Chlorine isotope pattern

The advantage of stable isotope patterns for metabolite identification is the ease of identification of metabolites with the same specific isotope patterns as parent drug in a complex biological sample. For example, chlorine and bromine exhibit unique natural isotopic patterns. Chlorine or bromine-containing compounds will have similar isotopic ratio patterns arising from Cl Cl... 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

For other elements that occur with major relative abundances of more than one isotope in the natural state, the isotope pattern becomes much more complex. For example, with chlorine and bromine, the presence of these elements is clearly apparent from the isotopes Cl and for chlorine and Br and Br for bromine. Figure 47.2a shows the molecular ion region for the compound chlorodecane. Now, there are new situations in that C, C, C1, and Cl isotopes all have probabilities of occurring together. Thus, there are molecular ion peaks for + Cl, C + Cl, + Cl, and so on. Even so, the isotopic ratio of 3 1 for Cl to Cl is very clear... 

Partial mass spectra showing the isotope patterns in the molecular ion regions for ions containing carbon and (a) only one chlorine atom, (b) only one bromine atom, and (c) one chlorine and one bromine atom. The isotope patterns are quite different from each other. Note how the halogen isotope ratios appear very clearly as 3 1 for chlorine in (a), 1 1 for bromine in (b), and 3 4 1 for chlorine and bromine in (c). If the numbers of halogens were not known, the pattern could be used in a reverse sense to decide their number. 

The mass spectrum of the unknown compound showed a molecular ion at m/z 246 with an isotope pattern indicating that one chlorine atom and possibly a sulfur atom are present. The fragment ion at m/z 218 also showed the presence of chlorine and sulfur. The accurate mass measurement showed the molecular formula to be C]3FI7OSCl R + DB = 10. 

The presence of chlorine and/or bromine is easily detected by their characteristic isotopic patterns (see Appendix 11). As in many aliphatic compounds, the abundance of the molecular ion decreases as the size of the R group increases. For example, in the El mass spectra of methyl chloride and ethyl chloride, the molecular ion intensities are high, whereas in compounds with larger R groups such as butyl chloride, the molecular ion peak is relatively small or nonexistent. 

Figure 17.2 is an example of a mass spectrum of an aromatic dichloro compound. The intensity of the molecular ion indicates that an aromatic compound is present. The isotope pattern is that of two chlorines, and subtracting 70 mass units from the molecular ion gives the formula QHj. (See Example 2.3 in Chapter 2 for another example of isotope abundances in the molecular ion region.)... 

The molecular ion is apparent in the mass spectrum of DDT (Figure 25.2) at m/z 352 with the classic isotope pattern for five chlorine atoms (see Appendix 11). The major fragment ion is the loss of CCI3 at m/z 235. 

Figure 19.9 compares the observed and library spectra for dichloro-methane (retention time 3.45 min in the GC-MS run). The prominent chlorine isotope pattern for the two chlorine atoms in this spectrum makes it readily identifiable. The primary fragmentation is loss of a chlorine atom, producing the m/z 49 fragment. While this fragment clearly manifests a chlorine isotope pattern still, it reflects the fact that only one chlorine atom remains. Library search identifies this spectrum as dichloromethane with quality-of-fit measures of greater than 95%. 

Fig. 3.1. Isotopic patterns of chlorine, bromine and xenon. The bar graph representations of the isotopic distributions have the same optical appearance as mass spectra.

Even if the analyte is chemically perfectly pure it represents a mixture of different isotopic compositions, provided it is not composed of monoisotopic elements only. Therefore, a mass spectrum is normally composed of superimpositions of the mass spectra of all isotopic species involved. [11] The isotopic distribution or isotopic pattern of molecules containing one chlorine or bromine atom is listed in Table 3.1. But what about molecules containing two or more di-isotopic or even polyisotopic elements While it may seem, at the first glance, to complicate the interpretation of mass spectra, isotopic patterns are in fact an ideal source of analytical information. 

Example The isotopic pattern of CI2 is calculated from Eq. 3.9 with the abundances a = 100 and = 31.96 as (100 + 31.96) = 10000 -1- 6392 + 1019. After normalization we obtain 100 63.9 10.2 as the relative intensities of the three peaks. Any other normalization for the isotopic abundances would give the same result, e.g., a = 0.7578, b = 0.2422. The calculated isotopic pattern of CI2 can be understood from the following practical consideration The two isotopes Cl and Cl can be combined in three different ways i) Cl2 giving rise to the monoisotopic composition, ii) Cl Cl yielding the first isotopic peak which is here X-i-2, and finally iii) Cl2 giving the second isotopic peak X+4. The combinations with a higher number of chlorine atoms can be explained accordingly. 

Note For a rapid estimation of the isotopic patterns of chlorine and bromine the approximate isotope ratios Cl/ Cl = 3 1 and Br/ Br =1 1 yield good results. Visual comparison to calculated patterns is also well suited (Fig. 3.3). 

Fig. 3.3. Calculated isotopic patterns for combinations of bromine and chlorine. The peak shown at zero position corresponds to the monoisotopic ion at m/z X. The isotopic peaks are then located at m/z = X+2, 4, 6,. .. The numerical value of X is given by the mass number of the monoisotopic combination, e.g., 70 u for CI2.

The isotopic patterns of sulfur and silicon are by far not as prominent as those of chlorine and bromine, but as pointed out, their contributions are sufficiently important (Fig. 3.5). 

Example The high-resolution spectrum in the molecular ion range of a zirconium complex is typified by the isotopic pattern of zirconium and chlorine (Fig. 3.22). Zr represents the most abundant isotope of zirconium which is accompanied by Zr, r, Zr and Zr, all of them having considerable abun-... 

Fig. 6.38. El mass spectrum of 4-chlorophenetole. The chlorine isotopic pattern is found in the signals corresponding to M, m/z 156, [M-C2H4] ", m/z 128, [M-OEt], m/z 111, and [M-C2H4-CO], m/z 100. Spectmm used by permission of NIST. NIST 2002.

The isotope patterns of chlorine and bromine are worth particular mention. Chlorine has two isotopes of mass 35 and 37, in a ratio of 75 25, respectively, while bromine has two isotopes of mass 79 and 81 in an approximately 50 50 ratio. If we examine the mass spectrum for 2-chlorobenzoic acid, with a molecular formula of C7H5CIO2 (Figure 5.19), we can see peaks at (MH + 1) and (MH + 2) corresponding to the presence of the and Cl isotopes, respectively. 

The isotope patterns of chlorine and bromine are particularly distinctive, such that analytes containing one or two chlorine atoms or one or two bromine atoms can be readily distinguished. 

The platinum has a complicated isotope pattern as shown. The spectrum corresponding to the peak A in the HPLC trace can be interpreted easily as corresponding to cisplatin. Indeed, its molecular weight calculated from the first isotopes of both platinum and chlorine is 194 + (2 x 35) + (2 x 17) = 298 u. Adding 23 u for a sodium ion adduct leads to the observed m/z 321 peak, accompanied by the expected isotopes. 

The fragmentation pathways for the methyl esters are strongly dependent on the type of acid, i.e. whether a derivative of phenoxyacetic acid, phen-oxypropionic acid, or phenoxybutyric acid, and on the nature of the substitution of the aromatic ring. The methyl esters of chlorinated phenoxyacetic and phenoxypropionic acids show reasonably abundant molecular ions (about 20% relative to the base peak) which, together with the chlorine isotope patterns, permit easy identification of these compounds. In contrast, the spectra of the methyl esters of chlorinated phenoxybutyric acids are dominated by the fragment ion at m/z 101, with only low abundance of molecular ions. However, the spectra also show... 

The mass spectra ofbromacil and terbacil and their A-methyl derivatives show only weak molecular ions, but they have the isotope patterns expected from bromine and chlorine compounds, respectively. The major ions arise from [M—55] and [M—56] and also reflect the halogen substituent. Less abundant, but highly characteristic ions, corresponding to [M-99Y and [M-98], are formed by retro Diels-Alder fragmentations. 

The isotopic pattern may complicate the molecular weight assignment on the other hand, it will also provide valuable reference for recognizing the type and number of element in a molecule. The characteristic patterns resulting from multiple isotopic contributions of the chlorine, bromine, and sulfur isotopes are shown in Table 7-5. One example is illustrated in the CI-MS spectrum of mometasone furoate (Figure 7-2), displaying prominent... 

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