What an HPLC Purity Number Actually Means
An HPLC purity percentage tells you what share of the sample is the target peptide versus impurities — and that number is only meaningful when the chromatogram behind it shows a single dominant peak at the expected retention time. A “99% pure” claim printed without the actual trace is not a test result, it is marketing.
The Three Numbers on a Real Report
A legitimate COA gives you three things: the HPLC purity percentage, the retention time of the main peak, and the peak area integration — all of which need to line up. If any of the three is missing, you cannot independently confirm the purity claim.
- Purity % — the target peptide’s share of total peak area (accept ≥98% for research)
- Retention time — where the main peak elutes; should be stable across the same column and method
- Peak area / integration — the raw data the percentage was calculated from
Why Mass Spectrometry Is Non-Negotiable
HPLC alone can tell you how pure a sample is, but only mass spectrometry confirms you actually received the right peptide with the correct molecular weight. A sample can be 99% pure and still be the wrong molecule — MS is what catches that. For research integrity, request both HPLC and MS on the same batch.
Matching the Expected Mass
The MS spectrum should show a peak matching the peptide’s theoretical molecular weight within a tight tolerance (±2 Da), which is how you rule out receiving a substituted or truncated sequence. Every peptide has a known mass — for example, Tirzepatide sits near 4813 Da — so a mismatch is an immediate red flag.
Third-Party Testing Adds Credibility
Independent verification through a lab like Janoshik or MZ Biolabs is the strongest authenticity signal a supplier can offer, because it removes the conflict of interest in self-reported COAs. Suppliers that openly encourage third-party testing and publish the results are materially more credible than those that don’t.
COA Red Flags to Catch Before Ordering
- No chromatogram or spectrum attached — the purity number is unverifiable
- Missing retention time or peak area — you can’t confirm the integration
- Recycled lot numbers — the same lot appearing across months suggests a reused template
- Refusal to allow third-party testing — a strong signal to walk away
FAQ
Q: What purity should I accept for a research peptide?
A: Accept ≥98% by HPLC with a single dominant peak and a matching MS spectrum; below 95% introduces impurities that can confound experiments. The percentage alone is insufficient — you need the chromatogram behind it.
Q: Is HPLC enough to confirm a peptide’s identity?
A: No — HPLC confirms purity, but only mass spectrometry confirms you received the correct molecule at the correct molecular weight. Both should appear on the same batch-specific COA.
Q: What is a normal retention time on a peptide HPLC run?
A: Retention time depends on the column and method, so there is no universal number — what matters is a single dominant peak that is stable and reproducible. A supplier should be able to state the column specs and method used.
Q: Why should I prefer a supplier that supports third-party testing?
A: Third-party testing removes the conflict of interest in self-reported COAs and is the strongest credibility signal available. Suppliers that encourage it are generally more transparent about their material.