Radiometric Assay

a method for analyzing the chemical composition of substances based on the use of radioisotopes and nuclear radiation. Radiometric instruments are used in these assays for a qualitative and quantitative determination of the composition of substances. There are several different types of assays. A direct radiometric assay is based on the precipitation of the ion to be determined through an excess of a reagent containing a radioisotope. Both the concentration of the reagent and the radioactivity of the isotope are known, and after precipitation the radioactivity of the precipitate or of the excess reagent can be measured.

Radiometric titration is based on the combination of the ion to be determined with the reagent to form a compound that is either poorly soluble or easily extractable. During titration, the change in the radioactivity of the solution with the addition of a reagent serves as an indicator in those cases where a poorly soluble compound is formed; where an easily extractable compound is formed, the indicators are the changes in radioactivity with added reagent of both the solution and the extract. The equivalence point is determined by a break in the titration curve, which expresses the relationship between the volume of reagent introduced and the radioactivity of the titrated solution (or precipitate). The radioisotope either can be introduced into the reagent or the substance being determined or can be introduced into both.

Isotope dilution is based on the identical nature of the chemical reactions of the isotopes of a given element. In this method, some amount of the substance to be determined m₀ is added to the mixture under analysis. The added substance contains a radioisotope whose radioactivity I₀ is known. A portion of the substance to be determined is then separated in a pure state by any convenient method, for example, precipitation, extraction, or electrolysis, and the mass m₁ and radioactivity I₁ of the separated portion are measured. The total amount of the unknown substance in the mixture can be found from the equality of two ratios. The ratio between the radioactivity of the separated portion and that of the added substance is equal to the ratio between the mass of the separated portion and the sum of the masses of the added substance and the substance originally in the mixture. Thus, I₁/m₁ = I₀/(m + m₀), from which m = (I₀/I₁)m₁ – m₀.

In activation analysis, the substance being analyzed is irradiated (activated) with nuclear particles or hard gamma rays, and the radioactivity of the resulting radioisotopes is then determined. This radioactivity is proportional to the number of atoms of the element to be determined, the amount of activated isotope, the intensity of the flux of nuclear particles or photons, and the cross section for the nuclear reaction producing the radioisotope.

The photoneutron method is based on the emission of neutrons upon the action of high-energy photons (gamma quanta) on the nuclei of atoms. The number of neutrons, which is determined by neutron detectors, is proportional to the content of the element being analyzed. The energy imparted by the photons must exceed the binding energy of the nucleons in the nucleus, which for most elements is about 8 megaelectron volts (MeV). (For beryllium and deuterium, however, the binding energy is only 1.666 and 2.226 MeV, respectively. Thus if the isotope ¹²⁴Sb having £_γ equal to 1.7 and 2.1 MeV is used as a source of gamma quanta, beryllium can be determined against a background of all other elements.)

Radiometric assays also employ methods based on the absorption of neutrons, gamma rays, beta particles, and quanta of characteristic X-ray emissions of radioisotopes. In the method of analysis based on the reflection of electrons or positrons, the intensity of the reflected beam is measured. Since the energy of particles reflected from light elements is several times less than that of particles reflected from heavy elements, the content of heavy elements in alloys with light elements and in ores can be determined.