“99% pure” is the number everyone quotes and almost nobody explains. It is a real and useful figure, but it answers only one question. This guide explains what it measures, what it leaves out, and what to ask a supplier so you are comparing like with like.
How purity is measured
Purity usually comes from HPLC, short for high-performance liquid chromatography. The lab dissolves a small sample and pushes it through a column that separates the mixture into its parts. Each part comes out at a different time and shows up as a peak on a chart. The target peptide is the big peak. Purity is the area of that peak divided by the area of all peaks, as a percentage.
So 99% purity means: of everything the detector saw, 99% was the target peptide. The remaining 1% is usually closely related material, such as shortened or slightly modified copies of the same sequence that formed during synthesis.
What purity does not tell you
It does not confirm identity. HPLC measures how clean a sample is, not what it is. Identity comes from mass spectrometry, which measures the molecular mass. A proper COA shows both. Read our COA guide for where to find each one.
It does not tell you how much peptide is in the vial. This surprises people. A 10 mg vial at 99% purity does not contain 9.9 mg of peptide. Lyophilised powder holds water and a salt (a counter-ion, often acetate or trifluoroacetate) left over from manufacturing. Those are not impurities in the HPLC sense, because they are not peptide, but they are part of the weight. The figure that accounts for them is called net peptide content, and it is typically somewhere between 70% and 90% depending on the peptide and the process.
Two vials can both read 99% purity, and one can hold noticeably more active peptide than the other. If your work depends on exact amounts, net peptide content is the number to ask for.
Why the number is never 100%
Peptide synthesis adds amino acids one at a time. Each step is very efficient but not perfect, so a small amount of the batch ends up one amino acid short, or carries a protecting group that did not come off cleanly. Purification removes most of this, not all of it. A lab reporting 99.3% is telling you the truth. A label claiming 100% is telling you the lab did not look hard, or the number was not measured at all.
What “pharmaceutical grade” and similar labels mean
Nothing specific, unless the seller defines it. These phrases are marketing. The things that carry information are: the purity percentage, the method used to get it, the identity result, and the batch number that ties the report to the vial. Judge suppliers on those.
Questions worth asking any supplier
- Is there a COA for this exact batch, and can I see it before I buy?
- Which lab ran it, and does the report include both HPLC purity and a mass-spec identity check?
- Do you publish net peptide content, or can you tell me the counter-ion?
- How is the product stored and shipped between the lab and me?
We publish every batch report on the Lab Tests (COA) page. If a report you need is not there, ask us and we will send it.
All products on this site are sold for laboratory research only. Nothing here is medical advice.
