Molecular ion cluster

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

Fig. 6.—One of the Molecular-ion Clusters Obtained from a Sample of Deuteropermethy-lated. Cyclic /3(l- 2)-Glucans (see Section VI,S). [All high-mass samples give unresolved clusters of this type if the mass spectrometer is operated at low resolution. The peak is 6 mass units wide at half height. The mass is assigned by using the mass marker, which gives marks every 4 mass units, as shown. The center of the peak corresponds to the chemical molecular weight of an [M + NKJ species.]...

Establishing the elemental composition based on the isotopic peaks may be problematic if, for example, the sample contains impurities with the masses in the region of the molecular ion cluster. In the El mass spectra of amines, alcohols, acids, and some other classes of organic compounds there is often a peak of [M + H]+ ion. It distorts the isotopic picture. It is worth mentioning as well that in real experimental conditions the peak intensity may vary slightly in each... 

Let us calculate the intensities of the molecular ion cluster of an acetophenone sample, which contains 10% of unlabeled molecules, 60% of molecules with three deuterium atoms, 20% with two deuterium atoms, and 10% with one deuterium atom. Let us create a table (Table 5.6), taking into account that the molecular ion cluster of pure unlabeled acetophenone is as follows 120 (100%), 121 (8.8%), 122 (0.54%). The calculations with the help of Table 5.6 are quite straightforward, while the final line represents the labeled sample spectrum. 

When all the available information is extracted from the molecular ion cluster it is possible to pass to the fragment ions. As mentioned above the fragment ions arise due to simple cleavages of the chemical bonds and to rearrangements. For the interpretation... 

DDT converts in part to p,p -DDE over time in the environment, especially in sediments [151, 171]. An example of the total aliphatic extract of a sediment from the Los Angeles Bight contaminated with p,p -DDE is shown in Fig. 11. The TIC trace shows a major UCM and the minor resolved peaks are normal alkanes (primarily from higher plant wax), with mature 17a(H),21/ (H)-hopanes (from petroleum residues as is the UCM). The mass spectrum ofp,p -DDE is shown in Fig. 12a, registering the molecular ion cluster at m/z 316-320. DDE is detected in the m/z 246 fragmentogram (Fig. lid), appearing as a small peak in the TIC trace and DDT is not detectable in this sample. 

Figure 6.29 shows mass spectra recorded during elution reduced to a two-dimensional contour plot. Each point is produced from pseudo-molecular ions, cluster formation or fragmentation. AU ions eluting in parallel with respect to time, at c. 29 min are assumed to belong to the main component, but there are some points clearly seen on the front edge of the main peak that indicate the presence of an impurity. This was confirmed by the production of a mass chromatogram of miz 486. 

FIGURE 2.12. p-Chlorobenzophenone. The M peak is arbitrarily set in the table above at intensity 100% for discussion of the molecular ion cluster. 

Figure 16.20—Multiply charged molecular ions. An electrospray spectrum of horse cytochrome c, a protein of molecular weight 12360 Da is shown. Between two consecutive peaks in the molecular ion cluster, the charge state varies by one unit. The second spectrum corresponds to a high-resolution spectrum in the 772-774 m/z range. In this isotopic cluster, all ions carry the same number of charges. It is possible from either of these spectra to calculate the approximate molecular weight and the number of charges carried by the ions (spectra reprinted with permission from F. W. McLafferty et al.. Anal. Chem., 1995, 67, 3802-5. Copright 1995 American Chemical Society).

Alteration of instrumental conditions may also provide evidence to confirm the recognition of the molecular ion. The use of maximum sensitivity may show up a very weak molecular ion. Alternatively, if the energy of the electron beam is decreased the intensity of the fragment ions will decrease relative to the molecular ion this also applies to fragment ions arising from impurities. Alternative methods of ionisation such as chemical ionisation and field ionisation are very much more likely to produce a molecular ion cluster than the electron ionisation method, and should be used if they are available. 

Table 8 Molecular Ion Clusters for Five-membered Heterocycles Containing More Than One Selenium or...

Chelation of copper with tetraphenylporphyrin was first reported as a technique for measuring copper in rat brain by isotope dilution O) This method appeared to be adaptable for measurement of isotopic enrichment in other biological samples Tetraphenylporphyrin reacts quickly and quantitatively to form chelates with nearly all of the transition metals (M The mass spectrum of a TPP chelate is dominated by the molecular ion cluster (Figure 1) the molecular ion is also the base peak so it is well-suited for quantitation of isotope ratios ... 

Table 8. Theoretical ion masses and abundances of molecular ion clusters of PCDEs...

Sometimes ligands themselves may fragment. For example, the electron impact spectrum of the dithiolate-bridged compound illustrated on the next page exhibits only a trace of a parent molecular ion cluster centered at tn/z 534, but has an intense cluster at 478 corresponding to loss of ethylenes (mass of 28 each)... 

The intensities of peaks with an abundance of more than 2% of the molecular ion clusters of (3) have been observed and compared to calculated intensities <80TL4565>. 

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