Isotopic cluster ions

The ehallenge in this analysis lies in the efficient extraction and analysis of the complex matrices in which PBDEs are found. The compounds are used in a variety of plastic materials found in consumer products, and samples are prepared typically using either Soxhlet extraction or microwave digestion, followed by few or no clean-up steps. MS/MS can be used to isolate M + and [M-2Br] + isotopic cluster ions for accurate identification and quantitation. Three stages of the MS/ MS analysis of a PBDE are shown in Eigure 15.43. Eigure 15.44 shows a TIC of the common PBDE isomers that are tested for under the RoHS regulations these common PBDE isomers are listed in Table 15.2. Excellent quantitative results... 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

Knowing what to look for with respect to isotopic clusters can aid in interpreting mass spectra How many peaks would you expect to see for the molecular ion in each of the following compounds At what m/z values would these peaks appear (Disregard the small peaks due to and )... 

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

Figure 2.17 Comparison of the isotopic clusters of the molecular ion for (a) methane, (b) ethane, (c) chloromethane and (d) decane...

Cover Illustration Atomic force microscopy image of molybdenum oxide particles on flat, silicon dioxide substrate, which serves as a model system for a supported catalyst. The area shown corresponds to one square micrometer the maximum difference in height is approximately 10 nanometer. The superimposed curve is the secondary ion mass spectrum of the model catalyst, showing the caracteristic isotopic patterns of single molybdenum ions and of molybdenum oxide cluster ions. 

The idea behind the use of chemical labeling is a straightforward approach to simplify peak assignment into the proper ion series. If either peptide s C- or N-terminus is labeled by specific isotopic cluster, the labeled ions are easily recognizable on the spectrum and determination of the sequence string is no longer a problem. 

Note Some authors use the term isotopic cluster, which is incorrect, as cluster refers to an associate of more atoms, molecules or ions of the same species, sometimes associated to one other species, e.g., [Ar ], [(H20) H], and [I(CsI) ] are cluster ions. 

The relevance of oxygen and sulfur isotopic patterns is nicely demonstrated by the cluster ion series in fast atom bombardment (FAB) spectra of concentrated sulfuric acid, where the comparatively large number of sulfur and oxygen atoms gives rise to distinct isotopic patterns in the mass spectrum (Chap. 9). 

Note Csl and Au both bear the advantage of being monoisotopic. This insures the peak top to exactly represent the theoretical isotopic mass of the respective cluster ion, independent of its m/z ratio or actual resolution (Chap. 3.3.5, 3.4). Csl, KI, and other alkali salts providing more narrow-spaced cluster ion series can alternatively be employed as saturated solutions in glycerol. [45-47]... 

Example The Bunte salt [ 3( 2)158-803] Na yields a very useful negative-ion FAB spectrum from NBA matrix (Fig. 9.11). NBA forms [Ma-H] and Ma " ions. The salt anion contributes the base peak at m/z 337.3. [Ch-2A] m/z 697.5, and [2C-I-3A] m/z 1057.6, cluster ions are observed in addition, their isotopic patterns being in good agreement with theoretical expectation. It is noteworthy that the matrix adduct at m/z 513.4 is a negative radical ion. 

Fig. 9.11. Negative-ion FAB mass spectra of a Bunte salt. The insets compare experimental and calculated isotopic patterns of the [C-I-2A] and [2C-I-3A] cluster ions. By courtesy of M. Grunze, University of Heidelberg.

In seeking to identify a newly prepared compound by mass spectrometry, the observation of the parent or molecular ion isotopic cluster is usually greeted with satisfaction, but observation of apparently lower or higher mje values for the parent ion causes initial confusion. However, even in a series of clotely related compounds, run in the same... 

A detailed theoretical analysis further revealed that this reaction can be considered to proceed as a barrier-free H atom abstraction without any significant additional interaction between the methyl radical formed and the remaining cluster ion. Consistent with this mechanistic interpretation, the kinetic isotope effect associated with C - H(D) bond activation of CH2D2 is rather low (KIE = 1.35). 

Fig. 2.17 Portion of a pulsed-laser time-of-flight spectrum showing the formation of doubly charged Mo diatomic cluster ions in pulsed-laser stimulated field evaporation of Mo. From a multinomial expansion analysis, it is concluded that few Mo+ ions are formed. Although there are a total of only 438 ions in the entire spectrum, all the expected 15 mass lines due to an isotope mixing of the 7 Mo isotopes are present. A mass line, even when it contains only three ions such as that of Min = 100, is clearly identifiable.

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).

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