Hmmm ... well, for mining purposes, copper compounds are separated using floatation agents, maybe they will separate copper compounds from dust, which is essentially dry soil blown in from out of doors. Or maybe you can separate the components with fluids of a particular density? Or can you subtract it out? For example, you know the total copper, and the total weight, maybe you can determine the percentage of the difference that is copper metal buy how much mass it gains by oxidation, assuming the copper compounds can't react further.