When looking at a Chemical, what does this mean... "99.9 atom % D" Hey, can someone explain to me what this means. Lets use D2o for example (Deuterium oxide) 1 store says "99.999 atom % D" And the other store simply says "DEUTERIUM OXIDE 100% D,99.96% " Can someone please shed some light on this for me? I am curious? Is it its purity? So, if one says (99.99%) and the other says (99.96%), what are they talking about? Thanks very much.