How to Read a Peptide Certificate of Analysis (and Spot a Fake One)

The Purity Number Is the Least Useful Part

A purity percentage on its own says little. The sections most buyers skip are the ones that show whether the vial contains what the label claims.

What HPLC Purity Actually Measures

Reversed-phase HPLC separates the compounds in a sample and measures UV absorbance as each one comes off the column, typically at 214 to 220nm. Purity is the area under the main peak divided by the total area under all peaks. Three things follow:

• •It is a relative measure. "98% pure" means the main peak is 98% of the UV-absorbing material the detector saw. Salts, solvents and water are largely invisible to this method.
• •It does not identify the compound. A 99% pure peak of the wrong molecule is still 99% pure.
• •It says nothing about sterility. Endotoxin and bioburden do not show up as peaks, so a 99% pure sample can still be contaminated.

Why Mass Spectrometry Matters Most

If a CoA has HPLC but no mass spec, treat it as incomplete.

Every peptide has a theoretical molecular weight computed from its sequence, and the mass spec result should match it within a small tolerance. It is the only section that answers whether this is actually the compound on the label. It is also the section most often missing from forged documents.

What Is Net Peptide Content?

A vial labelled "10mg" contains 10mg of lyophilised powder, not 10mg of peptide. The powder also holds the counterion (usually TFA or acetate), residual water, and leftover salts. Net peptide content is the share of the powder that is actual peptide, typically around 70 to 85% for a synthetic peptide.

Most vendors do not report it. Asking for it is the quickest way to gauge how seriously a supplier takes testing.

What Should a Complete CoA Contain?

The three in bold are the ones to check first.

How Do You Spot a Fake CoA?

Forged CoAs are usually lazy. These checks cost nothing:

• •The batch number does not match your vial. A CoA describes one specific batch. A generic CoA, or a lot number not printed on your vial, is not evidence about your product.
• •The chromatogram has no axes. A real one has a labelled time axis, an absorbance axis, retention times on the peaks, and usually an integration table.
• •The same chromatogram appears on different products. Peak shape and retention time work like a fingerprint for a run. Compare two CoAs from the same vendor side by side.
• •Purity is stated but no data is attached. A percentage with no chromatogram is a claim, not a result.
• •The PDF metadata is off. A creation date after the stated analysis date, or an author field naming the vendor instead of the lab, deserves a second look.
• •Nobody is named. A real testing lab puts its name on its work.

Is Third-Party Testing Better Than a Supplier CoA?

A CoA supplied by the seller comes from the party with the strongest incentive for it to say something specific. Most are genuine, but they are not independent evidence. Buyer-commissioned testing through an analytical lab closes the loop, especially when results go from the lab directly to the buyer. Janoshik Analytical is the most widely used option. A supplier who offers independent results without being asked is giving a meaningful signal.

What Purity Is Acceptable?

For research-grade synthetic peptides, 98% or higher by HPLC is the practical standard. Rank the questions in this order:

1. Is it the right molecule? (mass spec)

2. How much peptide is in the vial? (net peptide content)

3. How clean is it? (HPLC purity). 98% versus 99% matters far less than most forum arguments suggest.

4. What is in the impurity fraction? A few small peaks near the main one are typically synthesis deletion sequences. Unrelated peaks are a different matter.

Disclaimer: This information is provided for educational purposes relating to laboratory research quality control. All products are supplied strictly for in-vitro laboratory and research use. Not intended for human consumption. Not medical advice.