What’s the Difference Between Purity and Net Peptide Content?
Published August 27, 2026 · Artemis Labs
Research Buyer FAQ › Peptide Purity, Testing & Stability › Net Peptide Content
Purity is about the peptide; net peptide content is about the powder. Purity asks how much of the detected material is the target sequence. Net peptide content asks how much of the vial’s total weight is peptide at all — the rest being counterion salt and bound water. The two come from different tests, and one never stands in for the other.
How this page was built. The definitions below describe standard analytical chemistry for lyophilised material from any vendor, in the sealed vial. Published values under References are transcribed from the papers themselves.
Why are purity and net peptide content different numbers?
Because they answer two different questions about the same vial. Purity comes from a chromatography run and asks: of the peptide-like material this method detected, how much is the target sequence? Net peptide content comes from a separate measurement and asks a blunter question: of the total weight of powder in this vial, how much is peptide of any kind?
The two can sit far apart without anyone lying. A sample can sit at 99% on the purity line while a quarter of the powder’s weight is something chromatography was never looking at. That extra weight is not an impurity in the chromatographer’s sense — the method detects peptide-like molecules, and this weight is not peptide. It is mostly two things: the counterion salt left over from purification, and water the dry powder pulled back in after freeze-drying. Both are ordinary, both are heavy, and neither shows up in a purity percentage.
What is net peptide content, exactly?
Net peptide content is the share of the powder’s total weight that is actually peptide, usually written as a percentage. A figure of 80% means that in 100 milligrams of powder from the vial, 80 milligrams are peptide chains and 20 milligrams are counterion, bound water, and residual salt. It is sometimes called “peptide content” or reported as “net peptide” on a Certificate of Analysis. One rule keeps buyers out of trouble: if the COA does not name a peptide-content test, that test was not run — the purity percentage does not stand in for it, because it measures something else.
How is net peptide content measured?
By methods that weigh the peptide rather than compare peaks — most commonly amino acid analysis, and sometimes a nitrogen determination. In amino acid analysis, the peptide is deliberately broken down into its individual amino acids, which are then counted against known standards; from the count, the lab calculates how much intact peptide the powder held. A nitrogen determination works from the other direction: peptides are nitrogen-rich and salts and water are not, so measuring total nitrogen estimates the peptide fraction.
This is not exotic vendor-audit machinery — it is how the field’s own reference materials are certified. When Canada’s national metrology institute assigned the content of a candidate reference standard of the peptide angiotensin II, it combined isotope-dilution amino acid analysis with quantitative NMR and a mass-balance accounting of every impurity. The final assigned value was 691 ± 9 milligrams of peptide per gram of material — and the trifluoroacetate counterion alone was measured at nearly 25% by mass (PMID 30143839). A metrology-grade peptide standard, handled with national-laboratory care, was 69% peptide by weight. That is what makes a vendor’s bare “99%” incomplete: it can be true and still describe less than three quarters of what the vial weighs.
What is the counterion, and why does it weigh so much?
A counterion is the charged partner a peptide picks up during purification. Most synthetic peptides finish the standard purification process as trifluoroacetate (TFA) salts — the purification acid stays behind, paired with charged positions on the peptide chain (PMID 40872554). This is industry-standard chemistry, not a defect. But every paired position carries its own TFA, so short peptides with several charged residues can carry a lot of salt relative to their own size — which is how a quarter of a powder’s weight can be counterion, as in the angiotensin II case above. Labs that need a different salt form can measure and exchange the counterion, and analytical methods exist specifically to determine which counterion a peptide carries and how much (PMID 22252914). For a buyer, the takeaway is simpler: if the COA says nothing about counterion, assume a TFA salt, and assume its weight is in the vial.
What does “acetate” on a peptide label mean?
It names the salt form — the counterion paired with the peptide — not a different compound. “BPC-157 acetate” is the same BPC-157 sequence with acetate as its counterion instead of the trifluoroacetate most synthetic peptides finish purification with; labs that need a different salt form can have the counterion exchanged (PMID 40872554). Two consequences follow. The peptide itself is unchanged, so the published literature on the molecule applies to either salt. The arithmetic is not: acetate and trifluoroacetate weigh different amounts, so the net-peptide-content reasoning above gives different numbers for the two salts of one peptide, and comparing vendors’ figures without knowing the salt form is comparing unlike things. The same convention appears on small-molecule labels, where “chloride” or “iodide” names the counterion the same way. If the label and the COA are silent, assume trifluoroacetate.
Why does the measured mg differ from the mg on the label?
Because the label figure is a nominal fill target and the measured figure is a measurement, with three ordinary gaps between them. First, fill tolerance: filling equipment aims at a target vial fill and lands near it, not on it. Second, net peptide content: the powder in the vial weighs more than the peptide it contains, so a vial’s total contents and its peptide mass are two different numbers — and a label may be quoting either gross fill or net peptide, which a good COA disambiguates. Third, moisture: lyophilised powder draws water back in from the air over time, and that water has weight.
None of this is unique to peptides; it is the normal behavior of freeze-dried solids. What separates vendors is disclosure. A careful COA tells you which figure is which — gross fill, net peptide content, water content — and a careless or dishonest one prints a single flattering number and lets you assume the rest. The vial-label figure describes the contents of that sealed vial, and nothing else; how the purity side of the ledger works, and what its percentage does and does not claim, is covered in what a 99% HPLC purity figure measures.
What should a buyer look for on the COA?
Three lines, ideally all present: a purity percentage with its method named, a net peptide content figure with its method named, and either a water content value or a note on the counterion form. Vendors reporting all three are describing the vial as it is. Vendors reporting only purity are describing the best-lit corner of it — the figure is not false, but it answers one question out of three, and the two unanswered ones are where the weight went.
References
- Melanson JE, Thibeault MP, Stocks BB, Leek DM, McRae G, Meija J. “Purity assignment for peptide certified reference materials by combining qNMR and LC-MS/MS amino acid analysis results: application to angiotensin II.” Anal Bioanal Chem. 2018;410(26):6719–6731. PMID 30143839 · doi:10.1007/s00216-018-1272-7
- Erckes V, Streuli A, Chamera Rendueles L, et al. “Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides and Its Influence on Membrane Permeation.” Pharmaceuticals (Basel). 2025;18(8):1163. PMID 40872554 · doi:10.3390/ph18081163
- Mrozik W, Markowska A, Guzik L, Kraska B, Kamysz W. “Determination of counter-ions in synthetic peptides by ion chromatography, capillary isotachophoresis and capillary electrophoresis.” J Pept Sci. 2012;18(3):192–198. PMID 22252914 · doi:10.1002/psc.1436
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