HPLC purity measures the fraction of the peptide chromatogram that is the main peak — i.e. how clean the peptide is relative to its impurities.
Net Peptide Content (NPC) measures the fraction of the dry mass in the vial that is actual active peptide — as opposed to TFA / acetate counter-ion, water of hydration, and inorganic salts.
A vial can have 99 % HPLC purity and 75 % NPC. Both numbers describe different aspects of the same vial, and both matter for different reasons.
HPLC purity is a relative measurement. The detector measures peak area in the chromatogram; the main peak's area is divided by the total integrated area to give a purity percentage. By construction, the answer can't tell you anything about the mass of the dry powder in the vial.
Think of it this way: imagine a 5 mg lyophilized vial labeled "Semaglutide 5 mg." Its HPLC purity comes back as 99 %. That sounds good. But the 5 mg of dry mass might break down as:
The HPLC main peak is 99 % pure Semaglutide relative to other peptide species — that's true. But the actual amount of Semaglutide in the vial is 3.8 mg, not 5 mg. That's a 24 % dose accuracy gap, invisible on HPLC alone.
Net Peptide Content is a mass-balance calculation that subtracts everything in the vial that isn't peptide:
The final NPC value is reported as a percentage: "76.4 % NPC" means 76.4 % of the dry mass is active peptide and the rest is counter-ion plus water plus other non-peptide content.
Industry-typical NPC for research-grade lyophilized peptides:
Below 70 % NPC usually indicates excess counter-ion or excessive residual water. Above 90 % NPC is rare for TFA-salt material and may indicate the vendor is reporting gross peptide salt rather than active peptide.
NPC requires three measurements on the same sample: ion chromatography for counter-ion content, Karl Fischer for water content, and UV-corrected quantification against a reference standard for the peptide concentration. That's three instrument stacks running in parallel, each with their own QC. Most peptide testing labs only run the HPLC stack and skip the others — it's cheaper to publish a COA with "99 % HPLC purity" than to accurately report "78 % NPC."
The result is the public COA landscape: most published peptide COAs have a purity number and not much else. The vials with the highest NPC and the vials with the lowest NPC look identical on the published documentation.
NPC matters most when dose accuracy matters. That's typically:
Every Identity + Potency panel ($499) and higher on our compound pages reports both HPLC purity and NPC, plus TFA / acetate residual (the underlying counter-ion content driving the NPC gap). The Purity-only panel ($199) reports HPLC purity but not NPC — useful for routine quality checks but not for dose-accuracy questions.
See specific examples on our compound pages: Semaglutide, Tirzepatide, Retatrutide, BPC-157, TB-500.
HPLC purity tells you how clean the peptide is. NPC tells you how much actual active is in the vial. Both matter; the public COA landscape typically only reports the first. When dose accuracy matters, ask for the NPC number — and if your current testing lab doesn't measure it, that's a signal worth acting on.