I was thinking about this in organic chemistry terms but soon found out it was a weed thing. Shoulda known.
Check out http://www.airgas.com/
. They sell small tanks, 99.9% pure, and I doubt you'll find anything better than that in terms of quality. They're one of the largest suppliers of research grade fluids. It'll probably be pretty expensive to get something that small and pure in such a small quantity, but it's worth a shot.