radiochemical method

RADIOCHEMICAL METHOD IN ACTIVATION ANALYSIS &

ISOTOPIC DILUTION METHOD

PRESENTS BY

PRADEEPKUMAR YADAV

RAMNIRANJAN JHUNJHUNWALA COLLEGE

MSC (II) (PHYSICAL CHEMISTRY)

SEM – (III) (2013 – 14 )

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.


Radiochemical methods are both sensitive and

specific. There are three general types of radio-

analytical methods of analysis:

1. Radiometric analysis

2. Isotope dilution, and

3. Activation analysis

Radiometric Analysis

Radiometric analysis – the use of a radioactive reagent ofknown activity to isolate the analyte from the othercomponents of the sample. The activity of the product isdirectly proportional to the amount of the analyte.

Example: Chromate has been determined by precipitating itwith radioactive Ag+ (Ag-111, , , 7.5d) of a known activity.The limited reactant is the analyte, here CrO4

-2. Determiningthe activity of the precipitate of Ag2CrO4 allows for thedetermination of the amount of chromate.

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.

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 ofthe 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 theradioactive compound added, Ai is the activity of the addedcompound, and Af is the activity of the final purifiedcompound.

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.

http://www.wcaslab.com/tech/isotope_dilution.htm

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.


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 isirradiated with neutrons. After the irradiation, the gammaor beta spectrum is obtained, depending on the type ofemission produced by the irradiated element. Forquantitative work, both may be used. The energies of thespectral peaks allow for the identification of the elementspresent and the areas of the peaks define the amounts ofeach element as shown in the next slide.


Gamma ray spectrum of a sample after neutron induced radioactivity


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.


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.


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

http://www.missouri.edu/~glascock/nna_over.htm


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.


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.


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).

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)

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