Minor isotopes

B By knowing the natural abundances of minor isotopes, it s possible to calculate the relative heights of M+ and M+l peaks. If 33C has a natural abundance of 1.10%, what are the relative heights of theM+ and M+l peaks in the mass spectrum of benzene, C6H6 ... 

Figure 2-19 shows the mass spectrum of the element neon. The three peaks in the mass spectrum come from three different isotopes of neon, and the peak heights are proportional to the natural abundances of these isotopes. The most abundant isotope of neon has a mass number of 20, with 10 protons and 10 neutrons in its nucleus, whereas its two minor isotopes have 11 and 12 neutrons. Example illustrates how to read and interpret a mass spectmm. 

Carbon is a mixture of two isotopes in its natural abundance. The major isotope, C, occurs in a natural abundance of 98.9%, but it is insensitive to the NMR experiment. The minor isotope, C, occurs in a natural abundance of 1.1% it is with this isotope that we are concerned in the INADEQUATE and many other NMR experiments. Thus there will be only about one molecule in a hundred with a particular carbon bearing the C isotope. To find two adjacent carbons bearing C isotope would be even less likely... 

Hulston and Thode (1965) showed that the relationship between 5 values on a three isotope plot can be made linear if the definition of the 5 s are modified so that the term - 1) is replaced by ln( i / i , j) where x refers to one of the minor isotopes, R is the ratio of the minor isotope to the major isotope and std refers to the standard ratio. In the case of Mg isotopes, the new definitions for 5 "Mg and 5 Mg are ... 

Several elements naturally exist in two isotopes. Within the context of mass spectrometry it is useful to deal with them as a class of their own. Nevertheless, the term di-isotopic element is not an official one. These elements can even be subclassified into those having one isotope that is 1 u heavier than the most abundant isotope and those having one isotope that is 2 u heavier than the most abundant isotope. (For the unit u cf. Chap. 3.1.4.2). The first group has been termed A-rl or Xh-1 elements, the latter ones have been termed Ah-2 or Xh-2 elements, respectively. [2,3] If we do not restrict our view to the elements typically encountered in organic mass spectrometry, the class of X-1 elements with one minor isotope of 1 u lower mass than the most abundant one should be added. 

Isotope ratios. For some elements (most notably bromine and chlorine), there exists more than one isotope of high natural abundance e.g. bromine has two abundant isotopes - Br 49 % and Br 51 % chlorine also has two abundant isotopes- Cl 25 % and Cl 75% (Table 4.1). The presenee of Br or Cl or other elements that contain significant proportions (> 1%) of minor isotopes is often obvious simply by inspection of ions near the molecular ion. 

Symbol Ba atomic number 56 atomic weight 137.327 a Group llA (Group 2) alkaline earth element electronic configuration [Xejs valence state +2 ionic radius of Ba2+ in crystal (corresponding to coordination number 8) 1.42 A first ionization potential lO.OOeV stable isotopes and their percent abundances Ba-138 (71.70), Ba-137 (11.23), Ba-136 (7.85), Ba-135 (6.59), Ba-134 (2.42) minor isotopes Ba-130 (0.106) and Ba-132 (0.101) also twenty-two radioisotopes are known. 

Symbol Ce atomic number 58 atomic weight 140.115 a rare-earth metal a lanthanide series inner-transition /-block element metaUic radius (alpha form) 1.8247A(CN=12) atomic volume 20.696 cm /mol electronic configuration [Xe]4fi5di6s2 common valence states -i-3 and +4 four stable isotopes Ce-140 and Ce-142 are the two major ones, their percent abundances 88.48% and 11.07%, respectively. Ce—138 (0.25%) and Ce—136(0.193%) are minor isotopes several artificial radioactive isotopes including Ce-144, a major fission product (ti 284.5 days), are known. 

The isotope dilution method consists of mixing a natural sample with an artificial spike and measuring the isotopic ratios of the mixture using mass spectrometry, providing very precise quantitative determination of the concentrations of elements in trace quantities. A spike is a solution that contains a known concentration of the element, artificially enriched in one of its minor isotopes. For example, natural Rb samples have 27.84% and 72.16% Rb (Fig. 11.lA). Rb spikes are made by artificially enrich the minor isotope Rb. And a solution with 90% Rb and 10% Rb (Fig. 11.IB) is a Rb spike. Of course, a solution with 99.99% Rb and 0.01% Rb is a better Rb spike. When the known quantities (mass) of the sample and spike are mixed, the resulting isotopic compositions can be used to calculate the concentration of the element in the sample. 

The spike in most cases is single spike, but it can be double spike. Single spike method provides the information of the concentration of the sample, whereas the double-spike method can yield the concentration of the sample and the mass fractionation factor that can be used to calculate the true isotopic ratios in the sample. Take Pb isotope as an example. Natural Pb samples have 1.4% ° Pb, 24.4% ° Pb, 22.1% ° Pb and 52.4% ° Pb. A single spike is made by artificially concentrating one of the minor Pb isotopes, for example Pb. A double spike is made by artificially concentrating two minor isotopes. For example, we can concentrate Pb and b isotopes to make ° Pb- Pb double spike or concentrate Pb and Pb to make Pb- ° Pb double spike. 

A single spike is enriched in one minor isotope whereas a double spike is enriched in two minor isotopes (Fig. 11.3). Double spike method needs two runs. The first run is to measure the un-spiked sample and the second run is to measure the spiked mixture. Double spike method not only provides the concentrations of the sample but also corrects the mass fractionation in mass spectrometers (and thus giving the true isotopic ratios). 

Nickel has two major and three minor isotopes. For the purpose of this problem, suppose that the only isotopes are 58Ni and Ni. The atomic mass of 58Ni is 57.935 3 Da and the mass of w Ni is 59.933 2 Da. From the amplitude of the peaks in the spectrum below, calculate the atomic mass of Ni and compare your answer with the value in the periodic table. 

The z8SiO emissions show parabolic line profiles, suggesting that they are optically thick thermal emissions. Optically thin emissions should display rectangular line profiles. -The line profiles of the 2,SiO and 3°SiO emissions seems to be rectangular, at most intermediate between them. The optical thickness of the line usually leads to underestimates of the abundances of the most abundant isotopes. Therefore we concentrate on the two minor isotopes. 

The molecular ion overlaps due to plasma and solvent species are most severe below mass/charge 82. A high-resolution, double-focusing mass spectrometer was used to identify molecular ions observed in ICP-MS (Table 3.3) [3]. Common molecular ions that produce intense signals from plasma and solvent species include ArO+, ArOH+, ArH+, ArN+, Ar+, Ar2H+, 0+, N2+, NO+, and 0+. Other molecular ions become a problem at lower analyte concentrations. These include CO, CO H+, NOt, ArO+ ions with minor isotopes of Ar or O, ArC+, ArN+, and minor isotopes of Ar as ArJ. 

Among the commonly observed spectral overlap problems due to molecular oxide and molecular hydroxide ions are those due to TiO+ (with 5 isotopes of Ti from mass 46 to 50) that result in overlaps with a minor isotope of nickel, 62Ni+ both isotopes of copper, 63Cu+ and 65Cu + and the two major isotopes of zinc, MZn+ and 66Zn+. Calcium oxide and hydroxide ions overlap with all five isotopes of nickel, both isotopes of zinc, and three of the four isotopes of iron. The analysis of rare earth elements is particularly complicated by molecular oxide and hydroxide ion spectral overlaps [141,142]. 

Sulfur Isotopes. Because of the large range of 634S values found in natural systems, favorable minor isotope abundances (34S is -4% of total sulfur), and... 

To reach temperatures below 4.2 K, one can partially evacuate a He reservoir using a high-capacity vacuum pump this works down to the lambda point of liquid He (2.1768 K) below this temperature the 2He4 turns into a superfluid quantum liquid, which cannot be cooled any further. The minority isotope, 2He3 remains a normal fluid down to 0.002491 K this allows cooling down to about 1K. 

Element Major isotope abundance Minor isotope abundance... 

Since the dissociation of the abundant H2 and CO molecules result from photoabsorption at discrete wavelengths, isotopically selective photodissociation based on self-shielding is possible. Two conditions are required for this (1) dissociation via line absorption for each isotopically substituted molecule, and (2) differential photolysis that depends upon the isotopic abundances. Self-shielding occurs when the spectral lines leading to dissociation of the major isotopic species optically saturate, while the other residual lines relevant for dissociation of the minor isotopes remain transparent. As a consequence, such photolysis depends on nucleic abundance rather than the mass of a molecule see Fig. 4.4 (Langer 1977 Thiemens Heidenreich 1983). 

The following is an examination of the potential of the stable Isotope %g (11.01%) as an alternative and complement to Mg. While the second minor isotope of magnesium, %g CIO 00%), has a lower natural abundance, has the advantage of being de-... 

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