Isotopic pattern distinguishing

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. 

While the introduction of stable isotope labels on either the protein or the peptide enables the ready identification of inter-molecular cross-linked peptides, intramolecular cross-linked peptides eannot be distinguished by this method. Thus, isotopic labels have been incorporated directly into the eross-linking reagent. It has been demonstrated that by reaeting with 1 1 (w/w) mixtures of stable isotope-labeled and non-labeled cross-linking reagents, cross-linked peptides can be detected readily by their distinctive 1 1 isotope pattern [148,149]. Isotope-labeled cross-linkers have been combined with other strategies for further identification of cross-link types [150,151]. 

MS has a unique advantage in that the two exchange mechanisms can be readily distinguished in the isotope patterns of the deuterated peptides (see also Sections 1.3.1 and 3.2.2). In the EX2 mechanism, the local structure will unfold and refold many times until an exchange event happens. Therefore, the deuterium labeling on each backbone amide site of a peptide will be independent to one another. As a result, the distribution of deuterium across the molecules will be uncorrelated and result in a unimodal isotope distribution. 

What, then, are the options currently available to the scientist, and is aeeurate mass measurement a help The latter question is largely rhetorieal, especially for anyone who has tried to make sense of product ion spectra obtained on instrumentation capable only of generating nominal mass data. Trivial neutral losses can usually be assigned on the latter instruments, but not always, for example, distinguishing between a neutral loss of methane from that of an oxygen atom is not possible. The presence or absence of a halogen isotope pattern can still be used to limit possible explanations for a product ion. On the other hand, the benefit of accurate mass measurement is dramatic, as noted earlier, in that it affords a dramatic reduction in elemental compositions that are possible explanations of the measured w/z value—and thus limits the number of possibilities that must be considered by the analyst. 

Such chromatograms may be recorded with MS analyzers that have a resolving power between 15,000 and 20,000 full width at half maximum (FWHM) for masses that differ by +0.002-0.004 amu, which permits compounds with the same nominal masses and different substituents to be distinguished (Figure 24.8). These instruments are able to record m/z values of protonated or deprotonated molecules up to the fourth decimal point with a precision better than 5 ppm, and therefore, the calculation of the elemental composition of compounds is possible. Additionally, the analysis of the isotopic pattern (relative intensities of [M + H] or [M - H] ions with different isotopic composition) enhances the possibility of an unambiguous identification of compounds. Retention time is the third parameter, which may be taken under consideration for the identification of target analytes. 

Boulyga and Prohaska evaluated the use of LA-MC-ICP-MS to analyze microsamples collected in the vicinity of Chernobyl [19]. The authors observed different isotopic patterns. While Nd isotopic abundances in the samples was not distinguishable from the natural isotopic composition, the ° Ru/f Ru + Tc) and Ru/( Ru + c) ratios were found to be significantly lower than natural ratios. That is because, while it is not possible to differentiate between Ru and c with ICP-MS, Tc levels in natural samples are negligible, while in these exposed samples the contribution of the Tc signal to that ratio was not insignificant. Monitoring of and also provided very sensitive indicators of... 

Fig. 3.4. Aid in distinguishing isotopic patterns by using orientation lines. Left side patterns with four peaks each right side patterns with six peaks.

Figure 9.70 shows the El mass spectrum of a trichlorinated molecule trichloroethane. We can distinguish an isotope pattern corresponding to three chlorine atoms m/z 117/119/121/123 (the peak of the 123 ion is difficult to see because of its weak abundance) a pattern corresponding to 2 chlorine atoms m/z 97/99/101 and a pattern corresponding to a monochlorinated ion m/z 61/63. 

Haegele et al. (269) have used exact isotope masses and isotope abundances together in determining the detailed fragmentation patterns of square planar rhodium (I) -diketonate complexes. They found that some species postulated by other workers were in error. High resolution is needed to distinguish the 28 mass units for loss of CO (27.9949) from C2H4 (28.0313) (269) or the 69 mass units for PF2 (68.9906) from CFa (68.9952) (90). 

---