Very cool Aidan, I wasn't aware of this particular test for gold. And if it works for ancient gold it must work for ancient silver as well since the two metals are always associated. Many denarii contain up to 1% Au straight into the era of the antoninianus. Not on purpose of course, just what the refining tech of the day allowed. It is unfortunately a destructive process so in most cases this would be inappropriate for testing coins. And on top the process appears to be able to narrow down only to within 1,100 years. With that margin of error you basically get only pass/fail status on authenticity, not really useful for dating ancient coins. And, yes, with this type of testing (mass spectrometry) the presence of any transuranic element - even a single atom - should immediately cast doubt on the authenticity of the coin.