Guide

How to read a peptide certificate of analysis

A certificate is the only document in this category that carries checkable information. Knowing how to read one is the difference between buying evidence and buying a claim.

Written by [[CONTENT AUTHOR NAME]]Reviewed by [[SCIENTIFIC REVIEWER NAME]], [[REVIEWER CREDENTIALS]]Updated July 28, 2026Editorial policy

A certificate of analysis reports what a named laboratory measured on one specific lot. A usable one names the lot, states each method, gives a result against a specification, and is traceable to the laboratory that issued it. A purity figure with no lot number and no issuing laboratory is a marketing claim, not a result.

A certificate belongs to a lot, not to a product

This is the point everything else follows from. Synthesis is a process with variation, so purity and net content are properties of a particular batch of material. A certificate describes the batch it was run on and nothing else.

Consequently, a supplier that shows the same certificate on a product page for years is either not producing new lots or not testing them. When a new lot is released it is tested again, and the certificate published against it is that lot's certificate.

The six things a usable certificate has

A lot number, matching the number printed on the vial in front of you.
The name of the issuing laboratory, so the result is attributable to someone who does not work for the seller.
A date of issue, and where applicable a retest or expiry date.
The method used for each result — not just a number, but how it was obtained.
A specification alongside each result, so you can see what the result was measured against.
The identity of the material as tested, confirmed rather than assumed.

Identity: mass spectrometry

Mass spectrometry confirms that the molecule present is the molecule named, by measuring its mass. For a peptide this is the check that the sequence assembled is the sequence intended, and it is the result that makes the rest of the certificate meaningful — a purity figure for the wrong compound is worse than no figure at all.

Certain structural features show up here specifically: a C-terminal amide, an N-terminal acetyl or acyl group, a disulfide or lactam bridge in a cyclic peptide. Each shifts the expected mass, so each is confirmable.

Purity: HPLC, and what the percentage means

High-performance liquid chromatography separates the target peptide from synthesis-related impurities — truncated sequences, deletion products, incompletely deprotected species — and reports the target as a percentage of total peak area at a stated detection wavelength.

That last clause is doing real work. Purity by peak area at 214 nm is not the same measurement as purity by mass, and it does not account for anything that does not absorb at the wavelength used, which includes residual water and counter-ions. A 99% figure means 99% of what the detector saw.

Net peptide content, which is not the fill weight

A vial labelled 10 mg contains 10 mg of lyophilized solid. That solid is peptide plus water plus counter-ions such as trifluoroacetate or acetate from purification. Net peptide content is the actual mass of peptide, and it is routinely lower than the fill weight.

This is the parameter most often missing from certificates in this category, and the one that most directly affects a researcher preparing a solution of known concentration.

Contaminant screening, where the panel includes it

Depending on the panel run, a certificate may also report heavy metals, residual solvents, endotoxin or sterility. Which of these are appropriate depends on the material and the intended research context — endotoxin, for instance, matters far more for a recombinant protein than for a short synthetic sequence.

A certificate that reports fewer parameters is not automatically worse. A certificate that reports parameters it did not test would be.

Checking a certificate against the vial

The lot number on the label is the key. It should let you retrieve the certificate without contacting the supplier, and the certificate you retrieve should be issued by the laboratory rather than reformatted by the seller. Every lot in this catalogue is looked up that way, and the certificate library is open — no account required, because evidence behind a login is not evidence.

Common questions

What should a peptide COA include?

A lot number matching the vial, the issuing laboratory's name, a date, and for each result both the method used and the specification it was measured against. Identity by mass spectrometry and purity by HPLC are the two core results.

Is 99% purity good?

It is a normal specification for a short synthetic peptide, but the figure is only meaningful with the method attached. Purity as a percentage of HPLC peak area at a stated wavelength is a different measurement from purity by mass.

Why is net peptide content lower than the label weight?

Because the lyophilized solid includes water and counter-ions from purification alongside the peptide. The label states the fill weight; net content states how much of that is peptide.

Can a supplier use one lot's certificate for another lot?

It should not. Purity and net content are properties of a batch, so a result for one lot says nothing about another. Carrying a previous lot's certificate forward is the most common way a certificate becomes decorative.