Purity has to do with the composition or makeup of a substance- it is a measure of the relative freedom from contaminants.
Quality is a general term applicable to a broad range activities.
Purity is a more specific term elated to chemical composition of materials.
Milo
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People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe