Reading a Peptide COA: What HPLC and Purity Numbers Actually Tell You

A Certificate of Analysis, or COA, is the single most useful document a peptide supplier can hand you — and also one of the most commonly skimmed rather than actually read. Understanding what's on it, and what it does and doesn't confirm, makes it a genuinely useful research tool instead of a box-checking formality.

What HPLC Purity Actually Measures

High-performance liquid chromatography (HPLC) separates the components of a sample based on how they interact with a chromatography column, producing a chromatogram — a plot of detector signal against time. Each distinct peak represents a different compound eluting from the column. The "purity" percentage commonly quoted on a COA is typically the relative area of the main peak (the target peptide) as a proportion of the total peak area detected, which is a reasonable proxy for compositional purity but not an absolute, all-encompassing measurement.

A few things worth knowing when you look at that number:

• It reflects what the detector can see, at the wavelength and method used. Compounds that don't absorb well at that wavelength can be underrepresented.
• A high purity percentage confirms the sample is compositionally clean relative to detectable impurities — it does not, by itself, confirm the sample is the correct peptide sequence.
• Method matters. Different HPLC gradients, columns, and detection wavelengths can produce slightly different results for the same sample.

What Mass Spectrometry Adds

This is where identity confirmation comes in. Mass spectrometry (MS) measures the mass-to-charge ratio of ionized sample components, allowing a lab to confirm that the compound detected actually matches the expected molecular weight of the target peptide. Pairing HPLC (purity) with MS (identity) is considered a more complete verification than either test alone — HPLC tells you how clean the sample is, MS tells you it's actually the compound it claims to be.

What to Check on a COA

When you're reviewing a Certificate of Analysis, a few fields are worth paying particular attention to:

• Batch or lot number — this should match the code printed on your physical vial, not just the product name generically.
• Test date — recent testing tied to your specific batch, not a generic certificate reused across unrelated production runs.
• Testing lab — ideally an independent third party rather than solely in-house testing, since in-house-only results carry an inherent conflict of interest.
• Method disclosed — HPLC alone, or HPLC plus MS. Both being present is a stronger signal than purity claims with no stated method.

Why Independent Testing Matters

A COA generated entirely in-house tells you what a supplier claims about their own product. A COA from an independent third-party lab is a separate party's assessment, with no direct financial stake in the result. That distinction is exactly why we publish independently generated COAs rather than relying solely on our own internal QA.

The Bottom Line

A COA is a valuable verification tool, but only if it's current, batch-matched, and methodologically transparent. When you're evaluating any supplier — including us — a good habit is to check that the batch code on the certificate matches what's on your vial, and that the testing method and date are both disclosed rather than vague.