I hope you have noticed that we devote an entire forum to this topic - So called Natural Vitamin C.
I'm not sure if/what the official purity definition is, but every "synthetic" product comes with an assay of its purity and what else (impurities) may be in the product, etc. Ascorbic acid comes in different grades, but as the number of producers has dwindled, I doubt there is all that much difference.
As far as the Foundation product, it is made in Ireland by Roche, is the highest quality vitamin C there is. We asked our bottler, Douglas Laboratories, if we can say pharmaceutical grade on the label, and they said yes.
One of the problems with so-called "Natural C" is that there is no grade on purity, because it is a plant extract.
Owen R. Fonorow, Orthomolecular Naturopath
My statements have not been evaluated by the Food and Drug Administration. Any product mentioned is not intended to diagnose, treat, cure or prevent any disease.”