Purity vs Quantity: What “99% Purity” Does and Doesn’t Mean
A certificate of analysis that says 99% purity sounds as though the powder in the vial is 99% peptide. It usually does not mean that. Purity and quantity are different questions, answered by different tests.
Purity is a ratio of peaks
HPLC purity compares the main peak with the other peaks the detector saw (see How Peptide Purity Is Tested). It is a statement about the relative composition of the detected components, not about the total weight of the powder.
A vial contains more than peptide
A freeze-dried peptide powder typically also contains water absorbed from the air, and counter-ions left over from synthesis and purification, most commonly trifluoroacetate (TFA) or acetate, bound to the peptide’s charged groups. These do not show up as impurity peaks, so they do not lower the HPLC purity figure. They do mean that the peptide itself may make up noticeably less than 100% of the powder’s weight.
Net peptide content
The proportion of the powder that is actual peptide is called the net peptide content. It is measured by methods such as amino acid analysis or elemental analysis, and a standard purity COA does not necessarily include it. If the exact quantity matters for your work, ask whether it was measured.
Labelled amount vs measured amount
A vial labelled with an amount, for example 10 mg, normally refers to the weight of material filled into it unless the label says otherwise. Whether that figure means total powder or peptide content is worth confirming with the supplier.
Practical takeaways
- Read purity as “how clean is the peptide that is there.”
- Read quantity as a separate question, answered by a separate test.
- Look for reports that state what was measured and by which method.
For laboratory research use only. Not for human or veterinary use. See our Research Use Only page.