Features of the Mass Spectrum

The molecular ion, represented by M , is the intact molecule with one electron missing. This should be the peak in the spectrum with the largest mass, but it is not always observed. All spectra have an ion that we call the base peak. This is the most abundant peak in the spectrum m/z 44 in the spectrum of N2O or m/z 69 in the spectrum of PFTBA). In the next section you will find that more than one peak may correspond to the M or fragment ion when that ion contains elements with different isotopes. We use the term nominal mass to describe the mass of the molecule in terms of the most abundant (and, generally, the lightest) isotopes of the element. Relative isotopic abundances for common elements are summarized in Table 8.36. 

If you look at Table 8.36, you will find that fluorine is an isotopicaUy pure element however, 1.1% of carbon is the isotope. You may recall that it is this low abundance of that you measure with NMR. For nitrogen there is a small amount of When a molecule contains more than one atom of an 

Compound molecular weight is a valuable piece of information that is not always available from the NMR or IR spectrum. In this section we discuss some things to consider as you examine a mass spectrum and attempt to identify the M ion. If you look back at the mass spectra that have appeared above, you will find examples of spectra where the molecular ion is the base peak in the spectrum. In some other cases the molecular ion may be weak or not observed at all For example, PFTBA fragments extensively and the molecular ion does not appear in the spectrum. The following are some things to consider as you attempt to identify the molecular ion. 

Another useful feature to consider is the nitrogen rule. For most elements found in organic molecules, the compound molecular weight will be even if the compound has an even number of nitrogen atoms (remember, zero is an even number). In contrast, the mass will be odd if the compound contains an odd number of nitrogen atoms. If you are sure that your product could not contain nitrogen, then an odd mass ion could not correspond to the molecular ion. 

Mass spectrometrists also use softer Tonization techniques to obtain MW information. These techniques included measuring El mass spectra at lower 

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Oxostephasunoline (4) was isolated from the roots of Stephania japonica(4). The UV spectrum of oxostephasunoline (4) showed an absorption maximum at 286 nm, and the IR spectrum depicted bands at 3550,3500, and 1670 cm, indicating the presence of a hydroxyl group and a y-lactam. The mass spectrum (Table VI) exhibited the most abundant ion peak at m/z 258, and the H-NMR spectrum (Table II) revealed the presence of three methoxyl and one N-methyl group. The downfield shift (53.06) of the JV-methyl resonance indicated that oxostephasunoline (4) was a y-lactam, which was further supported by the IR band at 1670 cm 1, significant features of the mass spectrum (Table VI), and the 13C-NMR spectrum (Table III). On exhaustive H-NMR analysis similar to the case of stephasunoline (17), the structure of oxostephasunoline (4) including the stereochemistry was practically proved (4). 

The main features of the mass spectrum of benzo[6]thiophene have been established by Porter,121 using the spectrum of 3-deutero-benzo[6]thiophene for comparison purposes. Triply charged ions have been detected in the mass spectra of benzo[6]thiophene and its 2- and 3-methyl derivatives.126... 

No evidence is available which indicates whether the neutral fragment eliminated in (1) is cyanic, isocyanic, or even fulminic acid or whether the neutral fragment eliminated in (3) is phenyl isocyanide or benzonitrile. These features of the mass spectrum of... 

In 1970, Clampiti and Jefferies [106] reported their molecular beam experiments for various ion clusters. For Ne Li", they found the largest peak for m = 1 followed by nearly identical signals for m = 2, 3, 4, 5, 6. The yield of ion clusters decreases abruptly beyond HcgLi". The same result was found for hydrogen clusters (H2) Li. A remarkable feature of the mass spectrum was the relative insignificance of clusters with m > 6. It was concluded [106] that six molecules around an ion probably constitute a complete shell . This result is important in comparison with theoretical studies of Ng Be " ions. 

In your answer, use features of the mass spectrum of Figure 20-27 to illus-Iraie your description. 

Another prevalent feature of the mass spectrum, which is often more of a problem than elemental isobaric overlap, is the occurrence of peaks attributed to ionized molecular species. The formation of molecular ion species is discussed in Chapter 2. As discussed there, usually only diatomic homogeneous molecular species, such as, and diatomic heterogeneous molecular species, such as ArO", are observed in the mass spectrum from the ICP, although some triatomic species, such as ArOH" ", are observed when the components that form these polyatomic molecules are present at high concentrations. When molecules are present, only singly charged ions are observed. The principal m/z value observed in the mass spectrum for the molecular species is determined by the sum of the most abundant iso-... 

Similarly, a common feature in the mass spectrum of thiirene oxides is the high abundance of the substituted acetylene ion (e.g. [PhC CPh]7) formed by elimination of sulfur monoxide. In fact, this ion constitutes the base peak in the spectrum of 18a whereas the molecular ion has a rather insignificant intensity (0.25% I of M+)91. 

The anomeric phenyl glycosides (5) and (6) afford practically identical mass spectra. A characteristic feature of the mass spectra of the phenyl glycosides is the strong increase of intensity of the series A fragments (m/e 219, 187, 155) and of the E (m/e 111) fragment. These ions are formed by elimination of the phenoxyl grouping as phenol or as the C H60 radical the latter is rather stable, due to interaction of the odd electron with the benzene ring. At the same time, peaks of the series B, C, D, and E (except E ) become less intense, or even disappear completely from the mass spectrum. 

General (PAF and lysoPAF). The basic objective here is to examine the spectrum of a PAF sample subjected to fast atom bombardment-mass spectrometry (FAB-MS). The predominant feature of the resulting spectrum is the formation of a protonated mass ion. In actual fact, there is some cleavage of the molecule, but it is not extensive. These ions can, however, be used for diagnostic purposes. 

Due to their volatility, a number of stannocene and plumbocene derivatives have been studied by mass spectroscopy. The dominant feature in the mass spectrum of Cp2Sn, Cp2Sn BF3 (39), CpSnCl (31), (MesC5)2Sn (2/), Cp2Pb, CpPbCl, CpPbBr, and CpPbl (18) is the R M ion (R = Me or H). Apparently, redistribution occurs when CpMX is volatilized since... 

After observation of a molecular or pseudomolecular ion, the most important feature of a mass spectrum is the fragmentation behavior, i.e., the differences between the observed ion mass-to-charge ratios and their relative abundances. The rules for structural elucidations for electron ionization (El) are well known and accessible. The reader is referred to McLafferty and Turacek (1993) either as an introduction or as a refresher.30... 

The second problem is that ion-ion or ion-neutral reactions can occur. Reactions (e.g., proton transfer) result in high abundance of protonated molecular ions in the mass spectrum. Thus QIT can be disadvantageous for determining chemical composition in manual spectral interpretation, because the presence of the (M + l)" ion tends to confuse the interpretation. Library spectral matching, however, is not affected if the spectral matching algorithm reflects the unique features of the QIT spectrum. An external ionization source with ion injection into the QIT is an alternative solution, because only ions are present in the trap (i.e., neutral analyte molecules that could participate in ion-molecule reactions are not present). The Thermo Finnigan PolarisQ GC/MS is an example of such an instrument. 

The raw and deconvoluted mass spectra from scans 2000-2025 of the eighth protein fraction are shown in Figure 5.9a and b, as an example of proteins that are not isotopicafly resolved. The most intense feature of the deconvoluted spectrum in Figure 5.9b, 16293 Da,... 

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