Peptide purity explained

A certificate of analysis for a peptide usually shows a purity figure such as 98%. It is easy to read that as “98% of what is in the vial is the peptide”. That is not what the number says, and the difference changes what a price per mg means. This guide explains the figure in plain words. The sourced version, with the documents behind each statement, is the article what a peptide CoA proves and does not.

What the purity figure measures

Purity on a peptide certificate normally comes from HPLC, a method that separates what is in a sample and measures how much signal each part gives. The purity is the share of the detected signal that belongs to the main peptide, expressed as a percentage of the total peak area. It describes what the instrument saw: how clean the main peak is compared with the other peaks it found.

What it is not

• Not the amount in the vial. It says nothing about how many milligrams of peptide the vial holds.
• Not a share of the powder’s weight. A freeze-dried peptide powder also contains water, salts (the counter-ions that go with the peptide) and traces of solvents. These are measured separately and by weight, and they are not part of the HPLC percentage.
• Not an identity test. Confirming that the main peak really is the right peptide takes another method, usually mass spectrometry, which reports a mass. A purity figure alone does not.

Peptide content: the second number

The share of the powder that is peptide by weight is called the net peptide content. A certificate may or may not state it. When it does, the main peptide’s share of the labelled mass is roughly the purity multiplied by the net content. The purity-adjusted price calculator does that arithmetic for you.

Made-up numbers to show the idea: a vial labelled 10 mg with a stated purity of 98% and a net peptide content of 80% contains about 10 × 0.98 × 0.80 = 7.84 mg of main peptide. A vial that costs 30 would then cost 30 ÷ 7.84 ≈ 3.83 per mg of peptide, not 3.00 per labelled mg.

What neither number covers

• Endotoxin, sterility, heavy metals and residual solvents are separate tests. A certificate covers them only if it lists them.
• The report describes one sample of one batch. It does not show that the vial you receive matches it, and a seller can submit the sample.
• Two laboratories can report different purities for the same material, for example because of method settings.

How to read the figure

• Look for the method (HPLC), the date, the batch number and the named laboratory. See the checklist.
• Check whether the certificate states net peptide content, and read a purity figure without it as one half of the picture.
• Compare prices per mg with that in mind: a lower price per labelled mg is not necessarily a lower price per mg of peptide.

Peptilyze links to the lab reports shops publish. It does not verify them, and nothing on this site rates a product by its purity figure.