How to read a peptide Certificate of Analysis

The core fields

Most CoAs follow the same skeleton. Reading them in order is the quickest way to spot gaps.

• Product name and sequence: the compound should be named precisely, ideally with its sequence or CAS number.
• Batch or lot number: the link between the document and the physical vial.
• Appearance: usually a white lyophilised powder. It is a weak check on its own.
• Purity by HPLC: the percentage of the main peak. See the guide on purity versus net peptide content.
• Identity by mass spectrometry: the observed molecular mass compared with the theoretical mass.
• Date and signature: when the testing was done and by whom.

Checking identity with the mass

Mass spectrometry tells you whether the molecule is the one that was ordered. Calculate the expected mass from the sequence, then compare it with the observed value on the CoA. A difference of more than about 0.1% deserves an explanation.

Be aware of which mass is being quoted. Monoisotopic mass matches the main peak in high-resolution instruments, while average mass weights every isotope. Our peptide mass calculator shows both.

Reading the HPLC trace

A purity figure is the area of the main peak divided by the total area of all peaks. It says nothing about water, salts or counter-ions, which do not show up in the same way.

If a chromatogram is included, look for a single dominant peak, a flat baseline and a stated detection wavelength. A purity table with no trace is much harder to trust.

Red flags

None of these proves a document is false, but each is a reason to ask the supplier questions.

• No batch number, or one that does not match the vial.
• Round-number purities such as exactly 99.0% with no chromatogram.
• A mass listed as the theoretical value, rather than an observed one.
• Generic templates that could belong to any peptide.
• No named analyst, laboratory or method.

Independent testing

A supplier's own CoA is a starting point. For work where identity is critical, a third-party analysis of a retained sample is the stronger form of evidence.