Two numbers, two questions
Purity asks what fraction of the detectable material is the target compound. Net content asks how many milligrams of the compound are actually in the vial. One is a percentage and the other is a mass, and a vial can score well on one and differently on the other.
What purity measures
HPLC purity is calculated from UV peak areas: the main peak as a share of all the peaks the detector sees. Anything the detector can't see doesn't count, so water and salt counter-ions don't lower the figure. That is why purity says nothing about how much of the weighed powder is peptide.
What net content measures
Net content is the measured mass of the compound itself. Labs determine it in a few ways, most often quantitative HPLC against a reference standard of known amount, and sometimes amino acid analysis, which breaks the peptide down and measures the building blocks. The result is reported in milligrams, often next to the labeled amount.
Why the powder isn't all peptide
Synthetic peptides are almost always isolated as salts. Charged groups on the peptide carry counter-ions picked up during purification, commonly trifluoroacetate, or acetate after a salt exchange. Those ions add weight without adding peptide.
Lyophilized peptides are also hygroscopic: the dry powder takes up moisture from the air, and some water stays bound even after freeze-drying. Between counter-ions and water, the share of the gross powder weight that is actual peptide is typically well below 100%, even when HPLC purity is very high. Suppliers account for this when they fill vials, which is why the content figure, not the gross weight, is the meaningful one.
Fill accuracy
Vials are filled before freeze-drying, and every fill has a small tolerance. A measured content slightly above or below the labeled amount is normal; what matters is that the figure is measured for that lot and printed on its report. This is also why one size's report can't stand in for another: a different vial size is a different fill.
Blends
For a vial that holds more than one compound, a good report measures each component separately and lists the content of each, rather than one combined figure. The BPC-157 + TB-500 certificates show how that looks on a real report.
Reading both on a report
- Purity printed as a percentage, with HPLC named as the method
- Net content printed in mg, ideally next to the labeled amount
- Both figures tied to the same lot number
- Per-component content for blends
Every certificate in the Lumina Labs COA Library prints purity and net content exactly as the lab reported them. For how purity and identity fit together, read HPLC vs. mass spectrometry.
Lumina Labs supplies research compounds for in vitro laboratory research use only. They are not intended for human or veterinary consumption, or to diagnose, treat, cure or prevent any disease. These statements have not been evaluated by the FDA.