Mass Spectral Interpretation Some Examples

The general appearance of the mass spectrum depends on the type of compound analyzed. Seeing the patterns that distinguish, say, a normal alkane from an aromatic hydrocarbon requires practice and lots of it. The following examples include simple gas molecules and simple compounds representative of a variety of organic chemicals. 

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An El spectrum comprises a mixture of ions of types A and C which may contain some molecular ion (Fig. 5.3). To complicate matters, the ions A and C may also fragment further to produce smaller ions. In these processes, the ion which fragments is known as the precursor or parent ion, whilst the smaller ions formed are known as the product ions. Understanding the relationship between precursor and product ions is at the heart of mass spectral interpretation and deducing the structure of the original molecule. A simple example is shown (Fig. 5.4). 

The need for a cationization reagent in MALDI analysis of polymers can also create some complications in mass spectral interpretation [42, 56]. For example, the spectrum in Figure 8.3b shows a secondary distribution of lower intensity in addition to the principal distribution. This secondary distribution could be due to cation adduction with different ionic species and/or the presence of other polymeric species with different end-group structures. In this case, the secondary distribution has oligomer mass shifts of -1-22.4 Da from the nearest oligomer of lower mass in the principal distribution. This is consistent with the generation of salt cluster complexes, similar to what has been observed in the ESI of polystyrene [57]. For polyisoprene, it takes the form of [polyisoprene-i-Cu(copper retinoate)]L This amounts to an actual mass shift of -1-363.0 Da with respect to the principal distribution of [polyisoprene-tCujL This is consistent with the observed mass shift of 22.4 Da plus six repeat units of 64.2 Da. 

The eudistomins from E. olivaceum extract readily into a non-polar phase, toluene (26-28) or dichloromethane (29), and require repeated chromatographic steps to separate one from another, especially within a related class. Separation in some cases is especially problematic (28, 29) for example, eudistomins N (3) and O (4) proved inseparable at the time they were reported. Structure elucidation relied heavily on interpretation of spectral data, especially 500 MHz NMR and mass spectra (26) several members of the series have now been synthesized (see Section 3). Eudistomin K has been crystallized and subjected to X-ray analysis (32) revealing the absolute configuration of the oxathiazepino-eudistomins. 

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