This document provides an overview of radiochemical methods in activation analysis and isotopic dilution methods. It discusses three main types of radioanalytical methods: radiometric analysis, isotope dilution, and activation analysis. Activation analysis involves irradiating a sample with neutrons to induce radioactivity in atoms. This allows identification and quantification of elements in the sample by measuring the gamma or beta spectra of the radioactive elements. Isotopic dilution involves adding a known quantity of a radioactive isotope to the sample and using the dilution to calculate the amount of the non-radioactive substance.

1. RADIOCHEMICAL METHOD IN ACTIVATION
ANALYSIS &
ISOTOPIC DILUTION METHOD
PRESENTS BY
PRADEEPKUMAR YADAV
RAMNIRANJAN JHUNJHUNWALA
COLLEGE
MSC (II) (PHYSICAL CHEMISTRY)
SEM – (III) (2013 – 14 )

2. 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 decay schemes of that
particular nuclide.

3. Radiochemical Methods
Radiochemical methods are both sensitive and
specific. There are three general types of radioanalytical methods of analysis:
1. Radiometric analysis
2. Isotope dilution, and
3. Activation analysis

4. 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, β, γ, 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.

5. Isotopic dilution
Isotopic dilution analysis was introduced by von Hevesy and Hofer
in 1934. It involves the preparation of the analyte in a radioactive
form. 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 as a chemically pure
substance.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

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

12. 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.

13. 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 nonelastic 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.

14. 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

15. 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.

16. 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.

17. 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).

18. REFERENCE
J. Ruticka and J. Stary, Substoichiometry in
Radiochemical Analysis,
Pergamon Press, (1968)
D. Brune, B. Forkman, B. Person, Nuclear
Analytical Chemistry, ChartwellBratt Ltd., (1984)
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