How to Read a Peptide HPLC Certificate of Analysis
Almost every peptide supplier advertises a purity figure. Far fewer publish the document behind it, and fewer still explain how to read one. A certificate of analysis is the only evidence you have that the vial contains what the label says — but only if you know which numbers matter and which are decorative.
This guide walks through what a COA actually reports, how to read an HPLC trace, the one figure people consistently misunderstand, and the warning signs that a certificate is not worth the paper it is printed on.
What a COA is, and what it is not
A certificate of analysis reports the results of tests performed on one specific batch of material. That last part is the whole point. A COA is not a general statement about a compound, or about a supplier’s typical quality — it describes a particular lot, made on a particular date, tested on particular instruments.
This is why the lot number is the most important thing on the page. A COA that cannot be matched to the number printed on your vial is telling you nothing about your vial.
What should be on the page
| Field | Why it matters |
|---|---|
| Product name and sequence | Confirms the identity claimed. The one-letter sequence should match the compound. |
| Lot / batch number | Ties the document to your vial. Without it the certificate is unattributable. |
| Date of analysis | Tells you how old the testing is relative to the material. |
| Molecular formula and mass | Cross-check against the expected values for the sequence. |
| Purity by HPLC | The headline figure, with the method and wavelength stated. |
| Mass spectrometry result | Confirms the molecule is what it claims to be, not merely pure. |
| Appearance | Usually “white to off-white lyophilized powder”. Compare against what you received. |
| Water content | Residual moisture affects both stability and the real mass of peptide present. |
A certificate missing the lot number, the analysis date, or the method behind the purity figure is incomplete regardless of how good the number looks.
Reading the HPLC trace
High-performance liquid chromatography separates the components of a mixture by pushing it through a column; different molecules travel at different speeds and emerge at different times. The detector records what comes off, and the result is a chromatogram — a mostly flat line with peaks where something eluted.
Three things to look at:
The main peak. One large, well-shaped peak dominating the trace is what you want. Your target compound is that peak.
Retention time. The horizontal position of the peak, in minutes. Consistent retention time across lots of the same compound, on the same method, is a sign of a consistent process.
Everything else. Smaller peaks are impurities — typically truncated sequences, deletion products, or degradation. A few tiny peaks are normal in synthetic peptide production. Several substantial ones are not.
Peak shape matters as well as size. A sharp, symmetrical peak indicates clean separation; a broad or badly tailing peak can hide co-eluting material that the percentage figure will not reveal.
What “≥99% purity” actually measures
This is the figure everyone quotes and the one most often misread.
HPLC purity is area percentage: the area under the main peak divided by the total area of all peaks, at a particular detection wavelength. Usually 214 nm, which detects the peptide bond itself, or 220 nm.
Two consequences follow, and both matter.
First, it is relative, not absolute. It tells you what proportion of the detected material is your compound. Anything the detector cannot see at that wavelength does not appear in the calculation at all.
Second, the wavelength is part of the claim. A purity figure quoted without the detection wavelength and method is an incomplete statement. Reputable certificates state both.
The figure most people miss: net peptide content
Here is the distinction that catches out even careful buyers.
Purity describes the proportion of peptide material that is the right peptide. Net peptide content describes how much of the powder in the vial is peptide at all.
Lyophilized peptides are not pure peptide by weight. The powder also contains counter-ions — commonly trifluoroacetate (TFA) left over from purification — along with residual water and sometimes salts. Depending on the sequence and the process, peptide can be somewhere around 70–90% of the total mass, with the balance made up of everything else.
So a vial can be legitimately 99% pure by HPLC and still contain noticeably less actual peptide than the label mass suggests, because purity and content are answering different questions. A certificate reporting both is being more transparent than one reporting only the headline purity number.
If net peptide content is not stated and it matters to your work, ask for it. A supplier who has the data will send it.
Mass spectrometry: the identity check
HPLC tells you how pure the material is. It does not, on its own, tell you that the pure thing is the right thing.
Mass spectrometry closes that gap by measuring molecular mass directly. The observed mass on the certificate should match the theoretical mass calculated from the sequence, within the tolerance of the instrument.
A COA with a strong purity figure and no mass confirmation has answered only half the question. Both belong on the page.
Warning signs
- No lot number, or one that does not match the vial. The document cannot be tied to your material.
- No date of analysis.
- No chromatogram — only a typed purity figure with no trace behind it.
- The same certificate reused across different lots. Compare lot numbers and dates if you order repeatedly; identical traces on supposedly different batches mean the testing is not per-lot.
- A purity figure with no method or wavelength.
- Suspiciously perfect numbers. Real synthetic peptide production yields figures like 98.7% or 99.2%, not a flat 100%.
- No testing laboratory named, in-house or third-party.
What to do with it
Three habits make a COA genuinely useful rather than decorative:
- Match the lot number on the certificate to the number on the vial, every time. This takes five seconds and is the whole basis of the document’s value.
- Keep them. File the COA with the lot. If material ever behaves unexpectedly, the certificate is your starting point.
- Compare across orders. Consistent retention times and purity figures across lots tell you a supplier’s process is under control — which is more informative than any single certificate.
Every lot we ship has a certificate of analysis behind it. You can see them on our certificates of analysis page, and the lot number on your vial will match the one on the document.
If something on a certificate does not make sense to you — ours or anyone else’s — that is a reasonable thing to ask a supplier about. How readily they answer tells you something too.