Ratio of labelled vs. unlabelled species in "uptake assay" - Whether to use a fixed amount or a fixed concentration of label (Dec/18/2007 )

[::cross-post from the Biochemistry forum::]

This might be straightforward, but please enlighten me:

I am doing an assay, where I want to see how much (moles) of a molecular species is taken up by my model organism (C. elegans) at various concentrations of that molecule. I want to use a radiolabelled species, but it is unfeasible to use radiolabelled species only. But which is the proper thing to do:

a: Use the same amount of radiolabelled species in all samples/at all concentrations - and add unlabelled species to the desired final concentration,
b: Prepare a mastermix of labelled and unlabelled species and use that for preparing all samples - meaning that the ratio labelled:unlabelled remains constant in all samples,
c: It doesn't really matter what I do - both approaches are valid.

So, which is it? What would you do?

see my reply in the "biochemistry" post.

and please don't cross-post