Polyisotopic elements

The majority of elements belongs to the polyisotopic elements because they consist of three or more isotopes showing a wide variety of isotopic distributions. 

Isotopic abundances are listed either as their sum being 100 % or with the abundance of the most abundant isotope normalized to 100 %. The latter is used throughout this book because this is consistent with the custom of reporting mass spectra normalized to the base peak (Chap. 1). The isotopic classifications and isotopic compositions of some common elements are listed below (Table 3.1). A full table of the elements is included in the Appendix. 

Note Care has to be taken when comparing isotopic abundances from different sources as they might be compiled using the one or the other procedure of normalization. 

Classifi- cation Atomic Symbol Atomic No. Mass No. Isotopic Composition Isotopic Mass [u] Relative Atomic Mass [ul 

Bar graph representations are much better suited for visualization of isotopic compositions than tables, and in fact they exactly show how such a distribution would appear in a mass spectrum (Fig. 3.1). This appearance gives rise to the term isotopic pattern. 

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What is a polyisotopic element Give examples of monoisotopic and of polyisotopic elements. 

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. 

The polynomial approach is the logical expansion of the binomial approach. It is useful for the calculation of isotopic distributions of polyisotopic elements or for formulas composed of several non-monoisotopic elements. [2,14] In general, the isotopic distribution of a molecule can be described by a product of polynominals... 

In a strict sense, oxygen, silicon, and sulfur are polyisotopic elements. Oxygen exists as isotopes and 0, sulfur as and and silicon as... 

The treatment of polyisotopic elements does not require other techniques as far as calculation or constmction of isotopic patterns are concerned. However, the appearance of isotopic patterns can differ largely from what has been considered so far and it is worth mentioning their peculiarities. 

Example The presence or absence of the polyisotopic element tin (Table 3.1) can readily be detected from its characteristic isotopic pattern. In case of tetrabutyltin, Ci6H3gSn, the lowest mass isotopic composition is CieHse Sn, 340 u. Due to the 16 carbon atoms, the isotopic abundance is about 17.5 %. This is superimposed on the isotopic pattern of elemental Sn, which becomes especially... 

In conventional mass spectra the isotope patterns deriving from the presence of polyisotopic elements are striking and quite noticeable. An excellent example is the ESP spectrum shown in Figure 17 for a compound with a C14H12NOFCI2 empirical formula. The zw/z 300 [M -I- H]" " and the m/z 269 neutral loss fragment (resulting from neutral loss of methylamine) clearly show CI2 isotope patterns which match well with simulations. The zzz/z 234... 

When the gaseous diffusion plant came into operation, the cost of separating U electromagnetically was found to be higher, and in 1946, the Y-12 plant was taken off uranium-isotope separation. Some of this equipment is now beii used to produce gram quantities of partially separated isotopes of most of the other polyisotopic elements, for research uses. These units have also been used to separate artificially produced isotopes, such as U from irradiated uranium, and the various plutonium isotopes. 

The curves in Figure 8.5 are relatively flat over a broad Ns/Nsp range from about 10 to 10 for spike enrichments >90%, which means that under these conditions the precision of the IDMS result is nearly independent of the analyte to spike mixing ratio. It also demonstrates clearly that a highly enriched spike isotope (relative isotopic abundance approaching 100%) is usually not necessary for IDMS analysis. For other polyisotopic elements, the curves are similar to those in Figure 8.5, as can be seen from Figure 8.6 for boron, chlorine, and bromine for... 

Figure 8.6 Error multiplication factors for IDMS determination of the polyisotopic elements boron, chlorine, and bromine and of the monoisotopic element iodine for specified spike isotope enrichments as a function of the analyte/spike atomic ratio.

The characteristic abundance patterns resulting from the combination of more than one polyisotopic element can be calculated from the relative abundances of the different isotopes. The following polynomial expression gives the isotope distribution of a polyisotopic molecule ... 

Inasmuch as the method depends upon mixing natural and synthetic isotopic compositions, MSID is applicable theoretically to all polyisotopic elements for which an adequate ion current can be generated. Thus all the even Z lanthanides, Ce, Nd, Sm, Gd, Dy, Er and Yb, and the odd Z lanthanides La, Eu, and Lu may be analyzed by this technique. Isotope dilution, theoretically, can be used in determining abundances of the monoisotopic rare earths. Sc, Y, Pr, Tb, Ho, and Tm, by utilizing artificial radioactive (unstable) isotopes this will not be pursued further in the present chapter. 

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