Mass Spectra of Common Compound Classes

Now we shall attempt to apply the principles set forth above to the interpretation of common types of organic compounds. It is important for you to gain an appreciation of the reliability—and lack of reliability—of the various types of mass-spectral information in providing evidence concerning various structural features. Note that there is an exponential increase in the number of possible fragmentation pathways with an increase in the number of reaction-initiating sites this can be seen by comparing the behavior of esters with that of either ketones or ethers. 

This chapter is intended to illustrate how the mechanisms can be applied to particular compounds, not to give a comprehensive catalog of the mass-spectral behavior of organic structures. E etailed correlations have now been published of the mass spectra of a wide variety of molecular classes. In many of these studies, isotopic labeling and other special techniques have been employed to elucidate mechanistic pathways. An excellent summary of this literature to 1967 is available in the comprehensive volume of Budzikiewicz, Djerassi, and Williams (1967). We recommend it as a basic reference the compound classifications in this chapter are patterned after it. A wealth of specific material is to be found in the specialized references of Section 1.9. 

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Since the reference mass spectra of known compounds have been run previously for a number of years, correlations of SI mass spectra with structure can be made for many of the common classes of organic compounds. Most of these correlations emphasize the spectral pattern or simple decomposition pathways to be expected for a particular molecular structure. This has led to the tabulation of mass spectral correlations to provide empirical and structural formulas of ions that might be found at a particular m/z in a mass spectrum, plus an indication of how each such ion might have arisen. Such a table has been reported elsewhere [10]. 

The mass spectra of compounds described in this chapter usually show the molecular ion in traditional techniques. For instance, 10 shows the molecular ion peak 237 (MH+) by FAB. Other mass spectroscopy techniques have been successfully used for compounds in this class, such as electrospray ionization (ESI) (MNa+) <2006T9043> or El (M+) <2000BMC557>. Nowadays mass spectroscopy has become a common tool in organic chemistry and mass spectral data are presented for most structures described in this chapter. 

Solvents and their impurities represent a wide class of compound types therefore, a discussion of common mass spectral features is meaningless. However, most of the mass spectra are listed in computer library search programs and The Eight Peak Index. ... 

Similarities between mass spectral and thermal fragmentations are particularly common in certain reaction types. Electrocyclic reactions, for example, are frequently similar in the two processes. The thermal process has in general a higher stereoselectivity (because of the higher aromaticity in even-electron systems). Retro-Diels-Alder reactions are typical examples for the similarity of the two processes. Internal displacement reactions may also be similar in the two processes, mainly in the case of internal radical displacements. The relationship between mass spectra and thermal fragmentation is complex, and it is useful to discuss it for separate classes of compounds. 

GC has been widely used for amine analyses because of its simplicity, high resolving power, high sensitivity, and low cost. Coupled with mass spectrometry (GC-MS), it is a technique most commonly employed for the analysis of volatile organic pollutants in environmental samples. In this combination, the GC separation usually provides isomer selectivity, while the MS shows compound class homologue specificity. The MS fragmentation pattern can provide unambiguous component identification by comparison with library spectra. 

It is evident from this chapter that there are many examples of methods for the analysis of antibiotic residues in food that utilize mass spectrometry. As a result, the fragmentation patterns for different classes of antibiotics have been proposed and described in several multi-residue methods, as well as in procedures for specific groups of compounds. Table 6.4 and Figure 6.14 provide examples of the common product ions and expected neutral losses seen in MS/MS spectra for major classes of antibiotics. Specific examples, along with relevant citations, are also provided. As MS methods begin to search for and identify more non-targeted analytes, it will become more important to be familiar with the fragmentation patterns of common analytes. 

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