How to read a peptide COA: HPLC, mass spec, purity

What is a certificate of analysis?

The structure: a batch identifier, a list of tests, the method behind each test, an acceptance criterion, and a result. Everything useful on the page hangs on those last three columns being filled in.

The part that carries the weight: the method. A purity figure with no method named is a number without a definition, because purity by area percent, purity by mass and purity by a different chromatographic system are three different quantities.

What it is not: a specification. In an approved product the specification comes first and the certificate reports against it. A pharmacopoeial monograph is a third thing again, a public standard a substance is required to comply with rather than a document any one manufacturer writes: the federal compounding statute phrases it as bulk drug substances that "comply with the standards of an applicable United States Pharmacopoeia or National Formulary monograph, if a monograph exists, and the United States Pharmacopoeia chapter on pharmacy compounding". A 2009 review of the peptide monographs found them covering identification, purity and assay, barely harmonized between the European and the United States pharmacopoeias, and not always consistent within a single pharmacopoeia (Vergote, 2009).

What a monograph does not depend on: an approved product existing. The same statutory sentence sets out three conditions in sequence, as alternatives: a bulk substance complying with an applicable monograph if one exists; or, if no monograph exists, one that is a component of a drug approved by the agency; or, if neither, one that appears on a list the agency develops by regulation. A monograph and an approved product are separate legal facts, and the first does not wait on the second. Where none of the three applies, the certificate defines its own scope.

The consequence of that: a short certificate is not a failing certificate. It is a narrower one, and a test it leaves out is a test the document returns no result for.

The lines, one at a time

How to read this: the middle column is what the method physically measures; the right column is the question it leaves open. Both matter, and the right one gets skipped.

The identity line's real limit: mass spectrometry returns a mass. Two sequences built from the same residues in a different order have the same mass, and so do certain residue substitutions, which is why sequence confirmation is a separate exercise from a mass match.

Why regulators ask for more than one method: FDA's draft guidance on active-ingredient sameness recommends that applicants apply orthogonal analytical methods to characterize primary sequence and physicochemical properties, secondary structure, oligomer and aggregation states, and biological activities. One method characterizing one property is the baseline, not the finish.

What the impurities in a synthetic peptide are

Where they come from: solid-phase synthesis is a cycle of coupling and deprotection repeated once per residue. Every cycle is a chance for the reaction to go incompletely or sideways.

The catalogue, from the review that assembled it: deletion sequences from incomplete deprotection, insertion sequences from excess reagent, racemized residues producing diastereomers, side-chain protection adducts from incomplete deprotection, oxidation of side chains, dimers and larger oligomers, residual counterions such as trifluoroacetate, and, where practice was poor, contamination by unrelated peptides (D'Hondt, 2014).

The second family: degradation products that form later, through routes the same review names, including beta-elimination, diketopiperazine formation, pyroglutamate formation and succinimide formation.

Why deletion sequences are the awkward ones: a molecule missing one residue is chemically close to the intended one, so it can sit near the main peak in a single gradient and carry a similar detector response. That is a separation problem, not a detection problem.

Why the impurity profile is the object of the exercise: the same guidance records that peptide-related impurities may create the potential for differences in immunogenicity, or may otherwise affect the safety or effectiveness of a peptide drug product, and that which kind of application a synthetic peptide needs turns largely on the data required to evaluate the difference in impurities against the previously approved product.

Which tests a certificate does not cover

The split: identity, purity, water and counterion are chemical determinations made on a dissolved or a weighed sample. The four below are separate determinations under their own compendial chapters, each with its own sample and its own acceptance criterion, and a certificate speaks to them only if it reports them.

• Sterility. The compendial sterility test, USP General Chapter 71, is a microbiological test with its own incubation period. No chemical result implies it.
• Bacterial endotoxin. USP General Chapter 85, the bacterial endotoxins test. FDA defines endotoxin as a pyrogenic product present in the bacterial cell wall, and treats depyrogenation as a process separate from sterilization, validated on its own with an endotoxin challenge, so a sterility result does not report it.
• Elemental impurities. Covered by USP General Chapter 233, Elemental Impurities, Procedures, and by ICH Q3D(R2), which sets out a risk-based process for assessing and controlling them.
• Residual solvents. The compendial chapter is USP General Chapter 467, Residual Solvents. ICH Q3C(R8) recommends acceptable amounts of residual solvents in pharmaceuticals. The compendial chapter defines them as organic volatile chemicals used or produced in manufacturing, with their own determination; a solvent or reagent peak that does appear in a purity chromatogram is disregarded by the normalization procedure, so neither route puts one inside the purity figure.
• Visible particulates. USP General Chapter 790, Visible Particulates in Injections, sets the standard, and General Chapter 1790, Visual Inspection of Injections, carries the lifecycle guidance behind it.

The reading: a certificate covering identity, purity and water has answered three questions out of a longer list, and the unanswered ones are the ones that decide whether a preparation is safe to put into a body.

What does a peptide COA test?

Only the tests it names, on the sample it names. The five lines taken apart above are what the solid looks like, what mass the molecule has, what share of the detected chromatogram is the main peak, how much water is in the solid, and which counterion is present. Peptide content is a sixth line, the only one that speaks to how much peptide a weighed sample contains, and the only one the arithmetic downstream can rest on.

The single most useful question to ask of any certificate: which method produced this number. Purity, identity and content each have several possible methods, and the number is only interpretable once the method is named.

What purity number is normal?

There is no compendial normal. An area percent is defined by the normalization procedure in the chromatography chapter, which disregards peaks from solvents and reagents, peaks arising from the mobile phase or the sample matrix, and everything at or below the reporting threshold. The figure is therefore a property of the method as much as of the material, and two certificates printing the same percentage have not necessarily measured the same thing.

The comparison worth making instead: the impurity profile. In the guidance above, which kind of application a synthetic peptide needs turns largely on the data required to evaluate the difference in impurities against the previously approved product, not on a headline percentage.

Why that reframing matters: two batches at the same area percent can carry entirely different impurities, and the impurities are what the immunogenicity question is about.

Does a certificate prove the vial is sterile?

Not unless it reports a sterility test. Sterility is established by USP General Chapter 71, a microbiological method with its own sampling plan and incubation. Bacterial endotoxin is a further, separate test under General Chapter 85, and it answers a different question again, because endotoxin is a heat-stable fragment that remains after the bacteria are gone.

The inference people make and should not: that a clean chromatogram implies a clean preparation. Chromatographic purity and microbiological quality are unrelated measurements on unrelated samples.

Can a certificate be wrong?

Labels demonstrably are. When a research group bought 44 products sold online in one unapproved category and analyzed them, the labeled amount matched the analyzed amount in only 18, eleven held substances the label never mentioned, and four contained no active compound at all (Van Wagoner, 2017). In a separate action, a regulator reported confirming by laboratory analysis that two consumer products contained an undeclared active ingredient.

The structural gap: a certificate describes a batch. Nothing in the document links it to the container in front of anyone. That is a chain-of-custody question, and an analytical certificate is not the kind of document that answers it.

What that does not mean: that certificates are worthless. A certificate naming methods, criteria and a batch is far more information than none, and it can be checked against the methods it names. It is evidence, weighted by who produced it and whether anyone can verify the chain.

What a certificate leaves unsettled

The open items a certificate does not close:

• Chain of custody. Which lot the vial came from, and whether the tested sample and the sold material share a history.
• Sequence. A mass match constrains composition, not order.
• Net mass of peptide. Without a peptide content line, nothing on the certificate states how much of the labeled milligram figure is peptide. That is a gap in what was reported, which is not the same as evidence that no such measurement was ever made.
• Microbiological quality. Absent unless a sterility and an endotoxin result appear.
• Method detail. Column, gradient, wavelength and system suitability are the parameters that make a purity number reproducible. Where a certificate does not print them, the figure cannot be reproduced or compared against another.

How this page is sourced

Methods from the method documents: the analytical descriptions come from the compendial chapters named by number, each linked to its official USP-NF record and DOI, and from FDA guidance read directly. Where a chapter has been revised more than once, the entry links the current USP-NF record rather than an earlier one. The impurity catalogue comes from a 2014 review of peptide-related impurities, and the monograph wording from the United States Code.

One citation was replaced on 29 July 2026: an earlier FDA guidance on synthetic peptide applications no longer resolves at its FDA locator, so every claim that had rested on it was either re-sourced to a document that does resolve, or cut. The 0.5 percent impurity threshold this page used to print was cut for that reason.

What changed on 30 July 2026: a citation audit found four sentences reaching past their sources. The claim that no solvent shows as a peak in a purity run was replaced by what the two chapters actually say, that residual solvents are volatile chemicals with their own determination and that a solvent peak appearing in a chromatogram is disregarded by the normalization procedure. The endotoxin line, previously an unsourced assertion about heat, now reports FDA's own definition and its separate depyrogenation requirement, and that guidance is added below. A supply-chain universal and a claim that an absent peptide content line proves the label mass was never measured were both narrowed to what a certificate can show.

Nine sources: three papers with DOIs and PubMed IDs, two FDA guidances, one FDA enforcement document, one federal statute, one compendial entry covering the chapters named in the text, and one ICH entry covering the two impurity guidances.

The standard: the full sourcing and citation-verification standard for this site, including who checks a page before it ships, lives on the methodology page.

Last reviewed: 29 July 2026.

1. 1

   U.S. Food and Drug Administration, Center for Drug Evaluation and Research. Sameness Evaluations in an ANDA, Active Ingredients. Draft guidance for industry, November 2022. Synthetic peptides section, lines 128 to 142 on physicochemical characterisation and the orthogonal analytical methods recommendation, and footnote 28 on peptide-related impurities, immunogenicity and the impurity-difference test. Accessed 29 July 2026. fda.gov, Sameness Evaluations in an ANDA, Active Ingredients.

2. 2

   D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. doi:10.1016/j.jpba.2014.06.012. PMID 25044089.

3. 3

   Vergote V, Burvenich C, Van de Wiele C, De Spiegeleer B. Quality specifications for peptide drugs: a regulatory-pharmaceutical approach. J Pept Sci. 2009;15(11):697-710. doi:10.1002/psc.1167. PMID 19750489.

4. 4

   Van Wagoner RM, Eichner A, Bhasin S, Deuster PA, Eichner D. Chemical composition and labeling of substances marketed as selective androgen receptor modulators and sold via the internet. JAMA. 2017;318(20):2004-2010. doi:10.1001/jama.2017.17069. PMID 29183075.

5. 5

   U.S. Food and Drug Administration. Warning letter reference 718252, 19 December 2025, reporting that laboratory analysis of product samples confirmed an undeclared active ingredient. Accessed 27 July 2026. fda.gov warning letter 718252.

6. 6

   United States Pharmacopeia, USP-NF general chapters, each cited by its official USP-NF record and DOI. All accessed 29 July 2026. 621 Chromatography, 2025 record, doi:10.31003/USPNF_M99380_10_01; 71 Sterility Tests, doi:10.31003/USPNF_M98810_01_01; 85 Bacterial Endotoxins Test, doi:10.31003/USPNF_M98830_02_01; 233 Elemental Impurities, Procedures, doi:10.31003/USPNF_M5193_03_01; 467 Residual Solvents, 2025 record, doi:10.31003/USPNF_M99226_09_01;790 Visible Particulates in Injections, doi:10.31003/USPNF_M7197_01_01; 1790 Visual Inspection of Injections, doi:10.31003/USPNF_M7198_07_01; 921 Water Determination, doi:10.31003/USPNF_M99710_05_01, which directs a titrimetric, an azeotropic or a gravimetric method as the individual monograph specifies.

7. 7

   International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use. Q3C(R8), Impurities: Guideline for Residual Solvents, Step 4 version dated 22 April 2021; Q3D(R2), Guideline for Elemental Impurities, adopted 26 April 2022. Both accessed 29 July 2026.

8. 8

   Pharmacy compounding, 21 U.S.C. section 353a(b)(1)(A)(i), which lists three alternative conditions for a bulk drug substance: compliance with an applicable United States Pharmacopoeia or National Formulary monograph if a monograph exists, together with the United States Pharmacopoeia chapter on pharmacy compounding; if no monograph exists, being a component of a drug approved by the Secretary; if neither, appearing on a list developed by the Secretary through regulation. Retrieved 30 July 2026, HTTP 200. govinfo.gov, 21 U.S.C. 353a.

9. 9

   U.S. Food and Drug Administration. Sterile Drug Products Produced by Aseptic Processing, Current Good Manufacturing Practice. Guidance for industry, September 2004. Section VI.B.1, Containers and Closures, Preparation, which states that containers and closures should be rendered sterile and, for parenteral drug products, nonpyrogenic, and that validation data should demonstrate the depyrogenation process reduces endotoxin content by at least 99.9 percent; and the glossary entries defining depyrogenation as a process used to destroy or remove pyrogens and endotoxin as a pyrogenic product present in the bacterial cell wall. Retrieved 30 July 2026, HTTP 200. fda.gov, Sterile Drug Products Produced by Aseptic Processing.

Related pages

Around this page: five neighboring pages on what a test covers and what a vial can be shown to hold.

• What an HPLC purity number misses: the same purity line taken apart in detail, with the compendial definition.
• What a lyophilized peptide is: why water content sits on a certificate at all.
• What a vial cake means: the visual check that runs before any of these tests, and its ceiling.
• Is ostarine a steroid: the regulatory reason an unapproved substance has no specification behind it.
• The dose-math hub: the arithmetic section, which assumes the mass on the label is real.

The certificate series: every method explainer in this run ships first inside The Decadewise briefing.