How to Read a Peptide Certificate of Analysis (COA)

A peptide COA is only as useful as your ability to interpret it. The four values that matter most: HPLC purity percentage, observed mass (from MS), retention time, and counterion / water content.

HPLC purity should be reported as area percent at a specified wavelength (typically 220 nm for peptide bonds). Anything below 98% indicates significant impurities; ≥99% is the standard for analytical-grade research peptides.

Mass spec confirmation is non-negotiable for any sequenced peptide. The observed mass should match the theoretical mass within 1 Da. A larger deviation suggests deletion sequences, oxidation, or counterion adducts that will affect dosing accuracy.

Retention time is your fingerprint for batch-to-batch consistency. If two batches of the same peptide show meaningfully different RT under identical chromatographic conditions, something has changed — sequence, modification, or impurity profile.

Counterion content (typically TFA or acetate) reduces the actual peptide mass per vial. A peptide with 15% TFA counterion delivers only 85% peptide by weight. Quality COAs disclose this; mediocre ones omit it.