Spectral Interpretation Some Examples

The general appearance of the mass spectmm depends on the type of compound analyzed. Seeing the patterns that distinguish, say, a normal alkane from an aromatic hydrocarbon. 

Source. From Mass Spectral Interpretation Quick Reference Guide, Varian, Inc., courtesy of Varian, Inc. (www.varianinc.com). 

Now consider the mass spectrum in Fig. 10.11 and compare it to the carbon dioxide spectmm. The spectrum in Fig. 10.11 is one that every mass spectrometrist should know. It is the spectmm of air, a mixture of components. (Why should a mass spectrometrist recognize this spectmm ) The peak at m/z = 44 is from carbon dioxide, while the other peaks are due to water (18), nitrogen (28), and oxygen (32). The peak at m/z 28 in the CO2 spectmm is due to a CO ion, whUe in the air spectmm, it is due to nitrogen plus any carbon monoxide that is in the sample. At this resolution, it is not possible to distinguish between N2 and CO. It is important to remember that not all mass spectra are of a single pure component. In the real world, mixtures and impure samples are much more commonly encountered. 

A mass spectrum of air, a mixture of gases. (Courtesy of Dr. Robert Kobelski, CDC, 

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In this chapter, we will first describe what the Mossbauer effect is, and then explain why it can only be observed in the solid state and in a limited number of elements. Next, we discuss the so-called hyperfine interactions between the nucleus and its environment, which make the technique so informative. After some remarks on spectral interpretation, we will pass systematically through a number of examples which illustrate the type of information that Mossbauer spectroscopy yields about catalysts. 

In the analysis of chemicals related to the CWC, spectral interpretation is not done with only one technique. Often MS or NMR spectra are used together with IR spectra in the elucidation of the structure of an unknown chemical. There are some examples of this type of elucidation available in the literature (5-25-26-51). 

Despite the general improvement in spectral dispersion observed at high field-strengths, some examples of monosaccharide derivatives have been reported86-88 for which 220-MHz spectra failed to give more readily interpretable results, or more information, than had been obtained at 100 MHz. 

Another piece of information is obtained from the relative area of the absorption peaks in the spectrum, which teUs us the relative number of protons in each group. So, a proton NMR spectmm should be examined for (1) the number of proton resonances which tells you how many different types of protons are in the molecule (2) the chemical shifts of the resonances which identifies the type of proton (3) the multiplicity of the resonances which identifies the adjacent equivalent protons and (4) the intensity (area) of the resonances which tells you the relative number of each type of proton. Some examples of common classes of organic compounds are discussed. Not every type of compound is covered in this brief overview. Students needing more detailed spectral interpretation should consult the references by SUverstein and Webster, Pavia et al., Lambert et al., or similar texts. 

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. 

Flowever, we have also seen that some of the properties of quantum spectra are mtrinsically non-classical, apart from the discreteness of qiiantnm states and energy levels implied by the very existence of quanta. An example is the splitting of the local mode doublets, which was ascribed to dynamical tiumelling, i.e. processes which classically are forbidden. We can ask if non-classical effects are ubiquitous in spectra and, if so, are there manifestations accessible to observation other than those we have encountered so far If there are such manifestations, it seems likely that they will constitute subtle peculiarities m spectral patterns, whose discennnent and interpretation will be an important challenge. 

If some iron(III) complexes undergo rapid spin interconversion on the Mossbauer time scale, and some undergo slow interconversion, then it is inevitable that a few will interconvert, at some accessible temperature, at a rate which produces dynamic effects on the Mossbauer spectrum. Such examples have now been found (109, 111). Rate constants have been extracted from these spectra and are necessarily of the order of 106 -107 sec"1. The interpretation of the spectral lineshapes is complex (153, 154), however, and further work will be needed to establish the reliability of the rate data obtained from such spectra. 

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