Isotope clusters

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... 

Isotopic clusters are especially apparent when atoms such as bromine and chlorine are present m an organic compound The natural ratios of isotopes m these elements are... 

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 )... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Knowing what to look for with respect to isotopic clusters can... 

Isotopic cluster (Section 13.22) In mass spectrometry, a group of peaks that differ in mh because they incorporate different isotopes of their component elements. 

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

In addition to the study of the isotopic cluster, useful information on analyte identity can be obtained by the nitrogen rule. It states that if a molecule contains an odd number of nitrogen atoms, its molecular weight must be an odd number. However, if an even number of nitrogen atoms or no nitrogen is present, its molecular weight must be an even number. 

Figure 5.20 represents partial spectra (high m/z region) of tetrachlorobiphenyl and tetrabromobiphenyl. Isotopic clusters with 4, 3, and 2 atoms of chlorine (bromine) are clearly visible. 

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. 

The retention time results for 35 chlorinated compounds found in sewage sludge are given in Table 9.9. Not all compounds could be identified. A large number of spectra contained what appeared to be chlorine isotope clusters which are not reported. This could be due to interferences, very low levels, or spurious peaks. Although no structure... 

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. 

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... 

Additionally, mass spectrometry methods have been successfully applied for the identification of other Zn-containing complexes. Among them are compounds containing two Zn atoms (30-32), as well as some complexes that exist as dimers in the condensed phase according to different physical methods " (33-42). For all these compounds, isotope clusters with isotopic distributions similar to the graphical representations in Figure lb (for [Zni] ) were expected, provided that the binuclear species or dimer entities are able to survive the ionization process. 

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).

Most elements possess stable isotopes. A compound analysed by mass spectrometry will lead to an isotope cluster whose relative intensities can be calculated. 

Now if a molecule contains more than one chlorine atom, the appearance of isotope clusters can be calculated by the probabilities of isotope distributions and the natural abundances of die isotopes. For example, if a molecule contains two chlorine atoms such as o-dichlorobenzene, dien diere will be peaks at M, M + 2, and M + 4 for molecules which have two 35C1, one 35C1 and one 37C1, and two 37C1 (Figure 11.48). 

The relative intensities of diese peaks can be calculated by taking into account die number of combinations diat can give die required isotopic substitution and die probability of an isotope being present. An M + 2 peak in die above example will result if either of die chlorine atoms is 37C1 tiius die intensity of an M + 2 peak will be 2 x (1/3.058). An M + 4 peak will occur only if both chlorine atoms are 37C1 thus die intensity of die M + 4 peak will be 2 x (1/3.058)2. The squared term follows from die necessity tiiat both chlorine atoms must be 37C1. Thus the intensities of die peaks in die isotopic cluster of the molecular ion are (approximately) given as... 

The presence of isotopic clusters is particularly clear for molecules containing chlorine or bromine because of die abundance of two isotopes. The same considerations are applicable, however, for other elements tiiat have smaller abundances of higher isotopes. These natural isotopic abundances are given in Table 11.2. As can be seen, 13C is present to die extent of 1.08% of 12C while 2H is present only to die extent of. 016% of 1H. Now if a molecule such as benzene is examined,... 

Measurement of the isotopic cluster of the molecular ion showed an M + 1 peak of 5.30% and an M + 2 peak of 0.15% of the molecular ion. From tables of isotopic abundance ratios it was found that the expected product C4H8N2 should give M + 1 and M + 2 peaks of 5.21 and 0.11%, respectively, while cyclohexane C6H12 should give M + 1 and M + 2 peaks of 6.68 and 0.19%, respectively. It is clear that the isolated product is most likely the expected cyclic azo compound and not cyclohexane. 

---