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.
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