Lab Quality & Testing
Net Peptide Content vs Purity Percent
The two numbers most often confused on a certificate — and the reason your actual concentration may be lower than your calculation assumes.
August 19, 2026 · 5 min read · Novanta Research
Two different denominators
Purity answers: of the peptide-related material present, what fraction is the intended sequence? Net peptide content answers: of the total mass in the vial, what fraction is peptide at all?
A lot can be 99% pure and still be only 80% peptide by mass, because the balance is counter-ion salt and adsorbed water.
Where the extra mass comes from
Peptides purified by reverse-phase chromatography usually carry trifluoroacetate counter-ions from the mobile phase. Basic residues each pair with one, so lysine- and arginine-rich sequences carry proportionally more salt.
Lyophilized powder is also hygroscopic and retains some water even after drying.
The practical consequence
If a 10mg vial is 80% net peptide, reconstituting it in 2ml gives 4mg/ml of peptide, not 5mg/ml. Protocols that ignore this systematically overestimate concentration.
Where net peptide content is reported, use it. Where it is not, note the assumption explicitly in your records rather than silently treating labelled mass as peptide mass.
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For laboratory and research use only. Not for human or veterinary consumption. Novanta Research is not a compounding pharmacy and nothing in this article is medical advice or a claim that any compound diagnoses, treats, cures or prevents any condition.