Hi there, biscuitses:, you've asked a common question, and I'm sorry to say, I don't suspect you're going to be successful in getting the information you want.
There's no grand master resource of extinction coefficients for every obscure chemical. You might have gotten lucky, one time, with a Google search, for a very commonly used reagent. But that's doen't make what I said false.
You'll get a different extinction coefficient for various solvents. Resources that give extinction coefficients pick one or two solvents. You even switched solvents mid-thread with nothing more than a "Oops." This is going to go nowhere even faster.
I was going to say you should be following a defined protocol. But now it seems you're developing a complete assay on your own. In which case its, as was said, up to you to develop all steps. But you don't have pure standard? Thedre's too many layers of uncertainty here.
Oh by the way, what wavelength? You didn't even mention that. A compound has no extinction coefficient at wavelengths it doesn't absorb. Did you pick a good wavelength? How will we know? How did you decide what wavelength to work at? What reference did you use for that?
You have to refine your question -- where did you come up with this procedure, how much of a procedure do you have, and why don't you have a more complete procedure.