Radioactivity Calculation for assay

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Aardvark
Aardvark's picture
Radioactivity Calculation for assay

We are currently runnining an assay, where the radiolabel splits into two from the reaction, with the radioactive bit in the aqueous phase, and the non – radioactive bit in the organic phase.
 
I now need to quantify the amount of radiation in the aqueous phase (in nM).
 
I know how to work out radiactive concentrations (from specific activity and cpm).

But, am I right in assuming that the specific activity of the split radiation molecule will be the same as the parent radiation molecule I put in the assay.
 
I need the specific activity to work out my concentrations.
 
Sorry if this is a daft question.

Thank you for your help!

Sami Tuomivaara
Sami Tuomivaara's picture
Aardvark,

Aardvark,

First of all, your assumption is correct: the radioactivity of carbon-14 and tritium (and other common radionuclides) don't depend on the molecular structure, so labels have the same activity whether in your original molecule or the product because they decay by beta emission. (I think there are minor exceptions though, with some nuclides that decay by electron capture, but they are of not concern here).

I think the most straightforward way is to use liquid scintillator for both solutions (before and after the reaction) and take the ratio of these. Because radioactivity doesn't depend on the molecular structure, the ratio of cpms is the same as the ratio of the molecules themselves. So, now you can calculate the amount of product molecules in the scintillation vial (in moles). From that you can calculate the concentration (in M) of the product by taking into account the aliquot volume you put in the vial and the total volume of the aqueous solution.

I don't think there's need to calculate specific activities because you work with molarities and not masses, but: By definition, specific activity is the radioactivity of the material per unit mass. Hence, your specific activity goes up (assuning 100% yield of the reaction) as you get rid of the non-radiactive part of the molecule. If you know the molecular weights of the starting molecule and the radiactive bit, you can calculate the expected specific radioactivity of the radioactive bit by dividing the original radioactivity with the ratio of the molecular weights of the starting molecule and the bit with the radioactivity.

Cheers,

Aardvark
Aardvark's picture
Thank you very much Suola.

Thank you very much Suola.