Interpreting a COA: Reading the Numbers Line by Line

This guide walks through a peptide certificate of analysis in the order you would check it. Have a real COA alongside if you can. The aim is to confirm that the certificate applies to your material, that the identity result supports the right molecule, and that the purity result means what you think it means.

1. Header: compound, lot and date

Confirm the compound name and, if given, the sequence. Then match the lot number to the label on your vial exactly. Note the date of analysis: a certificate from long ago describes the material when it was tested, not now.

2. Identity: the mass spectrum

The COA should state an expected (theoretical) mass and an observed mass. Small differences are normal; large ones need explanation.

• Average or monoisotopic? Larger peptides are often reported as average mass, small ones as monoisotopic. The two differ by about 1 Da per ~1,800 Da of mass. Check which the COA uses.
• Charge states. Electrospray produces ions with several charges. A peptide of mass 1,419 Da can appear at m/z of about 1,420 (1+), 710.8 (2+) and 474.2 (3+). The certificate should either deconvolute these or show them clearly.
• Adducts. Peaks 22 Da above the [M+H]⁺ ion are usually sodium adducts, not impurities.
• Telling peaks. +16 Da often indicates oxidation; a loss of one residue’s mass suggests a deletion sequence.

See Mass Spectrometry Explained for more detail.

3. Purity: the chromatogram

Purity is usually given as an HPLC area percent: the main peak’s area divided by the total area of all integrated peaks, at a stated UV wavelength (often 214–220 nm, where the peptide bond absorbs). Read the trace, not just the number:

• Is there a single dominant, reasonably symmetrical main peak?
• Are the impurity peaks small and few, or is there a broad hump?
• Is the integration shown, with peak table and retention times?
• Is the method stated: column type, gradient, wavelength?

4. Other results

If the COA reports peptide content, water or counterion levels, use them for concentration calculations. If it reports appearance, compare with your vial. Endotoxin and sterility are only relevant to specific applications.

5. Sign-off and source

Look for a named testing laboratory, a signature or approval, and a date. Note whether testing was in-house or by an independent laboratory, and whether the raw data is available on request.

6. Compare across lots

A single good COA is reassuring; a consistent series is better. Comparing certificates for several lots of the same compound shows whether purity, mass results and methods are stable over time, and whether the certificates look genuinely lot-specific.

Practise on real certificates

Seeing real certificates side by side is the quickest way to learn the format. As one example, NuLife Peptides publishes its batch certificates on a public page, NuLife Peptides’ COA library, where each certificate shows a lot number, an HPLC trace and a mass spectrum. Practise the checks above on any of them: match the lot, find the observed mass and charge states, read the chromatogram rather than only the headline purity, and compare several lots of one compound.