A very common task in analytical chemistry is the separation of the different com­ponents to be analyzed. The components are typically separated by distribution reactions using phase separation. For example, the precipitation, extraction, ion exchange, electrolysis, and different chromatographic methods are mentioned.

In order to have accurate analytical results, the distribution ratio (in other words, the yield of the separation procedure) must be determined. Radioactive tracers can assist in determining the yield. The application of the radioactive tracers is espe­cially useful in multicomponent systems if the radioactive isotopes of components have distinguishable radiation properties, such as different gamma energies.

The following two examples demonstrate how this method can be used to deter­mine distribution ratios. First, the separation of the 90Sr—90Y parent—daughter pair is mentioned. These are fission products of 235U in nuclear reactors (see Figure 6.5 and Section 7.3). Since their fission yield is relatively high, and 90Sr has a rela­tively long half-life, it is important to be able to determine precisely the activity concentration of 90Sr in different samples. However, as a result of the negative beta decay of 90Sr, 90Y is formed:

90 Sr! 90Y! 90Zr

30 years 64 h

Both isotopes have pure negative beta radiation, and the maximum beta energies are 546 and 2284 keV for 90Sr and 90Y, respectively. As a result, the activity of the parent nuclide, 90Sr, can be measured directly only in secular equilibrium (as discussed in Section 4.1.6). The time needed to reach the secular equilibrium is determined by the half-life of the daughter nuclide, 90Y. This is 64 h, so the secular equilibrium is reached after about a month, which is too long to obtain the analyti­cal result. To avoid this problem, 90Sr and 90Y isotopes are separated chemically using several different procedures. Each separation procedure requires the determi­nation of the yield of separation. This can be done easily using the 85Sr isotope as a tracer that has beta and gamma radiation. Since neither 90Sr nor 90Y has gamma radiation, the gamma radiation of 85Sr (514 keV) is used for the analysis. 85Sr with

Solid sample

I

Tracers: 242Pu, 243Am, 232U, (229Th)

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Digestion: cc. HNO3/(HF) Dissolution: 8 M HNO3

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Extraction chromatography by UTEVA resin

known activity/intensity is added to the mixture of 90Sr—90Y before the separation. The yield is calculated on the activity/intensity measurement of the pure strontium fraction after the separation.

The other example illustrating the use of radioactive isotopes to determine sepa­ration yields is the analysis of the uranium (sometimes thorium) and transuranium elements. As discussed in Section 6.2.1 (Eqs. (6.22) and (6.23)), these isotopes are produced from 238U in nuclear reactors. For their quantitative analysis, precipita­tion, extraction, and ion exchange separations are used. The separation yields are determined by radioactive tracers that are not present in the samples, i. e., nonfission products, such as 242Pu, 243Am, 232U, and 229Th. An example for the separation and sample preparation is shown in Figure 10.1.

In these two examples, the radioactive tracers are applied in radioactive samples. Obviously, similar procedures can be applied for nonradioactive samples. The sepa­ration yield of different processes (precipitation, gravimetry, extraction, ion exchange, chromatography, etc.) can be determined using radioactive tracers, both in the analytical and in the preparative scale. The traditional analytical methods coupled with radioactive tracers include radiogravimetry, radiochromatography, and so on.