Sequence ions, mass spectra

The importance of linked scanning of metastable ions or of ions formed by induced decomposition is discussed in this chapter and in Chapter 34. Briefly, linked scanning provides information on which ions give which others in a normal mass spectrum. With this sort of information, it becomes possible to examine a complex mixture of substances without prior separation of its components. It is possible to look highly specifically for trace components in mixtures under circumstances in which other techniques could not succeed. Finally, it is possible to gain information on the molecular structures of unknown compounds, as in peptide and protein sequencing (see Chapter 40). 

An alternative approach to peptide sequencing uses a dry method in which the whole sequence is obtained from a mass spectrum, thereby obviating the need for multiple reactions. Mass spec-trometrically, a chain of amino acids breaks down predominantly through cleavage of the amide bonds, similar to the result of chemical hydrolysis. From the mass spectrum, identification of the molecular ion, which gives the total molecular mass, followed by examination of the spectrum for characteristic fragment ions representing successive amino acid residues allows the sequence to be read off in the most favorable cases. 

Examples illustrating the reactions 21-23 are given in Figures 10-12. Shown in Figure 10 is the CID mass spectrum for the desolvation of Ni2+(H2O)10. The sequence of product ions Ni2+(H20) where n = 9 to n = 4 illustrates the sequential solvent loss represented by equation 21. The CID spectra in Figure 11 demonstrate that for the n = r = 4, charge reduction via internal proton transfer (see equation 23)... 

As an example of the form of the information that may be derived from a pyrolysis-MS, Figure 26 [69] shows the structure of the polycarbonate (PC) and the EI-MS spectra of pyrolysis compounds obtained by DPMS of poly(bisphenol-A-carbonate) at three different probe temperatures corresponding to the three TIC (total ion current) maxima shown in Figure 27(b) Figure 27 compares the MS-TIC curve with those obtained from thermogravimetry. (The TIC trace is the sum of the relative abundances of all the ions in each mass spectrum plotted against the time (or number of scans) in a data collection sequence [70].)... 

Fourier transform ion cyclotron resonance (FTICR) analyzer is excellent for MSn measurements (see Section 2.2.6), perhaps even more so, since the ions remain in the cell after detection. In principle one injection of ions is enough for a whole MSn sequence, including acquisition of a mass spectrum of each step. 

The assignments of sequence ions observed in the FAB mass spectrum of CB-l and its methyl ester are given in Table I, and suggested that the sequence of CB-l was ... 

Figure 6.2B shows the mass spectrum of the second major chromatographic peak (residue 60-70 peptide), which contains the triply charged (m/z 451.3) and doubly charged (676.7) molecular ions. Thus, a molecular weight of 1351 is assigned. The next step involves the selection of either one of the molecular ions (m/z 451.3, 676.7) to obtain amino acid sequence information by using MS/MS. The... 

The ion CsHsMnCNCoHn from the cyclohexyl isocyanide complex undergoes successive losses of hydrogen cyanide and cyclohexene in either possible sequence. The following processes exemplify unusual fragmentations observed in the mass spectrum of the dimethylsulfoxide complex ... 

The mass spectra of some cyclopentadienylrhodium olefin complexes have been investigated 80>. The molecular ion in the mass spectrum of the ethylene complex CsHsRh H undergoes fragmentation to the bare metal ion by the following sequence ... 

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