What is Purity? (Peptide Glossary)

Purity measures how much of the material in a vial is actually the peptide on the label versus synthesis byproducts. Peptides are built amino acid by amino acid, and each coupling step has a small failure rate. The failures produce deletion sequences and truncated chains - molecules that look similar but are not the target compound. Purification (usually preparative HPLC) removes most of these, and the purity percentage tells you how well that worked.

The practical thresholds: 98% or higher is research-grade and what any reputable vendor should deliver. 95-98% is acceptable for some applications but below par for a premium supplier. Anything under 95% suggests poor synthesis or skipped purification. Claims above 99.5% are achievable for short peptides but should be viewed with mild skepticism for long, complex sequences.

Remember that purity is only half the quality picture. A vial can be 99% pure and still be the wrong compound (identity is confirmed by mass spectrometry), underfilled (quantity is confirmed by weighing or quantitative testing), or contaminated with endotoxins (measured separately). Complete quality assessment requires purity, identity, quantity, and sterility data together.