Obtaining 100% pure metal
The only way to get a "100% pure" sample of an element is by mass spectroscopy, or something similar, which acts on each atom (or ion, actually) one at a time.

So you could get a sample of pure rubidium of a few dozen atoms. Caveat: "100 % pure" applies only to those atoms actually observed... there could be other elements in the vicinity that have not been observed.

Practically, though, there is no way to prevent a single atom of hydrogen, for example, from eventually contaminating your "100 %" pure sample.