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Purity

The proportion (%) of active drug in a preparation: a different concept topotency, which is quantitative measure of the activity or strength of a drug. Most laboratories determine purities with respect to the base because in a sample sent for analysis the particular salt form cannot be determined without further, often unnecessary, investigation. So, for example, pure amphetamine base has a purity defined as 100%. When amphetamine base reacts with, for example, sulfuric acid to form the sulfate salt, then the purity of that salt, with respect to the base, is 79%; the remaining 21% is the sulfate residue. If the purity is expressed with respect to a specific salt form, then pure amphetamine sulfate has a purity of 100%.

EMCDDA Drug profiles glossary