What “>99% purity” means: HPLC area percent and net peptide content

Last reviewed 9 August 2026

Two different numbers are routinely reported as a peptide’s purity. One is a ratio of detector responses in a chromatogram. The other is a fraction of the mass that was weighed into the vial. They are produced by different methods, they answer different questions, and on the same material they can sit twenty percentage points apart with nothing whatever wrong.

The figure printed on a research certificate is, in the ordinary case, the first of the two. Purity and identity of synthetic peptides are typically evaluated by reversed-phase chromatography with ultraviolet detection near 214 or 220 nm, alongside mass spectrometry (Hoofnagle et al., 2016). The figure most readers believe they are being given is the second.

The two numbers, and what each is a percentage of

The second term has a settled definition. Hoofnagle and colleagues (2016), writing for a multi-laboratory working group, put it directly: Net peptide content is a measurement, usually in the form of a percentage that represents the amount of actual peptide within a gravimetrically measured sample. The measurement excludes the weight of water and counter ions that exist in all peptides.

Note the last four words. Not some peptides, and not badly made ones.

Area percent: a ratio of detector responses

The calculation is the integrated area of the main peak divided by the summed area of every peak the detector recorded, expressed as a percentage. Detection is by ultraviolet absorbance in the region where the peptide bond absorbs. Everything the number is capable of saying is bounded by that sentence: one chromatogram, one wavelength, one separation, one integration.

The equal-response assumption, and a sequence that breaks it

Area normalisation assumes detector response is proportional to quantity, so that equal areas mean equal amounts. For peptides the assumption is measurably false. Kuipers and Gruppen (2007) determined molar extinction coefficients at 214 nm for the twenty amino acids and for the peptide bond itself, reporting the peptide bond at 923 M⁻¹ cm⁻¹, tryptophan at approximately thirty times that value, and phenylalanine, tyrosine and histidine at approximately six times. Response therefore tracks the number of peptide bonds in whatever is eluting and the aromatic residues it happens to carry.

The consequence is directional rather than random. A deletion sequence carries fewer peptide bonds than the target, so it under-contributes relative to its mass, and the main peak’s share rises accordingly. If the residue lost was aromatic, it under-contributes by a much larger margin. The impurity classes at issue are not exotic: D’Hondt and colleagues (2014) catalogue deletion and insertion sequences, diastereomeric impurities from racemisation, protection adducts, oxidation products and oligomeric species as the ordinary output of solid-phase synthesis.

Nor is the difficulty solved by switching detector. Stocks and colleagues (2018), quantifying low-level impurities in an angiotensin II reference material, note that peptides of differing sequence can exhibit widely disparate electrospray responses, and report that external calibration frequently overestimated impurity amounts relative to standard addition.

A sequence in this catalogue makes the point concretely. BPC-157 is H-Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val-OH (PubChem CID 9941957). Fifteen residues, fourteen peptide bonds, and no tryptophan, tyrosine, phenylalanine, methionine or cysteine at all. Two things follow. Its absorbance at 214 nm arises from the backbone and from its four internal prolines, which Kuipers and Gruppen report absorb roughly three times a peptide bond when they sit inside a chain, with no aromatic contribution at all; so an impurity differing by one aromatic residue would be weighted quite differently from the parent; and it cannot be quantified by absorbance at 280 nm at all, because nothing in the molecule absorbs there.

Three things the denominator never contains

1. Anything that does not absorb at the detection wavelength. This is the category that matters most, because it includes the counter-ion and the water.
2. Anything that does not elute under the gradient. Strongly retained material and anything left on the column is absent from the arithmetic rather than counted against it.
3. Anything that co-elutes with the main peak. Hoofnagle and colleagues are explicit that a shallow gradient is required — no more than 2% change in organic concentration per minute — otherwise the presence of impurities can be masked by co-elution of contaminant synthesis byproducts. Two species arriving together are integrated as one, and the merged peak is attributed to the target.

The European Medicines Agency addresses the same problem from the regulatory side, noting that development of these procedures must take into account the complexity of the structure of these molecules and the risk of co-eluting impurities, and that where one method cannot separate everything, additional independent method(s) may be needed.

The number moves with the method

Because the denominator is defined by what the method detects, the same vial yields different area percentages under different conditions. Wavelength changes the relative weighting of aromatic species; gradient slope changes what resolves and what merges; column chemistry changes elution order and retention; and the integration threshold decides which small peaks count as peaks at all. An area percent quoted without its method is therefore not comparable with any other area percent, which is why the method reference belongs on the certificate rather than in a laboratory’s files — a requirement set out with the rest of the document’s structure on the page on reading a certificate of analysis.

Net peptide content: a fraction of the weighed mass

A lyophilised solid is peptide, plus counter-ions carried through from synthesis and purification, plus residual water, plus residual solvents and any inorganic material. Net peptide content is what remains when the rest is subtracted. Each component is measured by its own method, and none of them is chromatographic area.

Counter-ions, and the arithmetic

Solid-phase synthesis relies on trifluoroacetic acid as a cleavage agent and as an ion-pairing reagent in purification, with the result that peptides are commonly isolated as trifluoroacetate salts (Erckes et al., 2025). Trifluoroacetic acid has a molecular weight of 114.02, and one anion pairs with each protonated basic site. That is enough to compute the mass consequence for any sequence.

Free-base masses are PubChem CIDs 9941957, 62707662 and 172198. The third row is included because it can be checked against an independently published calculation, and then against the measurements that follow: Erckes and colleagues (2025) worked on a series of angiotensin peptides and report the expected weight percentage for the TFA⁻ and Cl⁻ salt for AT 1–4 was determined at approximately 25% for TFA⁻. Angiotensin II is AT 2 in that series, and the 24.6% above is the same quantity arrived at independently.

Two findings in that paper matter more than the agreement. First, measurement exceeded stoichiometry: Before counterion exchange, AT 1–4 all showed a higher number of TFA⁻ ions per peptide than expected from the number of positive charges, with content reaching up to 35% of total weight, and one peptide measured at 0.333 ± 0.008 mg of trifluoroacetate per mg of salt. Stoichiometry is a floor for this quantity, not an estimate. Second, measuring it took three validated techniques — ¹⁹F-NMR, HPLC with evaporative light-scattering detection, and FT-IR — none of which is a UV chromatogram.

Counter-ion identity changes the answer substantially. The same BPC-157 sequence as a diacetate rather than a bis-trifluoroacetate carries roughly 7.8% counter-ion by mass instead of 13.8%, because acetic acid is a little over half the molecular weight of trifluoroacetic acid. The EMA guideline records that acetate is the usual counter-ion for pharmaceutical synthetic peptides, with trifluoroacetate and chloride among the alternatives, and requires that The type of counter ion should be defined, and the amount of counter ions should be controlled in the active substance specification with a justified upper limit. Salt form is a property of the material that has to be read, not assumed.

Residual water

The second subtraction is water, and it is not a stable quantity. The EMA guideline states plainly that Peptides are often very hygroscopic powders, requires precautions against moisture uptake during storage and during analysis, and expects water content to form part of stability testing for hygroscopic powders. Water content is determined by a water-specific method, conventionally Karl Fischer titration.

The practical consequence is that a vial opened, weighed and closed in ambient air is not the same mass it was beforehand. Why the solid is presented as a cake at all is covered on the page on lyophilisation and cake appearance, and the storage side on the page on lyophilised and in-solution stability.

The two numbers on one material

Melanson and colleagues (2018) assigned a purity value to a candidate certified reference material of angiotensin II, combining quantitative NMR on the intact peptide, amino acid analysis by LC-MS/MS after hydrolysis, and a mass balance in which the trifluoroacetate counter-ion was quantified by validated ¹⁹F-qNMR. They report the counter-ion at nearly 25% by mass and a final assigned value of 691 ± 9 mg/g.

691 mg/g is 69.1%. That is a reference material prepared to the most exacting standard available, characterised by three orthogonal primary methods, and slightly more than three-tenths of its mass is not peptide. A chromatographic purity in the high nineties on that same material would not contradict it. The two figures are answers to different questions, and only one of them is normally printed.

McCarthy and colleagues (2023) describe the underlying approach as a two-step mass balance: chromatographic area is one input, and the summed weight-for-weight contributions of counter-ions, residual solvents, water and inorganic material are the others. Area percent is a term inside a mass balance. It is not the mass balance.

What the regulators require, and what research material is not made to

The Guideline on the Development and Manufacture of Synthetic Peptides (EMA/CHMP/CVMP/QWP/367182/2025, adopted 1 December 2025, in effect 1 June 2026) is the current European statement, and it is unusually direct about all of this.

On impurity limits, it removes the framework most people assume is operating: Synthetic peptides are excluded from the scope of ICH Guideline Q3A/VICH GL10, ‘Impurities in New Drug Substances’, and consequently the limits laid down in this guideline are not applicable. The thresholds that apply instead come from the European Pharmacopoeia general monograph Substances for Pharmaceutical Use, under which, as the guideline records, peptide-related impurities are reported above 0.1%, identified above 0.5% and qualified above 1.0%.

On the two numbers, it separates them in the specification itself, listing purity and assay/content as distinct tests, with assay obtainable by LC, elemental analysis, amino acid analysis, nitrogen analysis by Kjeldahl, or qNMR, and requiring counter-ion identity and content, residual trifluoroacetate, and water content as further specification parameters in their own right. The United States Pharmacopeia covers the same ground in general chapter ⟨1503⟩ Quality Attributes of Synthetic Peptide Drug Substances, whose scope likewise separates peptide content and assay from related-substance impurities, counter-ion content, water content and residual solvents.

On how the assay is expressed, it is categorical: Limits of assays determined by LC are expressed in terms of the counter-ion free, anhydrous substance, unless otherwise justified. And on what a stated strength means: the strength of the finished product should be defined with respect to the mass of peptide base (not including salt or counter-ion).

That last sentence is the one worth sitting with. For a medicinal product, a stated mass means peptide, with salt and water already subtracted, and correction factors applied at dispensing to achieve it. Laboratory research material is not manufactured under that guideline and carries no equivalent obligation, so a mass on a research label is a gravimetric fill and not a peptide-base figure unless the certificate says otherwise. The wider question of what research grade is not made to is covered on the page on endotoxin, sterility and research grade.

This also has a consequence one step downstream. Concentration after reconstitution is vial mass divided by diluent volume, and the numerator of that fraction is whichever mass the label refers to. The arithmetic itself is unaffected — it is set out on the page on calculating concentration after reconstitution — but what the resulting figure is a concentration of depends on this distinction.

Why a floor is a different kind of statement from a point value

A purity figure quoted as a point value — 99.4%, 98.7% — belongs to one lot, one method and one certificate. Quoted about a catalogue, it asserts something about lots that have not been made yet, using a measurement taken from one that has. The precision is real and its scope is not, and nobody re-measures it when the next lot arrives.

A floor claim behaves differently. It states a boundary that every lot must clear, and it is falsified by any single lot that does not. It concedes the resolution a certificate genuinely warrants rather than borrowing more. NovoVita’s published statement is written in that form: Third-party tested at greater than 99% purity. The reasoning behind stating it in that form is set out on the page on that position.

Neither form, it should be said, converts one number into the other. A floor on chromatographic purity remains a statement about chromatographic purity.

Questions that establish which number is on the page

1. Which quantity is it? A row labelled purity or chromatographic purity is area percent. A row labelled assay, content or peptide content is a mass fraction. A certificate reporting only one of them has answered only one question.
2. At what wavelength, and on what gradient? Without both, an area percent cannot be compared with any other area percent.
3. Is counter-ion content reported, and is the salt form named? Trifluoroacetate and acetate differ by roughly a factor of two in mass contribution for the same sequence.
4. Is water content reported, by a water-specific method? On a hygroscopic solid this is not a rounding term.
5. Is the assay stated on an anhydrous, counter-ion-free basis? This is the phrase the EMA guideline uses, and its presence indicates the subtraction has been done.
6. Was identity established by two orthogonal methods? The guideline recommends at least two for identification. What a conforming mass does and does not establish is covered on the page on mass spectrometry confirmation.

One further boundary is worth stating, since it is where the whole distinction stops applying: a peptide purity specification has no meaning applied to a diluent. A purity row on a water certificate would be a category error rather than a reassurance.

Purity is a statement about composition. It is not a claim that a compound does anything.

References

1. European Medicines Agency, Committee for Medicinal Products for Human Use and Committee for Veterinary Medicinal Products. Guideline on the Development and Manufacture of Synthetic Peptides. EMA/CHMP/CVMP/QWP/367182/2025. Adopted by CHMP 1 December 2025, by CVMP 4 December 2025; date of coming into effect 1 June 2026. Sections 2, 4.4.1, 4.4.2, 4.4.5, 4.5, 4.7.1 and 5. Regulatory document.
2. European Pharmacopoeia. General monograph Substances for Pharmaceutical Use (2034). Peptide-related impurity reporting, identification and qualification thresholds, as stated in the EMA guideline above. Pharmacopoeial monograph — thresholds cited via the EMA guideline; the monograph text itself is not open access.
3. International Council for Harmonisation. Impurities in New Drug Substances, Q3A(R2). Step 4 version dated 25 October 2006. Cited for its scope exclusion of peptides. Regulatory document.
4. United States Pharmacopeia. General Chapter ⟨1503⟩ Quality Attributes of Synthetic Peptide Drug Substances, covering peptide content and assay, related-substance impurities, counter-ion content, water content and residual solvents. Pharmacopoeial general chapter — not open access; cited for scope, not quoted.
5. Hoofnagle AN, Whiteaker JR, Carr SA, Kuhn E, Liu T, Massoni SA, Thomas SN, Townsend RR, Zimmerman LJ, Boja E, et al. Recommendations for the generation, quantification, storage, and handling of peptides used for mass spectrometry-based assays. Clin Chem. 2016;62(1):48–69. PMID 26719571. DOI 10.1373/clinchem.2015.250563. Consensus recommendations, analytical methodology.
6. Kuipers BJH, Gruppen H. Prediction of molar extinction coefficients of proteins and peptides using UV absorption of the constituent amino acids at 214 nm to enable quantitative reverse phase high-performance liquid chromatography-mass spectrometry analysis. J Agric Food Chem. 2007;55(14):5445–51. PMID 17539659. DOI 10.1021/jf070337l. Analytical measurement.
7. Erckes V, Streuli A, Chamera Rendueles L, Krämer SD, Steuer C. 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. Analytical method development and in vitro liposomal assay.
8. 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):6719–31. PMID 30143839. DOI 10.1007/s00216-018-1272-7. Analytical method development, reference material.
9. Stocks BB, Thibeault MP, Meija J, Melanson JE. Assessing MS-based quantitation strategies for low-level impurities in peptide reference materials: application to angiotensin II. Anal Bioanal Chem. 2018;410(26):6963–72. PMID 30128809. DOI 10.1007/s00216-018-1302-5. Analytical method development.
10. McCarthy D, Han Y, Carrick K, Schmidt D, Workman W, Matejtschuk P, Duru C, Atouf F. Reference standards to support quality of synthetic peptide therapeutics. Pharm Res. 2023;40(6):1317–28. PMID 36949371. DOI 10.1007/s11095-023-03493-1. Review.
11. D’Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. PMID 25044089. DOI 10.1016/j.jpba.2014.06.012. Review.
12. National Center for Biotechnology Information. PubChem Compound Summaries for CID 9941957 (BPC-157, C₆₂H₉₈N₁₆O₂₂, 1419.5, monoisotopic 1418.7042), CID 62707662 (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH, C₃₈H₆₈N₁₀O₁₄, 889.0, monoisotopic 888.4916), CID 172198 (angiotensin II, C₅₀H₇₁N₁₃O₁₂, 1046.2) and CID 6422 (trifluoroacetic acid, C₂HF₃O₂, 114.02). Chemical database records.