How to read a certificate of analysis

Most buyers check one number on a COA. There are four that matter, and the relationship between them tells you more than any of them alone.

12 August 2026 · 7 min read

A certificate of analysis is the only object in this transaction that carries information rather than persuasion. It is also, in this category, frequently decorative — a PDF with a logo, one number, and no laboratory named on it.

Here is what a useful certificate contains, and how the parts relate.

The four figures

1. Chromatographic purity

Almost always the headline. It comes from reverse-phase HPLC: the material is separated on a C18 column, detected by UV absorbance, and the area under the main peak is divided by the total integrated area.

Two details decide whether the number means anything.

The detection wavelength. Look for 214 nm. That is the absorbance of the peptide bond itself, so every residue in the chain contributes to the signal. At 280 nm you are largely detecting aromatic residues — tryptophan, tyrosine, phenylalanine — which flatters peptides rich in them and under-reports impurities that lack them. A purity figure measured at 280 nm is not comparable to one measured at 214 nm.

What “total area” includes. Purity is a ratio, and ratios can be improved by shrinking the denominator. A short gradient that never elutes late-running hydrophobic impurities produces a beautiful number about an incomplete picture.

2. Observed mass against theoretical mass

This is the identity check, and it is the one people skip.

Purity tells you the material is one thing. It does not tell you it is the right thing. A 99.5% pure sample of the wrong peptide is 99.5% pure. Mass spectrometry compares the observed mass against the mass calculated from the stated sequence; agreement within tolerance is what makes the name on the vial a claim rather than a label.

Check that the certificate prints both numbers. A certificate showing only “mass confirmed” is asking you to accept someone’s arithmetic.

3. Net peptide content

Lyophilized peptide is not pure peptide. What is in the vial also includes counter-ions from purification — typically trifluoroacetate — plus residual solvent and adsorbed water. Net peptide content, usually by elemental nitrogen determination, states how much of the vial mass is actually peptide.

Typical values run 70–85% w/w. This is normal and not a defect. But it means a 10 mg vial at 80% net content holds about 8 mg of peptide, and if you are calculating concentrations from vial mass rather than from net content, every figure downstream is wrong by a fifth.

A supplier who reports purity but never reports net content has told you about the quality of the peptide without telling you how much of it there is.

4. Residual moisture

Usually by Karl Fischer titration. Water drives hydrolysis, so residual moisture is the main determinant of how long lyophilized material stays intact in storage. Low single-digit percentages are what make a stated storage life a measurement rather than an estimate.

How the figures interact

The four are not independent, and the interesting information is in the relationships.

  • High purity with low net content means a clean peptide carrying a lot of salt. Common, not alarming, but it changes your mass calculations.
  • High purity with high moisture means material that was clean at analysis and may not stay that way. The purity figure has a shorter shelf life than the vial does.
  • Purity reported without identity is the one combination that should stop you. It is the only case where the headline number can be entirely true and entirely irrelevant.

What is not on the certificate

Equally important, and rarely stated plainly.

Standard peptide COAs do not establish sterility, and do not establish endotoxin levels. Neither is measured by any of the four assays above. Material supplied as a research chemical is not sterile, and no purity figure implies otherwise. If your protocol depends on either property, it must be commissioned as separate testing.

Three questions worth asking

  1. Who ran the assay? A certificate produced in-house by the seller is a self-assessment. That is not worthless, but it is a different kind of evidence from an independent laboratory’s report, and the certificate should let you tell which one you are holding.

  2. Does the lot number on the certificate match the vial? A certificate for a representative lot is a certificate for material you did not receive.

  3. What happens to lots that fail? A supplier whose archive contains only passing results is not showing you a record. They are showing you a selection.

Aminopia publishes the full certificate for every lot against its lot number, including lots withheld from sale. Look one up.

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