Protocol Online logo
Top : New Forum Archives (2009-): : Biochemistry

How to reduce the specific activity of a radioactive compound? - (Aug/16/2011 )


I was wondering if anyone could shed some light on how to reduce the specific activity of a compound.

I have a tritiated compound required for my experiment but I worked out at the concentrations I need I would be using a vial for every replicate!

So I need to reduce the specific activity but I am not sure how to do this. Presumably you do this with unlabeled compound? I have some unlabeled compound but its in a powder, I assume I add this to the radioactive compound but won't it affect the concentration?

Also, will you need extra time to count each sample? I currently count at 1 min per sample.

Thanks for the clarification!!


you add cold carrier (unlabeled compound) to the labelled compound.

yes, the concentration of the compound will be changed (it should be higher). recalculate for your reactions.

you probably will be adding sufficient labelled compound to count as you were, but, counting time is usually determined by the label rather than the amount of label (e.g.- 1 minute for 32P, 4 minutes for 14C and 10 minutes for 3H). if you're counting 3H for 1 minute then you are using too much label in the reaction.

also, especially when using 3H, you should count for dpm (corrected for quenching) rather than cpm.


Thanks for your help!! I will start by trying 10min per sample then.