Polyisotopic elements, isotope

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

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. 

Most practitioners of the precise art and subtle science of mass spectrometry acknowledge the field to have originated with the work of J.J. Thomson (Fig. 19.1) and associates, [6] published in 1910-1912, using the parabola mass spectrograph. Seminal discoveries that he and his coworkers made include the fact that the elements could be polyisotopic, by discovering the isotopes of neon. Thomson was awarded the Nobel Prize in physics of 19062 for his work on investigations on the conduction of electricity by gases. ... 

It is seen that also in the general polyisotopic case the ratio of sample and spike amounts can be determined solely from isotope ratio measurements. Isotope dilution mass spectrometry measures the change in the ratio of two isotopes of the element of interest, induced by the addition of a known amount of the same eiement with an artificially altered isotope ratio of the same isotopes to a weighed aliquot of a sample. Since only isotope ratios in the different materials are measured, it follows that sample treatment or recoveries for chemical separations need not be quantitative once isotopic equilibration after spiking has been achieved. Also variability of chemical separation recovery is generally not important. 

The calculations described are simple but lengthy, especially when corrections for interference are required. This tedium may be lessened by the use of automatic data processing. In any case, the values obtained are examined critically for errors and inconsistencies. In particular, if the lanthanide yields polyisotopic results, these are compared, more weight being given to values judged to have the least potential for interference. Likewise, more weight is attached to values derived from the more abundant natural isotopes of the element. 

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