Understanding Peptide Purity

What purity actually measures

A stated purity of 99% almost always refers to HPLC peak area, the proportion of UV-absorbing material eluting as the target peptide. It is not the same as mass content, which also includes counterions, residual solvent, and water.

Reading an HPLC chromatogram

• Retention time identifies the compound against a known standard.
• Peak area percentage is the purity figure. Look for a single dominant peak.
• Shoulder peaks next to the main peak usually indicate deletion sequences or oxidised variants.
• Baseline noise should be flat. A drifting baseline can hide small impurity peaks.

Mass spectrometry

The MS report confirms identity, not purity. The observed molecular weight should match the theoretical weight for the sequence within about 1 Dalton. A mismatch means the wrong sequence was synthesised, regardless of how clean the HPLC trace looks.

What a complete COA contains

1. Product name and full amino acid sequence
2. Lot or batch number matching the vial label
3. Test date and the testing laboratory
4. HPLC chromatogram with the purity percentage
5. Mass spectrometry trace with observed and theoretical mass
6. Net peptide content, where reported

Red flags

A COA with no lot number, no chromatogram image, or a lab name that cannot be verified independently should be treated as unverified. Third-party testing means the analysis was run by a laboratory with no commercial interest in the result.