Purity Is Not Potency: Three Numbers a Certificate Reports and What Each Means

Research-use-only context. This article summarizes published third-party scientific literature and laboratory practice. It is not medical advice, not a therapeutic or performance claim, and not a usage guide. US Prime Peptides products are supplied strictly for in vitro laboratory research and are not for human or veterinary use.

A peptide certificate can report three numbers that are often confused: purity, content, and potency. They answer three different questions, come from three different kinds of measurement, and can move independently of one another. This article defines each, shows how it is measured, works through the arithmetic that connects purity to content, explains what potency means in the regulatory vocabulary, and sets out why almost no research-use certificate reports it.

Three questions, three measurements

Purity answers "of the peptide-related material in this vial, what fraction is the target sequence?" Content answers "of the total mass in this vial, how much is peptide?" Potency answers "how much biological activity does a given amount produce?" A vial can be 99% pure and 75% peptide by mass; it can be 99% pure and biologically inert if the one percent of impurity is the wrong one, or if an isobaric epimer is hiding inside the 99%.

ICH Q6B, the specifications guideline for biotechnological products, draws the content-versus-potency line explicitly: potency "is the quantitative measure of biological activity based on the attribute of the product which is linked to the relevant biological properties, whereas, quantity (expressed in mass) is a physicochemical measure of protein content." That sentence is the whole argument of this article in regulatory form.

Purity: what the HPLC number counts and what it ignores

HPLC purity is a ratio of peak areas, not a mass fraction. The detector sees only species that absorb at the chosen wavelength and elute within the integrated window, so water, trifluoroacetate, acetate, chloride and inorganic residues contribute nothing to the denominator. A powder that is 20% TFA salt by mass can still show 99.5% purity, because the TFA is not a peptide-related peak. The mechanics of area percent are covered in HPLC purity: what area percent does and does not tell you.

Purity also has a stereochemical blind spot. A D-amino acid substituted at one position produces a diastereomer with exactly the target's mass and often nearly the same retention time, so it can sit under the main peak on a standard method. Strege and colleagues at Lilly published a chiral HPLC-electrospray MS/MS method in 2023 that hydrolyzes the peptide in deuterated acid and quantifies each D-amino acid, recovering substitutions at 0.1 to 1.0% at every position of an octapeptide; a method of that kind, not the routine purity trace, is what detects epimers. Lian and colleagues' 2021 review of LC-MS characterization of synthetic peptides singled out structural isomers and epimers as the impurity class most likely to escape conventional analysis. A certificate that reports purity and identity has still not reported chiral purity, one of the items listed in what a certificate of analysis does not test for.

Content: the mass-balance arithmetic

Net peptide content is the mass fraction of the powder that is peptide, and it is measured either directly or by subtraction. Direct methods include amino acid analysis, in which the peptide is hydrolyzed to its amino acids and each is quantified against isotope-labeled standards (Qasrawi, Petrotchenko and Borchers described a reversed-phase LC-MS version without derivatization in 2023), and quantitative NMR, which integrates a peptide proton signal against a certified internal standard on the intact molecule. The subtraction method is mass balance: measure water (Karl Fischer), counter-ion (ion chromatography or ¹⁹F NMR for TFA), inorganic residue (sulfated ash or ICP) and related peptide impurities, and assign the remainder to the target.

Real reference-material work shows how large the corrections are. Chen and colleagues reported in 2026 that a purified sublancin sample had a mass-balance purity of 78.60%, after subtracting 5.46% water, 15.48% trifluoroacetate, 0.4% related organic compounds and 0.056% inorganic residue, with an independent amino-acid isotope-dilution value of 77.58% and a certified value of 78.1%. Melanson and colleagues found TFA at nearly 25% by mass in an angiotensin II candidate reference material in 2018 and combined qNMR, amino acid analysis and mass balance to assign a single value. Li and colleagues' 2019 USP multi-laboratory study of oxytocin compared HPLC assay, qNMR and amino acid analysis and found that the HPLC assay against a shared bulk standard had the lowest between-laboratory variation, with the caveat that the standard's own purity uncertainty was not included. McCarthy and colleagues describe the resulting two-step USP practice: assign the bulk by mass balance, then use it as the standard for the vialed material.

The table uses label masses only to show the arithmetic: target peptide mass equals label mass multiplied by content and by purity. The counter-ion and water fractions in the last row are the ranges the reference-material papers report, and they are why net peptide content vs purity is its own article.

Potency: what a bioassay measures and why it is different

Potency is measured by comparing the response a sample produces in a biological system with the response produced by a reference standard, and it is reported as relative potency rather than as an absolute number. ICH Q6B defines it as "the measure of the biological activity using a suitably quantitative biological assay (also called potency assay or bioassay), based on the attribute of the product which is linked to the relevant biological properties." The assay may be a receptor-binding measurement, a reporter-gene readout in cultured cells, or a functional cell response; the sample's concentration-response curve is fitted against the reference's, and the horizontal shift between them is the relative potency.

Two properties make potency unlike the other two numbers. First, it is defined by the assay: a different cell line or endpoint can give a different relative potency for the same vial, so the assay must be validated and its reference material must have an assigned activity. Wilson described in Bioanalysis in 2018 how designed experiments are used to establish robustness and qualify a cell-based potency assay; Novick, Sondag and Schofield proposed a simulation-based qualification approach in the same year. Second, potency can change while purity and content do not. Hedayati and colleagues reported in 2017 that three lots of recombinant erythropoietin with 6.8%, 16.7% and 27.3% methionine oxidation, all sequence-confirmed by mass spectrometry, showed progressively lower specific activity in a mouse bioassay; the activity loss tracked the shift toward acidic isoforms rather than any change in the amino acid sequence. Modifications that a purity method barely registers can therefore carry most of the activity difference between two lots.

Why most research certificates report no activity assay

Research-use certificates almost never include potency because a potency assay requires three things that do not exist for most research compounds: a reference standard with an assigned activity value, a validated product-specific bioassay, and a defined "relevant biological property" to measure. Pharmacopeial reference standards with assigned content exist for a handful of peptides, as the USP program described by McCarthy and colleagues shows, but assigned-activity standards exist only for products with an approved use and a defined mechanism. For a compound whose biology is still being characterized in the literature, there is no agreed assay to validate.

Cost and variability add to this. A compendial content assay has a between-laboratory coefficient of variation of a few percent; cell-based bioassays commonly carry relative-potency acceptance ranges of tens of percent and need replicate plates, which is why their validation is governed by designed experiments rather than a single calibration. For a research buyer the consequence is simple: a certificate's identity, purity, content, endotoxin, sterility and metals lines describe the chemistry of the material, and no line on it describes biological activity. US Prime Peptides' lot-matched certificates report exactly those chemistry lines from an independent US laboratory and do not report, and do not imply, an activity value; any activity question belongs in the researcher's own assay, designed for the research question at hand. The validation vocabulary for the chemistry methods, including accuracy, precision, specificity and range, is set out in USP General Chapter <1225> and ICH Q2(R2).

Reading the three numbers together

Read purity as the quality of the peptide, content as the quantity of the peptide, and potency, when it is reported at all, as a separate biological statement that depends on its assay. A certificate is internally consistent when its purity comes with a chromatogram and method, its content comes with a stated method (amino acid analysis, qNMR, UV, or mass balance with the subtracted fractions listed), and its identity line shows a mass that matches the sequence. Three habits follow: multiply content by purity rather than reading either alone; ask how content was measured, since mass balance and amino acid analysis can differ by a few percent on the same powder; and do not infer activity from either number, because the published cases above show activity changing with oxidation and stereochemistry that the standard panel does not resolve. The certificate's purpose is to describe the material; what the material does is the research.

Frequently Asked Questions

What is the difference between peptide purity and potency?

Purity is the fraction of peptide-related material that is the target sequence, measured by HPLC as area percent. Potency is biological activity per unit of material, measured by a validated bioassay against a reference standard and reported as relative potency. ICH Q6B separates them explicitly: potency is a measure of biological activity, quantity is a physicochemical measure of content.

Does 99% purity mean the vial contains 99% peptide by weight?

No. HPLC purity ignores water, counter-ions and salts because they produce no UV peak, so a 99% pure powder can be 70 to 80% peptide by mass. Reference-material studies have measured trifluoroacetate at 15 to 25% of powder mass. Net peptide content, measured by amino acid analysis, qNMR or mass balance, gives the mass fraction.

Why do research peptide COAs not report biological activity?

A potency assay needs a reference standard with an assigned activity value, a validated product-specific bioassay and an agreed biological property to measure. Those exist only for compounds with an approved use and defined mechanism; for most research compounds there is no agreed assay, and cell-based assays carry far more variability and cost than chemical tests.

Can a peptide be chemically pure but biologically inactive?

Yes. A D-amino acid epimer has the same mass and nearly the same retention time as the target and can hide inside a 99% main peak, and oxidation at a single methionine has been shown to lower the specific activity of a sequence-confirmed protein lot. Purity and identity describe chemistry; activity has to be measured in a biological assay.

Citations

1. ICH. Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products Q6B. 1999. database.ich.org (PDF)
2. United States Pharmacopeia. General Chapter <1225> Validation of Compendial Procedures. USP-NF. doi.usp.org/USPNF/USPNF_M99945_04_01.html
3. FDA. Q2(R2) Validation of Analytical Procedures: Guidance for Industry. 2024. fda.gov
4. Li C, Bhavaraju S, Thibeault MP, et al. Survey of peptide quantification methods and comparison of their reproducibility: A case study using oxytocin. J Pharm Biomed Anal. 2019;166:105-112. PMID 30640042
5. 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;410:6719-6731. PMID 30143839
6. Chen W, Yan J, Yang M, et al. SI-traceable purity assignment for peptide sublancin using mass balance approach and isotope dilution mass spectrometry based on amino acid analysis. J Chromatogr B. 2026;1277:125038. PMID 41916078
7. Qasrawi DO, Petrotchenko EV, Borchers CH. Amino acid analysis for peptide quantitation using reversed-phase liquid chromatography combined with multiple reaction monitoring mass spectrometry. Anal Bioanal Chem. 2023;415:5261-5267. PMID 37468754
8. McCarthy D, Han Y, Carrick K, et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharm Res. 2023;40:1317-1328. PMID 36949371
9. Strege MA, Oman TJ, Risley DS, Muehlbauer LK, Jalan A, Lian ZY. Enantiomeric purity analysis of synthetic peptide therapeutics by direct chiral high-performance liquid chromatography-electrospray ionization tandem mass spectrometry. J Chromatogr B. 2023;1219:123638. PMID 36857849
10. Lian Z, Wang N, Tian Y, et al. Characterization of Synthetic Peptide Therapeutics Using Liquid Chromatography-Mass Spectrometry: Challenges, Solutions, Pitfalls, and Future Perspectives. J Am Soc Mass Spectrom. 2021;32:1852-1860. PMID 34110145
11. Hedayati MH, Norouzian D, Aminian M, et al. Identification of methionine oxidation in human recombinant erythropoietin by mass spectrometry: Comparative isoform distribution and biological activity analysis. Prep Biochem Biotechnol. 2017;47:990-997. PMID 28825868
12. Wilson R. Design of experiment in assessing robustness and for qualification of a cell-based potency assay. Bioanalysis. 2018;10:737-746. PMID 29771136
13. Novick S, Sondag P, Schofield T. A Novel Method for Qualification of a Potency Assay through Partial Computer Simulation. PDA J Pharm Sci Technol. 2018;72:249-263. PMID 29444993

This article is for laboratory research reference only. US Prime Peptides products are supplied strictly for in vitro research. Not for human consumption.

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