Isotope Dilution Method

Isotope Dilution Method

Isotope dilution analysis is used to determine the unknown amount of a given species of matter mixed up in a large sample.
Examples are asserting the amount of an active component as auromycin in large fermentation broth, or the volume of blood in a living being.
Isotope dilution analysis is of three types-
1. Direct Isotope Dilution Analysis (DIDA)
2. Inverse Isotope Dilution Analysis (IIDA)
3. Sub Stoichiometric Isotope Dilution Analysis (SSIDA)
1. Direct Isotope Dilution Analysis (DIDA)
In this analysis to the sample containing x gm of the species, y gm of a radioisotope form of the species of initial specific activity S_i counts min^-1 mg^-1 is added and thoroughly mixed. Then a small amount of the species from the mixture is isolated and the final specific activity (S_f) is measured.
According to the principle of the conservation of total activity, initial and final total activities should be equal.
Hence (x + y)S_f = ys_i
x = y[(_i − _f)/_f]
2. Inverse Isotope Dilution Analysis (IIDA)
This is just reverse of the DID analysis. In this analysis to a small amount of a radioactive substance x gm having initial specific activity S_i, y gm of the nonradioactive compound of the same substance is added and thoroughly mixed. Then a small amount of the mixture is isolated and final specific activity S_f is determined.
(x + y)S_f = xS_i
x = y[_f/(_i − _f)]
Using this method the amount of a radioactive substance 'x' initially present can be measured.
3. Sub Stoichiometric Isotope Dilution Analysis (SSIDA)
In this method, an equal amount of extractant is added to two solutions one containing known amount (m₁) of labelled isotope and the other containing unknown original solution (m₂) and same amount (m₂) of labelled solution. After the extraction the specific activity is measured in both the cases and from these the mass of the unknown substance is calculated.
Let S₁ be the specific activity of known stock solution containing the mass m₁ (known) and S₂ be the specific activity of mixture containing m₂ amount of unknown and m₁ amount of isotope.
(m₂ + m₁)S₂ = m₁ S₁
m₂ = m₁[(S₁ − S₂)/S₂]
This technique finds extensive application in radio immunoloassay.

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