen-bond-like interactions [102, 110, 129, 145 147] must be mentioned in this context. In a broader sense, this also applies to isolated ra-alkanols, where a weak Cy H O hydrogen bond stabilizes certain conformations [69,102]. Microwave techniques can also be used to unravel the information contained in the IR spectrum of clusters with high sensitivity [148], Furthermore, high-resolution UV spectroscopy can provide accurate structural information in suitable systems [149, 150] and thus complement microwave spectroscopy. 

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