The proportion of a peptide sample that consists of the intended compound, as measured by high-performance liquid chromatography (HPLC). A purity of 98% means 98% of the sample is the target peptide; the remaining 2% consists of synthesis byproducts, truncated sequences, or other impurities.

Research-grade peptides typically range from 95–99%+ purity. Higher purity costs more because additional purification steps are required. For research purposes, purity matters because impurities can produce biological effects of their own—confounding experimental results or, in the case of self-experimentation, introducing unknown risks. A certificate of analysis from an independent lab is the standard way to verify purity claims.