Position specific mass spectrum

The problem of non-specific decomposition is exemplified by the study of 4-methyl-l, 3-dioxolane [657]. Following El the d2 molecule labelled at the 2 position (the ring C between the O s) gave a metastable peak for loss of D- but not for loss of H In the mass spectrum, however, the peak intensity for H loss was greater than that for D loss, suggesting that there were not just one, but at least two, reaction channels for atomic hydrogen loss. 

The structure of pumiliotoxin 25ID is shown in Fig. 14. Positional disorder in the crystal, as evidenced by the increasing size of the experimentally determined thermal parameters for each succeeding atom in the side chain, was sufficiently large that reliable values for the bond lengths and bond angle for the three terminal atoms could not be determined. However, the mass spectrum of the compound, including data recorded from the specific crystal used for the X-ray analysis, shows that the side chain does... 

Other ions in the spectrum are the result of ion fragmentation at the IMS-MS interface. The ion fragments are easy to recognize in the spectrum because they produce a mass spectrum at a single mobility. Within the metabolite trend band, individual metabolite ions are positioned above or below the average trend line as a result of differences in their SCS (0/m). Thus, isomers and isobars are separated within the metabolite trend band. Hundreds of specific metabolites can be identified and quantified from a single ion mobility MS run in a matter of seconds. 

Curtis and Boyd utilized this feature to the full by optimizing NICI conditions for achieving hard ionization, viz., DEA, by which chloride ions are predominantly produced from chlorinated compounds and detected by SIM at m/z 35 and 37. Operated in this way, NICIMS is effectively turned into a chlorine-specific GC detector with selectivity and sensitivity comparable to those of the ELCD. By using this technique, Milley et al. successfully identified dichlor-otetradecanoic acid in a GPC-enriched sample from lobster digestive gland lipids (but with the chlorine position remaining undetermined). The identification of this compound was supported by the mass spectrum obtained with soft NICI optimized for molecular anions, which resembled that of a synthesized 9,10-dichlorotetradecanoic acid. 

The choice of a specific matrix is mainly experimental. The analyte must be miscible with the matrix and they must cocrystallize. For positive ionization acidic, proton donor matrixes are favored, and for negative ionization mode, basic matrixes. Hot spots may be generated during cocrystaUization if a nonuniform mixture is produced. Heterogeneity of the sample poses a major difficulty and in such a case the position of the laser beam may influence the spectra. In general, the selection of appropriate MALDI matrix, cationization salt, and sample concentration and sample preparation technique are critical success factors for obtaining a reliable mass spectrum that influence the polymer distribution. 

Positive-ion MALDI mass spectrum of PE species is characterized by the presence of a specific fragment ion corresponding to the loss of the phosphoethanolamine head group [93]. PE species can also be ionized in the negative-ion mode with a lower sensitivity in comparison to that in the positive-ion mode. Negative-ion MALDI mass spectra of PE species are usually dominated by the matrix adducts of PE. 

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