Net Peptide Content Explained: Why Purity % Isn’t the Whole Picture
HPLC purity and net peptide content answer different questions — and mixing them up leads to under- or over-estimated stock concentrations.
A vial listing 99% HPLC purity is not necessarily 99% peptide by weight. Purity and net peptide content (NPC) measure different things, and conflating them is one of the more common — and consequential — mistakes in interpreting a Certificate of Analysis.
What HPLC Purity Actually Measures
HPLC (high-performance liquid chromatography) purity describes the proportion of peptide-related material in a sample that corresponds to the target sequence, relative to other peptide-related impurities (truncated sequences, deletion products, oxidised variants). It says nothing about how much of the vial’s total mass is peptide at all.
What Net Peptide Content Measures
Lyophilised peptide powder is rarely 100% peptide by mass. Counter-ions from the synthesis and purification process (commonly acetate or trifluoroacetate salts), residual moisture, and other non-peptide material make up the remainder. Net peptide content is the percentage of the total vial mass that is actually peptide — determined by amino acid analysis, elemental analysis, or mass balance, not by HPLC.
A peptide can show 99% HPLC purity and still have an NPC of 75–85% — meaning roughly 15–25% of the vial’s weighed mass is salts and moisture, not peptide.
Why This Matters for Stock Concentrations
If you weigh out 1 mg of lyophilised material and reconstitute assuming 100% NPC, your actual stock concentration will be lower than intended — proportionally, by the gap between the assumed and true NPC. For experiments sensitive to absolute concentration (dose-response curves, binding assays with defined Kd targets), this gap can meaningfully shift results.
- Effective peptide mass = vial mass × (NPC% / 100)
- Example: 5 mg vial at 82% NPC contains 4.1 mg of actual peptide
- HPLC purity and NPC should both appear on a complete COA — one without the other is an incomplete picture
Reading Both Figures on a COA
A rigorous Certificate of Analysis reports HPLC purity (sequence-level quality) and net peptide content (mass-level composition) as separate figures, typically from separate analytical methods. If a COA lists only "99% pure" with no NPC or amino acid analysis figure, that is a gap worth asking the supplier about directly before treating the nominal vial weight as the true peptide mass in a calculation.