Normal mass spectrum

Each bin is connected to a memory location in a computer so that each event can be stored additively over a period of time. All the totaled events are used to produce a histogram, which records ion event times versus the number of times any one event occurs (Figure 31.5).With a sufficiently large number of events, these histograms can be rounded to give peaks, representing ion m/z values (from the arrival times) and ion abundances (from the number of events). As noted above, for TOP instruments, ion arrival times translate into m/z values, and, therefore, the time and abundance chart becomes mathematically an m/z and abundance chart viz., a normal mass spectrum is produced. 

The steps (reactions) by which normal ions fragment are important pieces of information that are lacking in a normal mass spectrum. These fragmentation reactions can be deduced by observations on metastable ions to obtain important data on molecular structure, the complexities of mixtures, and the presence of trace impurities. 

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). 

The normal-mass spectrum of the mass interval selected. The graph corresponds to a collection of signals in the form of peaks of different width, depending upon the resolution of the instrument (Figure 16.1c). These peaks lead to the determination of the ion masses and can attain a precision better than 10 parts in a million (10 Da). The upper mass limit, which is constantly being improved, can exceed 10 Da. 

In gas chromatography coupled with mass spectrometry a procedure has been given, how to separate the inadequate resolved peaks [7,8]. It is remarkable that the mass spectra of the individual peaks that are overlapping need not to be known. Such a procedure is only possible, because in a mass spectrum of an individual compound there is so much information inside. The mass spectrum in a gas chromatographic peak is, for each mass number, the sum of the concentration of each individual compound, weighted by the relative intensity of the respective mass number and the relative amount of the respective compound. In the procedure, a normalized mass spectrum is constructed as a vector with the relative intensities of the mass numbers. Here it is necessary that the number of the mass numbers constituting a compound is equal for each compound, i.e., all the vectors should have the same dimension. Because of the formalism of the matrix multiplication, the overlapped spectrum is the product of the matrix of the spectra and the concentration vector. The overlapped spectrum is also addressed as matrix of the observable intensities... 

The possibility of comparing disappearance rate constants of reactant ions with appearance rate constants of product ions offers obvious advantages for the characterization of reaction pathways. In general, however, the task of collecting useful rate data is harder at high pressures than at low pressure, since additional complications arise. Additional assumptions are also necessary, for example, that it is valid to plot the normalized mass spectrum as a function of pressure. Surprisingly, perhaps, in view of what has been said about the uncertainty of detection efficiencies, this particular procedure seems to work rather well. ... 

Figure 3.22. (q) Time-domain free-ion decay signal and (b) Fourier-transformed normal mass spectrum. (Reproduced from ref. 61 by permission of WUey-Interscience, cop3right 1996.)... 

The product-ion scan (the old, now-unaccepted term, still used by some, is daughter-ion scan) is the most common mode of MS/MS operation. That spectrum is useful in the structure elucidation of a specified analyte. Information obtained in this scan is similar to that derived from a normal mass spectrum, except that the spectrum contains only those product ions that are formed exclusively from a mass-selected precursor ion. To acquire this spectrum, the first mass analyzer is set to transmit only the precursor ion chosen, and the second mass spectrometer is scanned over a required miz range. As an example, the ketene radical cation reacts with neutral ketene to form a cycloadduct, the structure of which was delineated by mass-selecting the mIz of the adduct (at 84) and acquiring the product-ion scan (see Figure 4.3). 

Since multiphoton absorption generally produces molecular fragmentation patterns quite different from those of the normal mass spectrum, this method additionally provides useful insight into the dynamics of multiphoton excitation. Multiphoton ionisation and laser mass spectrometry have been reviewed [236]. 

FIGURE 9.8 MALDI mass spectra of a-cyano-4-hydroxy-cinnamic acid recorded on the dual-stage reflec-tron tandem mass spectrometer, (a) Normal mass spectrum obtained through both reflectron analyzers, (b) mass spectrum after gating of the molecular ion, and (c) product-ion mass spectrum after activating the pulsed valve. (Reprinted with permission from reference 6). 

The fragment ion M2+ from metastable Mi + will thus appear in the normal mass spectrum at an apparent mass... 

The limitations of the structural information in the normal mass spectrum can be partly offset by special mass-spectral techniques. Although a complete description of these is beyond the scope of this book, you should have a basic familiarity with their capabilities. 

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