Analytical methods · 6 min read
Net Peptide Content vs. Purity: Why a Peptide Has Two Different Numbers
Why HPLC purity and net peptide content are different measurements, how each is determined, and how to read both on a research peptide certificate.
Two percentages appear on most peptide certificates and they are often confused. Purity describes what share of the peptide material is the intended sequence. Net peptide content describes what share of the powder in the vial is peptide at all. A lot can be 99% pure and still contain only 70 to 85% peptide by weight. Both numbers are correct; they answer different questions.
What purity measures
Purity is determined by reversed-phase HPLC with UV detection, usually at 214 or 220 nm, where the peptide bond absorbs. The target peak's area is divided by the total area of all peptide-related peaks. Impurities such as deletion or truncated sequences lower the figure. Water and salts do not, because they do not produce peaks at that wavelength. See HPLC vs. mass spectrometry for how purity and identity work together.
What else is in a lyophilized vial
- Counter-ions bound to basic residues (commonly acetate or trifluoroacetate) that balance the peptide's charge
- Residual water that lyophilization cannot fully remove and that the powder re-absorbs from air
- Trace residual solvents from purification
Peptides rich in arginine, lysine, or histidine carry more counter-ions, so their net content is usually lower. Read more in counter-ions: acetate vs. TFA.
How net peptide content is measured
- Amino acid analysis (AAA): the peptide is hydrolyzed and the free amino acids are quantified against standards
- Elemental nitrogen analysis: total nitrogen is measured and converted to peptide mass using the sequence's nitrogen content
- Quantitative HPLC or UV against a characterized reference standard
Reading the two numbers together
Suppose a 10 mg label, 99.2% HPLC purity, and 82% net peptide content. The vial holds about 8.2 mg of peptide material, of which 99.2% is the intended sequence. A certificate that reports only purity leaves the researcher unable to calculate how much analyte is actually present in a sample preparation for an assay.
What to look for on a certificate
- Purity by HPLC with the chromatogram attached
- Net peptide content with the method named
- Measured mass per vial alongside the label claim
- Counter-ion form stated (acetate or TFA)
North Wall reports purity, identity, and measured content for every lot. Compare examples on the BPC-157 and TB-500 reference pages, or see the full eight-assay panel.
Educational content for laboratory professionals. North Wall products are for in-vitro research only and are not for human or veterinary use.