I Google the the ACR (refrigerant)tubing standard ASTMB280- ACR tubing is required to be 99.9% copper per that standard.
As I understand it the only difference between plumbing and refrigerant tubing is that the refrigerate tubing has been cleaned and dehydrated to a higher standard so as not to contaminate a refrigerant system. Also, the refrigerant guys use O.D. as opposed to I.D. like the plumbing guys. Keep this in mind when buying fittings.
99.9% is as pure as it gets in my world. I probably am getting more chem exposure typing on this keyboard....
