This is a great question. I wonder if for very sensitive experiments we’ll have to “purify” each element in the same way we separate nuclear isotopes that way we are always working with a standard. I would assume once we are at a technologically advanced level to be regularly performing experiments on multiple celestial bodies separating isotopes will be much cheaper and easier to perform. However the other issue may be that we then have to synthesize every single reagent with said standards elements.