How to Read a Peptide Certificate of Analysis

A certificate of analysis is the document that tells you what is actually in the vial. Two things matter on it: whether the compound is the one named on the label, and how much of what is in there is that compound rather than something else.

Everything else on the page supports those two questions. Once you know what to look at, a COA takes about a minute to read.

The two tests that carry the document

HPLC tells you how pure it is

High performance liquid chromatography pushes the dissolved sample through a packed column under pressure. Different molecules move through at different speeds, so they come out separated in time. A detector at the far end records what leaves the column and when.

The output is a chromatogram: a flat baseline with peaks rising off it. Your target peptide is the big peak. Anything else that shows up is an impurity, which might be a fragment, a truncated sequence from synthesis, or residual solvent.

Purity is calculated from area. If the main peak accounts for 98.6% of the total peak area, the certificate reports 98.6%. That is what the number means. It is a proportion of what the detector saw, not an absolute measure of everything in the vial.

Mass spectrometry tells you what it is

Purity alone does not confirm identity. You could have a 99% pure sample of entirely the wrong molecule. Mass spectrometry closes that gap.

The instrument ionizes the sample and measures mass to charge ratio, which gives you the molecular weight. Every peptide has a molecular weight you can calculate from its sequence. If the measured weight matches the theoretical weight, the molecule is what the label says. If it does not match, something is wrong regardless of how clean the HPLC trace looks.

This pairing is the whole point. HPLC answers “how much of this is the thing,” and MS answers “is the thing the right thing.” A certificate with only one of the two is doing half the job.

What to check, in order

1. Compound name and sequence. The single letter or three letter amino acid sequence should be printed. It should match the compound you ordered.
2. Batch or lot number. This ties the document to a specific production run. A COA without one cannot be traced to anything.
3. Date of analysis. A certificate from four years ago describes a batch from four years ago.
4. Purity percentage and method. The number plus how it was measured, normally HPLC.
5. Molecular weight, theoretical and observed. These two should agree.
6. The actual chromatogram and MS trace. Not just a summary table. You want to see the data.
7. Who ran the test. An in house lab or an outside one, named on the document.

Reading the chromatogram itself

You do not need to be an analytical chemist to get value out of the trace.

Look for one dominant, sharp, symmetrical peak. That is what a clean sample looks like. Then look at the baseline around it. Small bumps are normal in any real sample. What you are watching for is a second peak large enough to be a meaningful fraction of the total, or a main peak that is broad and lopsided rather than crisp.

Also check that the axes are labeled and readable. A chromatogram cropped so tightly that you cannot see the baseline, or scanned so poorly that the numbers are illegible, is not evidence of anything.

Things that should make you slow down

• No batch or lot number anywhere on the page
• No date
• A purity figure stated with no chromatogram behind it
• A chromatogram with the axis labels cropped off
• No mass spectrometry data at all
• The same certificate reused across different products, which you can spot by identical lot numbers or identical trace shapes
• Molecular weight that does not match the published weight for that peptide

None of these prove bad material on their own. They mean the document is not doing what a certificate is supposed to do, which is let someone outside the company verify a claim.

What purity numbers actually mean in practice

Research grade peptides are typically reported at 98% or higher, and it is worth understanding what the remaining fraction is.

Peptides are built one amino acid at a time on a solid support. Each coupling step is efficient but not perfect, so a small number of chains end up missing a residue. These deletion sequences are chemically very similar to the target, which is exactly why they are hard to separate and why they turn up as small neighboring peaks.

There is also water and residual salt. Freeze dried material holds some moisture and often counterions left from purification, and that mass is part of what is in the vial without being part of the peptide. This is normal, expected, and one reason net peptide content can differ from the HPLC purity figure.

Common questions

Is a higher purity number always better?

Above a certain point the difference stops being meaningful for most work, and a number is only as good as the trace behind it. A documented 98% with a clear chromatogram and matching mass spec tells you far more than an undocumented 99.9%.

What is the difference between third party and in house testing?

In house testing is run by the producer. Third party testing is run by an independent lab with no stake in the result. Both use the same instruments and methods. The difference is who is checking the work, and independent testing is the stronger position.

What does mg on the label refer to?

It refers to the peptide content of the vial. Because freeze dried material also contains some water and salt, the total powder mass can be slightly higher than the stated peptide mass. This matters when you calculate strength during reconstitution.

Does a COA say anything about how the vial was stored?

No. It describes the batch at the time of testing. What happened afterward is down to handling, which is why storage conditions matter as much as the certificate does.

TL;DR

Check the sequence, the lot number and the date. Confirm there is an HPLC trace and a mass spec result, and that the observed molecular weight matches the theoretical one. Look at the chromatogram rather than just the summary number. If the document will not let you check the claim yourself, it is marketing rather than data.

Everything we supply is lab tested for purity and identity, and sold as research grade material for laboratory and research use. You can browse the full range in the shop, or start with the supplies you will need to work with it.