8.1 History of Radioactive Tracer Methods

Radioactive tracing was discovered by George Hevesy in 1911. He was working in Rutherford’s laboratory, where radium was prepared from uranium ore by copreci­pitation with lead chloride. His goal was to separate RaD from lead chloride. At that time, the term “isotope” had not been defined; the decay series of uranium was described as in Figure 8.1. All radioactive isotopes were considered to be “new radioactive elements,” and correspondingly named after the parent element and the place of the given product in the decay series. Thus, RaD indicated the sixth ele­ment of the Ra decay series. As comparison, refer to the “modern” decay series of 238U shown in Figure 4.4.

He attempted to separate RaD by many chemical methods but did not succeed. The separation factor was found to be 1 in every method, so Hevesy concluded that RaD could be suitable for labeling lead. Nowadays, we already know that RaD is a radioactive isotope of lead (210Pb); that is, lead and RaD are chemically the same. Using RaD as a radioactive tracer, Hevesy, with F. Paneth, determined the solubility of lead salts (sulfide and chromate) which are very little; the solubility products are about 10-33 mol2 dm-6. In 1943, Hevesy received the Nobel Prize in Chemistry for “his work on the use of isotopes as tracers in the study of chemical processes.”

Similar to radioactive isotopes, stable isotopes can also be used as tracers. The determination of the stable isotopes, however, requires expensive instrumentation (nuclear magnetic resonance and mass spectrometers), while it is much simpler and cheaper to measure radioactive isotopes. In addition, the radioactive isotopes can be measured easily in very small quantities. Depending on the decay constants, as small quantities as 10-16—10-6g of the radioactive isotopes can be detected. The application of the radioactive tracers/indicators is independent of the physical and chemical properties (pressure, temperature, chemical species, etc.) because the energy of the nuclear radiation is 6— 8 orders of magnitude higher than the energy of the aforementioned physical and chemical effects. Since the radioactive isotopes are chemically the same as the studied inactive isotopes, they do not change the studied system. They can be applied in dry analytical methods. If the radioactive indicator is chemically pure, no contaminants are added to the investigated system.

Nuclear and Radiochemistry. DOI: http://dx. doi. org/10.1016/B978-0-12-391430-9.00008-1

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