How to Read a Peptide Certificate of Analysis (COA)

A certificate of analysis (COA) is the test report for one specific production lot of a compound. It tells you what was tested, how it was tested, who tested it and what the results were. A COA only describes the lot it names, so the first thing to check is whether it matches the vial in front of you.

The fields that matter

Product name and lot number. The lot (or batch) number on the COA should match the lot number printed on your vial label exactly. A COA with no lot number, or one that doesn't match your vial, says nothing about the material you have.

Test date and testing laboratory. A COA should state when the analysis was performed and which lab performed it. If the lab is a third party, its name should appear on the report.

Purity by HPLC. Purity is normally reported as a percentage from high-performance liquid chromatography (HPLC) with UV detection. It is the share of the target peptide relative to all UV-absorbing material detected, which includes peptide-related impurities (Bachem). A good COA also includes the chromatogram itself, not just the number.

Identity by mass spectrometry. Mass spectrometry (MS) confirms the compound is what the label says by measuring its molecular mass (JPT Peptide Technologies). Look for the observed mass listed next to the expected (theoretical) mass for that sequence. They should agree within the method's tolerance.

Appearance and quantity. Many COAs note the physical form (for example, white lyophilized powder) and the stated amount per vial.

What a purity number does not tell you

HPLC purity and mass spec answer two different questions. Purity tells you how much of the UV-absorbing material is the target peptide. Mass spec tells you the target peptide is present. Neither one measures how much of the powder's total weight is peptide. Lyophilized peptides also contain counter-ions and residual water, which is described separately as net peptide content (Bio-Synthesis). See our guide to peptide purity testing for more detail.

Warning signs on a COA

• No lot number, or a lot number that doesn't match the vial
• No test date or no named laboratory
• A purity figure with no method stated and no chromatogram
• No identity test (mass spec) at all
• The same COA reused across different lots

Zerilyx certificates

Zerilyx publishes certificates by lot on the Certificates of Analysis page. Our Quality page explains how lots are verified. If you can't find the certificate for your lot, email help@zerilyx.com with the lot number from your vial.