RADIOCHEMICAL METHOD IN ACTIVATION ANALYSIS & ISOTOPIC DILUTION METHOD

Radiochemical Method… Nuclei of some atoms are unstable and emit particles/radiation , the branch of chemistry which is concerned with the decay of unstable nuclei and the products of decay is Radiochemistry. Analysis of stable nuclei using the radio chemicals/radioisotopes is called radiochemical methods. Radiochemical methods of analysis depend on the specific properties of certain. These properties include the type and energy of the radiation emitted, the half-life (t1/2), and the deecay schemes of that particular nuclide. Furthermore, detection limits tend to be high for long-lived isotopes using radiochemical methods . Fine Particle Technology of Radio Chemical Method of Analysis 9/20/2018

Fine Particle Technology of Radio Chemical Method of Analysis Radioactive Decay… Radioactive Decay Alpha Decay Beta Decay X-Ray Emission Gamma Decay Fine Particle Technology of Radio Chemical Method of Analysis

Fine Particle Technology of Radio Chemical Method of Analysis Radioactive Decay… Radioactive Decay Beta Decay Alpha Decay X-Ray Emission Gamma Decay Alpha () decay is a common radioactive process encountered with heavier isotopes. The alpha particle is a helium nucleus having a mass of 4 and a charge of +2. Fine Particle Technology of Radio Chemical Method of Analysis

Fine Particle Technology of Radio Chemical Method of Analysis Radioactive Decay… Radioactive Decay Gamma Decay Beta Decay Alpha Decay X-Ray Emission Beta () decay is a radioactive process in which, the atomic number changes but the mass number stays the same. The high-energy electrons have greater range of penetration than alpha particles, but still much less than gamma rays. Fine Particle Technology of Radio Chemical Method of Analysis

Fine Particle Technology of Radio Chemical Method of Analysis Radioactive Decay… Radioactive Decay X-Ray Emission Gamma Decay Beta Decay Alpha Decay Gamma () rays are produced by nuclear relaxations. Gamma rays are more dangerous than radio waves Fine Particle Technology of Radio Chemical Method of Analysis

Fine Particle Technology of Radio Chemical Method of Analysis Radioactive Decay… Radioactive Decay Beta Decay X-Ray Emission Gamma Decay alpha Decay X-Ray emission are formed from electronic transitions in which outer electrons fill the vacancies created by the nuclear process. Fine Particle Technology of Radio Chemical Method of Analysis

Radiochemical Methods Radiochemical methods are both sensitive and specific. There are three general types of radio- analytical methods of analysis: Radiometric analysis Isotope dilution, and Activation analysis

Radiometric Analysis Radiometric analysis – the use of a radioactive reagent of known activity to isolate the analyte from the other components of the sample. The activity of the product is directly proportional to the amount of the analyte. Example: Chromate has been determined by precipitating it with radioactive Ag+ (Ag-111, , , 7.5d) of a known activity. The limited reactant is the analyte, here CrO4-2. Determining the activity of the precipitate of Ag2CrO4 allows for the determination of the amount of chromate.

Isotopic dilution Isotopic dilution analysis was introduced by von Hevesy and Hofer in 1934. It is a kind of quantitative analysis based on the measurement of the isotopic abundance of a nuclide after isotope dilution with the test portion. Isotopes: Nuclides having the same atomic number but different mass numbers. Isotope Dilution Mixing of a given nuclide with one or more of its isotopes. A known weight of this compound labeled with isotope (such as an acid with O-18 or a hydrocarbon with H-2) is then mixed with the mixture containing the compound to be analyzed. After treatment to ensure homogeneity between the labeled and unlabelled species, a portion is recovered or separated as a chemically pure substance.

Isotopic dilution The pure substance is weighed and its radioactivity measured. The extent of the dilution of the radioactive sample may then be calculated and related to the amount of the nonradioactive substance in the original sample. Quantitative recovery (100% yield) is not required for a successful analysis.

Isotopic dilution The mathematical relationship for calculating the amount of the material in the original sample is Wm / Wa = Ai / Af - 1 or Wm = Wa { Ai / Af – 1} where Wm is the mass of the analyte, Wa is the mass of the radioactive compound added, Ai is the activity of the added compound, and Af is the activity of the final purified compound.

Isotopic dilution Isotopic dilution is especially useful in the analysis of complex biochemical substances, such as vitamins D and B, insulin, steroids, that occur a complicated matrix such that methods of separation and analysis are difficult. In a recent work (www.wcaslab.com/tech/isotope_dilution.htm), the isotopes are detected by a highly sensitive mass spectrometer; the radioactive form of the compound then acts as an internal standard for the analysis because the mass of the isotope used for labeling is different than the non-radioactive form of the element.

Activation Analysis Neutron activation analysis (NAA) was discovered in 1936 when Hevesy and Levi found that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons. From this observation, they recognized the potential of employing nuclear reactions on samples followed by measurement of the induced radioactivity to facilitate both qualitative and quantitative identification of the elements present in the samples.

Radiochemical Methods There are several ways of inducing the radioactivity in the atoms present in the sample for analysis. The most common is neutron activation in which the sample is irradiated with neutrons. After the irradiation, the gamma or beta spectrum is obtained, depending on the type of emission produced by the irradiated element. For quantitative work, both may be used. The energies of the spectral peaks allow for the identification of the elements present and the areas of the peaks define the amounts of each element as shown in the next slide.

Radiochemical Methods Gamma ray spectrum of a sample after neutron induced radioactivity

Radiochemical Methods The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons, instrumentation suitable for detecting gamma rays, and a detailed knowledge of the reactions that occur when neutrons interact with target nuclei.

Radiochemical Methods The sequence of events occurring during the most common type of nuclear reaction used for NAA, namely the neutron capture or (n, gamma) reaction, is illustrated in Figure 1. When a neutron interacts with the target nucleus via a non-elastic collision, a compound nucleus (metastable) forms in an excited state. The excitation energy of the metastable nucleus is due to the binding energy of the neutron with the nucleus. This nucleus will almost instantaneously de-excite into a more stable configuration through emission of one or more characteristic gamma rays.

Radiochemical Methods Fig. 1. Diagram illustrating the process of neutron capture by a target nucleus followed by the emission of gamma rays. The above figure is from http://www.missouri.edu/~glascock/nna_over.htm

Radiochemical Methods In many cases, this new configuration yields a radioactive nucleus which also decays by emission of one or more characteristic delayed gamma rays, but at a much slower rate according to the unique half-life of the radioactive nucleus. Depending upon the particular radioactive species, half-lives can range from fractions of a second to several years.

Radiochemical Methods Although there are several neutron sources such as reactors, accelerators, and radio isotopic neutron emitters, nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes due to the materials used to moderate the primary fission neutrons.

Radiochemical Methods There are 3 types of neutrons, classified according to their energies: 1) thermal (low energy, < 0.5eV) 2) epithermal (mid energy, 0.5eV to 0.5MeV) , and 3) fast (> 0.5 MeV) An NAA technique that employs nuclear reactions induced by fast neutrons is called fast neutron activation analysis (FNAA).

J. Ruticka and J. Stary, Substoichiometry in Radiochemical Analysis, REFERENCE J. Ruticka and J. Stary, Substoichiometry in Radiochemical Analysis, Pergamon Press, (1968) D. Brune, B. Forkman, B. Person, Nuclear Analytical Chemistry, Chartwell- Bratt Ltd., (1984) POWERSHOW.COM