Understanding Certificates of Analysis for Research Peptides: HPLC, Mass Spec and Red Flags
How to read a peptide certificate of analysis: the HPLC and mass-spec sections, what each proves and misses, and how to spot a fabricated or recycled COA.
By the APL Research Team · Updated · First published · 7 min read
A certificate of analysis (COA) is a laboratory's record of what it measured on a sample from one specific batch. For a research peptide that usually means an HPLC purity result and a mass-spectrometry identity check. A COA is only as useful as two things: its traceable link to the vial on the bench, and the raw data behind its numbers. This guide works through each section, shows how to check the MS arithmetic, sets out what a genuine COA still cannot tell you, and lists the signs of a fabricated or recycled document.
What a COA is, and what it is not
A COA reports results against a specification. The specification is the acceptance limit ("≥98% by HPLC"); the result is what was measured ("99.1%"). A document that shows only the specification, or a round number with no trace, is a statement of intent rather than a measurement.
The depth of testing varies with the material's purpose. Pharmaceutical peptide reference standards are characterised for identity, purity and strength using NMR, mass spectrometry and chromatography, with a mass-balance approach to assign quantitative content [1]. Research reagents are rarely tested that deeply, but expectations exist outside pharma too: a consensus paper from the US National Cancer Institute's proteomics consortium set out recommendations for how research peptides used in quantitative MS assays are procured, characterised, quantified and stored [2].
| Field | What it should show | Why it matters |
|---|---|---|
| Product and sequence | Full sequence with modifications (Ac-, -NH2, disulfides, non-standard residues) | The reference for the MS check |
| Batch or lot number | A unique code matching the vial label | Ties the data to the material in hand |
| Formula and theoretical mass | Stated as average or monoisotopic | The basis for the MS comparison |
| Test date | Plausible relative to manufacture and shipping | Exposes recycled or back-dated documents |
| HPLC result | Result, specification, chromatogram, integration table, method | The purity evidence |
| MS result | Instrument type, observed m/z, charge state, expected value, spectrum | The identity evidence |
| Appearance | For example, white lyophilised powder | A basic consistency check |
| Optional tests | Net peptide content, counter-ion, water, endotoxin | Present only if actually measured |
| Issuer | Laboratory name, analyst or approver, report number | Accountability and verifiability |
Reading the HPLC section
The HPLC section answers one question: what fraction of the UV-absorbing material in the sample elutes as the main peak? Generic peptide quality control uses a C18 column, an acetonitrile gradient containing 0.1% trifluoroacetic acid and UV detection at about 215 nm [3]. Purity is the main peak's area as a percentage of all integrated peak areas.
A credible HPLC section lets a reader check that calculation. Look for:
- The chromatogram itself, with labelled axes, a time scale and the main peak's retention time.
- An integration table listing each peak's retention time and area, summing to 100%.
- Method conditions: column, mobile phases, gradient, flow rate, detection wavelength.
- A run long enough for late-eluting, more hydrophobic impurities to appear after the main peak.
Two limitations apply even to a genuine result. Structurally close impurities that were not resolved during preparative purification can co-elute with the main peak in the QC run as well, so a single generic method can overstate purity [3]. And UV detection is blind to anything that does not absorb: quantitative NMR found undeclared, UV-transparent mannitol at 20% and 43% w/w in two custom synthetic peptides bought commercially [4]. The HPLC testing guide covers method dependence in detail.
HPLC purity is also not peptide content. In a metrology study of synthetic glucagon, water measured by Karl Fischer titration, TFA by ion chromatography and inorganic ions by ICP-MS were each subtracted alongside related peptide impurities, leaving 896 mg of glucagon per gram of material [5].
Reading the mass spectrometry section
Mass spectrometry is the identity test. When the intended sequence is known, MS is the method of choice for confirming a synthetic peptide's authenticity and integrity [6]. The check is arithmetic: the observed mass must match the mass calculated from the sequence, within the instrument's accuracy.
Compare like with like
Three conventions trip readers up:
- Monoisotopic or average mass. The monoisotopic mass uses the most abundant isotope of each element; the average mass weights all isotopes. They differ by about 0.6 Da for Semax and by more for larger peptides.
- Neutral mass or ion. Instruments measure ions. The singly protonated ion, [M+H]⁺, is about 1.007 Da heavier than the neutral molecule.
- Charge state. Electrospray produces multiply charged ions, so a peptide of mass M also appears at (M + 2 × 1.007)/2, (M + 3 × 1.007)/3 and so on.
Worked example: Semax
Semax is Met-Glu-His-Phe-Pro-Gly-Pro (C37H51N9O10S). Calculated from the formula, and reproducible with the peptide molecular weight calculator:
| Species | Shift vs [M+H]⁺ | Expected m/z |
|---|---|---|
| Neutral molecule, monoisotopic | — | 813.348 (average 813.92) |
| [M+H]⁺ | — | 814.355 |
| [M+2H]²⁺ | — | 407.681 |
| [M+Na]⁺ sodium adduct | +21.98 | 836.337 |
| Met oxidised to Met sulfoxide, [M+H]⁺ | +15.995 | 830.350 |
| Des-Gly deletion, [M+H]⁺ | −57.021 | 757.334 |
| Des-Pro deletion, [M+H]⁺ | −97.053 | 717.303 |
| D-His epimer, [M+H]⁺ | 0 | 814.355 |
A COA reading "observed 813.9, theoretical 813.92" compares average masses and is internally consistent for a low-resolution instrument. One reading "observed 814.35, theoretical 813.92" mixes a protonated monoisotopic ion with a neutral average mass; the numbers may still describe the right molecule, but the report was not prepared carefully. High-resolution instruments typically report to within a few parts per million, which at m/z 814 is less than 0.005.
What MS cannot show
- Isomers. D-amino acid epimers have exactly the same mass as the all-L peptide, and because D and L forms differ only in configuration they are hard to separate and quantify [7]; a review of LC–MS characterisation of synthetic peptide therapeutics gives isomers and epimers particular attention for that reason [8]. Leucine and isoleucine, or a scrambled sequence, likewise give an identical intact mass.
- Quantity. A clean spectrum says nothing about how much peptide is in the vial.
- Trace contaminants. An immunology group found a contaminating peptide too dilute to see by direct MS, detectable only after concentrating it by LC, that nonetheless produced T-cell responses [9].
What a genuine COA still does not prove
Even an authentic, well-prepared COA describes a sample from the batch on the test date. It does not show how that vial was stored afterwards, how accurately it was filled, or whether the peptide is biologically active. Impurity classes that commonly accompany synthetic peptides, including deletion sequences, diastereomers, protecting-group adducts, oxidation products and residual trifluoroacetate, are only partly visible to HPLC-UV and MS [10]; peptide purity explained and how peptides are made cover where they come from. If counter-ion, water, endotoxin or content results are not printed, they were not measured, or at least not reported.
How to spot a fabricated or recycled COA
The gap between printed and measured figures can be large. Semaglutide vials test-purchased from illegal online pharmacies measured 7.7–14.4% purity against a 99% label claim [11], and falsified peptide drugs bought online in Belgium included cysteine-containing products of 5–75% purity [12]. Those were illegal medicines rather than research reagents, but the lesson transfers: a COA costs nothing to type, whereas consistent raw data are hard to fake.
| Red flag | Why it matters | How to check |
|---|---|---|
| No batch number, or one that differs from the vial | A generic document can be reused for every shipment | Compare the code on the vial, label and COA |
| A percentage with no chromatogram or spectrum | There is nothing to verify | Ask for the trace and spectrum |
| Identical purity across products or batches | Real batches vary; repeated identical figures suggest a template | Compare several COAs from the same source |
| The same chromatogram for different peptides | Different sequences would be expected to elute at different times; random baseline noise never repeats exactly | Overlay or compare the images |
| MS values that do not match the sequence | The document may belong to another peptide | Recalculate with the molecular weight calculator |
| Mixed or impossible mass conventions | Copied numbers often lose their context | Check average vs monoisotopic, ion vs neutral |
| Dates out of order | Testing before synthesis, or one date on many batches | Read the dates against each other and against the order |
| No named, contactable laboratory | Nobody is accountable for the result | Find the laboratory's contact details independently and ask it to confirm the report number |
| Signs of editing | Mismatched fonts, misaligned fields, text pasted over images | Inspect the PDF closely |
| No answer to a method question | A supplier holding real data can name its column, gradient and detector | Ask |
When the stakes justify it, independent testing settles the question. Analytical laboratories accredited to ISO/IEC 17025 (in Australia, through NATA) can run HPLC and MS on a sealed vial, and peptide content if quantity matters. The supplier evaluation guide covers when that is worth doing.
For the bench
- Record the batch. Note the batch number in the lab notebook beside every experiment, and file the COA with it.
- Recheck the MS arithmetic on receipt. It takes two minutes with the peptide molecular weight calculator and catches mismatched documents.
- Ask for what the experiment needs. Molar work needs content data, not just purity (the molarity calculator shows why); cell assays need the salt form, covered in TFA vs acetate peptide salts.
- Request raw data, not just a summary. Australian Peptide Lab, for example, tests each batch by HPLC against its purity specification with MS identity confirmation and supplies the batch documentation on request; whichever supplier is used, the trace and spectrum are the parts worth asking for.
- Re-test old stock. A COA describes the batch on its test date; long-stored or repeatedly handled material can be rechecked by HPLC before a critical experiment.
Frequently asked questions
What should a research peptide COA include at a minimum?
A batch number that matches the vial label, the sequence with any terminal modifications, the theoretical mass, an HPLC result with its chromatogram and method conditions, a mass spectrum with observed and expected m/z values, the test date and the issuing laboratory. Reference-standard work adds content and impurity assignments by several techniques [1], but for a research reagent the HPLC trace and mass spectrum are the core evidence; a bare percentage is not.
How can I check that the mass spectrometry result matches the peptide?
Calculate the expected mass from the sequence with the peptide molecular weight calculator, then compare like with like: monoisotopic against monoisotopic, average against average, and the right charge state. For Semax (MEHFPGP), the singly protonated monoisotopic ion should appear at m/z 814.36 and the doubly protonated ion at m/z 407.68. A value 16 Da high suggests methionine oxidation; one 57 Da low suggests a missing glycine.
Does 99% HPLC purity mean 99% of the powder is peptide?
No. HPLC purity is the share of UV-detected peak area belonging to the main peak. Water, counter-ions such as trifluoroacetate and inorganic salts are not seen by that method. In a metrology study of synthetic glucagon, the peptide accounted for about 896 mg per gram once water, TFA, inorganic ions and related impurities were measured and subtracted [5]. See peptide purity explained.
Can mass spectrometry detect every impurity?
No. Isomers such as D-amino acid epimers have exactly the same mass as the target and differ from it only in configuration, which makes them hard to separate and quantify [7], and reviews of LC–MS impurity characterisation give isomers and epimers particular attention because intact mass cannot distinguish them [8]. Very small contaminants can also fall below detection: one immunology group found a peptide contaminant that direct MS could not see but that still stimulated T cells [9].
Is a COA from a third-party laboratory automatically more trustworthy?
Independence helps only if the document can be verified. A third-party report still needs a batch number that matches the vial, raw data rather than a single figure, and a laboratory that can be contacted to confirm it issued that report number. A fabricated document can name a real laboratory, so contact details are best found independently rather than copied from the PDF.
How common are mislabelled peptide products?
In regulated-market surveillance, very common. Semaglutide vials bought from illegal online pharmacies measured 7.7–14.4% purity against a 99% label claim [11], and falsified peptide drugs bought online in Belgium included cysteine-containing products with purities of 5–75% [12]. These are studies of illegal medicines rather than research reagents, but they show how far a printed figure can sit from a measured one.
References
- 1.McCarthy D, Han Y, Carrick K, et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharm Res. 2023. PubMed 36949371
- 2.Hoofnagle AN, Whiteaker JR, Carr SA, et al. Recommendations for the Generation, Quantification, Storage, and Handling of Peptides Used for Mass Spectrometry-Based Assays. Clin Chem. 2016. PubMed 26719571
- 3.Karongo R, Ikegami T, Stoll DR, et al. A selective comprehensive reversed-phase×reversed-phase 2D-liquid chromatography approach with multiple complementary detectors as advanced generic method for the quality control of synthetic and therapeutic peptides. J Chromatogr A. 2020. PubMed 32823119
- 4.Choules MP, Bisson J, Simmler C, et al. NMR reveals an undeclared constituent in custom synthetic peptides. J Pharm Biomed Anal. 2020. PubMed 31671336
- 5.Wang X, Zhang F, Li H, et al. Purity determination of synthetic glucagon using a mass balance approach. Sci Rep. 2020. PubMed 32157163
- 6.Chrone VG, Lorentzen A, Højrup P. Characterization of Synthetic Peptides by Mass Spectrometry. Methods Mol Biol. 2024. PubMed 38997482
- 7.Badgujar D, Paritala ST, Matre S, et al. Enantiomeric purity of synthetic therapeutic peptides: A review. Chirality. 2024. PubMed 38448043
- 8.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. PubMed 34110145
- 9.Brezar V, Culina S, Østerbye T, et al. T cells recognizing a peptide contaminant undetectable by mass spectrometry. PLoS One. 2011. PubMed 22194932
- 10.D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014. PubMed 25044089
- 11.Ashraf AR, Mackey TK, Vida RG, et al. Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription: Market Surveillance, Content Analysis, and Product Purchase Evaluation Study. J Med Internet Res. 2024. PubMed 39509151
- 12.Janvier S, Cheyns K, Canfyn M, et al. Impurity profiling of the most frequently encountered falsified polypeptide drugs on the Belgian market. Talanta. 2018. PubMed 30029448
This article summarises published research for educational purposes. It is not medical advice. Compounds sold by Australian Peptide Lab are research reagents for in-vitro laboratory use only, not for human or veterinary use.
