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“99% pure” is probably the single most repeated phrase in peptide marketing, and it’s also one of the least explained. This post breaks down what that number is actually measuring, and what it doesn’t tell you.

Purity Is a Relative Measurement

The purity percentage on a COA almost always comes from HPLC peak-area integration: the target peptide’s peak area is divided by the total area of all peaks detected, then expressed as a percentage. This means “99% purity” is a statement about the proportion of detected material that matches the main peak — it is not an absolute measurement of chemical perfection, and it depends heavily on what the test is capable of detecting in the first place.

What Can Make a Purity Number Misleading

A few structural limits are worth understanding. First, HPLC can only report on what elutes and gets detected under the specific run conditions used — a different column, gradient, or detection wavelength can reveal impurities an earlier test missed, so purity numbers aren’t perfectly comparable across different labs or methods. Second, peak-area percentage isn’t the same as mass or mole percentage; different substances can have different detector responses at a given wavelength, meaning peak area doesn’t always translate directly to weight-for-weight purity. Third, related substances (truncated sequences from incomplete synthesis, deletion sequences, or oxidized variants) can sometimes co-elute closely enough with the main peak to be undercounted as impurity.

What a More Trustworthy Purity Claim Looks Like

Look for a COA that shows the actual chromatogram (not just the number), states the testing method and conditions, and ideally pairs the purity result with a mass spectrometry identity check (see our companion post on HPLC vs mass spectrometry). A batch-specific COA tied to a real lot number is more trustworthy than a generic purity claim reused across every listing on a supplier’s site.

Common Misunderstandings

A high purity percentage says nothing about whether the peptide is actually the correct sequence — that’s an identity question, answered by mass spectrometry, not purity testing. It also doesn’t say anything about sterility, endotoxin levels, or heavy metal contamination, which require separate, specific tests. “99% pure” is a real and useful number when reported properly, but it answers a narrower question than it’s often assumed to answer.

Related Reading

See our full guide to reading a peptide Certificate of Analysis and HPLC vs mass spectrometry for the underlying test methods. Browse COAs, Lab Testing & Documentation for related discussion.

Questions for Discussion

Have you compared purity numbers for the same peptide across different suppliers and seen meaningful discrepancies? What additional tests (beyond HPLC and MS) do you think should be standard for a trustworthy COA? How do you weigh a supplier’s in-house purity claim against independent third-party testing?

This post is educational information about analytical chemistry and does not constitute a recommendation to purchase or use any research compound.

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