Radionuclides isotopic distinction

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

There are in principle two sources for anthropogenic radioactivity in rivers in Sweden, 1954-58 (20%) and 1961-62 (80%), and the Chernobyl accident, 1986. These events are quite distinct in time and by using isotopic ratios and radionuclide ratios the two sources can be distinguished from each other. China and France also conducted nuclear tests during 1960 -1980. 

Radioanalytical chemistry was first developed by Mme. M. Curie, with contributions by many other distinguished researchers, notably E. Rutherford and F. Soddy. These pioneers performed chemical separations and radiation measurements on terrestrial radioactive substances during the 20 years following 1897 and in the process created the very concept of radionuclides. Their investigations defined the three major radiation types, confirmed the emission of these radiations by the nucleus and the associated atomic transformations, established the periodic table between bismuth and uranium, and demonstrated the distinction between stable and radioactive isotopes. 

Another distinction pertains to radionuclides that have no stable isotopes. Chemical analysis for these radionuclides has no basis in conventional analytical chemistry except as studies performed with the usual small amounts, based on similarities in chemical behavior to homologous stable elements according to their location in the periodic table. When sufficiently large amounts of these radionuclides are produced and purified to permit observation by microchemical manipulations, any conclusions must consider the impact of the intense radiation on the observed chemical reactions. 

The atomic mass difference between the radionuclide and the mix of its stable isotopes in nature, although minor in terms of its effect on chemical equilibrium and reaction rates, provides opportunities for separation, identification, and quantification at low concentration by mass spectrometer, as discussed in Chapter 17. The mass difference ratio is at its extreme for tritium (T or H) relative to the stable isotopes and H. This distinction causes minor separation between ordinary water with molecular mass 18 and tritiated water (HTO) with molecular mass 20 during distillation, and can be applied to enriching tritiated water in the laboratory by electrolysis. 

Measuring alpha particles with an LS counter is an attractive option because the counting efficiency is near 100% and no self-absorption problem exists. After the usual sequence of separations for radionuclides such as thorium, uranium, and transuranium isotopes, the radionuclide is prepared in the final solution for counting and yield determination. A tracer that emits alpha particles at a sufficiently distinct energy is added initially to measure yield. The factor that controls detection sensitivity is the background, typically of 1-2 c/m in the alpha-particle energy region of the LS counter. 

Chemical separations may be specific for the analyte of interest (see Chapter 3), such as liquid or gas chromatography, or scavenging (such as by precipitation) to remove the major interfering substances. Addition of carrier, as practiced in radioanalytical chemistry to assist in purifying radionuclides, usually is not appropriate for mass spectrometric analysis. Such addition undermines the isotopic ratio measurements that are often at the heart of this procedure, and also overloads the system for ion generation and peak resolution (but carrier addition is used for accelerator mass spectrometry). Addition of tracers, known as isotope dilution, is often employed for yield determination (see Section 17.2.9). Interferences are distinctly different in radiometric and MS analyses of radionuclides, and may be the deciding factor in selecting one method versus the other. 

Extraterrestrial materials In some extraterrestrial material such as meteorites, elements may show isotopic compositions that are distinct from all terrestrial material investigated. This is related to decay of radionuclides that may already be extinct, due to half-lives which are very short compared with the age of the solar system of 4.6 x 10 years. Such variations are rare for terrestrial materials, in large part due to preferential sampling of the crust, whereas some extraterrestrial material, such as iron meteorites, resemble the Earth s core, in which parent to daughter element ratios may be much higher than in the cmst. 

Radionuclidic purity is only of concern in the context of dual-isotope labeling, or if crosscontamination from a laboratory mishap is suspected. Radionuclidic purity is best measured by liquid scintillation counting modem LSC instruments have detectors and analysis software designed to discriminate quantitatively between the different isotopes used in the life sciences, except at very low counting levels. Radionuclidic purity is entirely distinct from isotopic purity, or content, of compounds labeled with stable isotopes, such as deuterium or carbon-13. Such information may be very important to the utility of stable-labeled compounds such as internal standards for mass spectrometric quantitation assays. ... 

---