Isotope ratio mass spectrometry analytical considerations

Mass spectrometry ICP-MS offers an accurate method for sulfur isotopic ratio measurements and a sensitive technique for elemental sulfur determination. The method may be applied as an absolute method, because the analyzed atoms themselves, and not the radiation they emit, produce the analytical signals. The technique is useful for the analysis of high-purity materials, such as metals and semiconductors. The detection limits obtainable in most cases by ICP-MS are considerably better than typical values reported for ICP-AES. However, the analytical precision and accuracy of the method are often poor. The utility of the method to determine sulfur is further complicated, as efficient ionization of sulfur is difficult to obtain because of the high ionization energy of the element. 

In principle, two fundamentally different methods can be applied to solve this task. The first one is determination of the residual concentrations of the fissile nuclides after irradiation and calculation of the burnup from the difference between final and initial values. For this purpose, the uranium and plutonium fraction has to be separated from the fission and activation products and from each other (e. g. by extraction chromatography) subsequently, the concentrations of the individual isotopes, in particular of the fissile isotopes, are analyzed by mass spectrometry. Well-established analytical techniques for performing such analyses are available, so that only small error margins are to be expected in the determination of the concentrations of the isotopes under consideration. However, there are two problems that can potentially cause systematic errors. The first one is the well-known question of the accuracy of results which have been obtained as a difference between two numbers, which limits the accuracy at lower burnup values in particular. The second problem is that the fissile nuclides are not only consumed by nuclear fission but by neutron capture as well in order to avoid systematic errors here, the capture-to-fission ratio valid for the particular irradiation conditions has to be taken into account in the calculation of depletion during irradiation. If one recalls the complicated buildup and decay mechanisms of actinide nuclides during reactor irradiation (see Fig. 3.5.), it is obvious that such correction requires complex calculations. On the other hand, the direct determination of the residual concentration of fissile nuclides is not influenced by errors due to inaccuracies in the fission yields of fission products to be measured nor by migration-induced inho-mogenities in the fuel. 

Emission spectrometry is generally thought of as a technique for elemental determinations, with isotope ratio determinations being the domain of mass spectrometry. In the manufacture, handling, and analysis of nuclear materials, there is considerable need for techniques to determine both elemental and isotopic concentrations of U, Pu, and other actinides in as near an on-line, process control fashion as possible. These elements emit many lines when excited in an ICP, as shown in fig. 9. Edelson and Fassel (1981) point out that some of these lines exhibit isotopic shifts of sufficient magnitude to be separated by a high-resolution monochromator. An example is shown in fig. 16. Separate lines from each isotope are clearly detectable. In these experiments, the fact that actinides are the heaviest elements actually helps resolution of isotopic lines, because the Doppler width of a line decreases as atomic weight increases. The plasma is operated inside a secure containment facility to prevent excretion of actinides into the environment. Since the analytical information is carried by photons, the optical instrumentation for the actual measurement is completely isolated from the radioactive source, so deposition of actinides in the spec-... 

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