A peptide certificate of analysis — the COA — is the analytical record for one specific batch of material: what it is, how pure it is, what the mass spectrometer says it weighs, and who measured it. It is the only document that connects a vial on your bench to evidence. This guide explains every field on a peptide COA, how to read an HPLC chromatogram and a mass spectrum, and where the common gaps and red flags appear, for qualified researchers handling research-use-only material.
Key takeaways
- A COA is batch-specific. A certificate that does not carry a batch or lot number matching the vial in your hand is not evidence about your material.
- The two load-bearing tests are RP-HPLC for purity and mass spectrometry for identity. Purity without identity, or identity without purity, is only half a certificate.
- HPLC purity is a chromatographic area percentage, not a fraction of the vial’s mass. It answers “of the peptide-related material detected, how much is the target sequence?”
- Net peptide content is the separate figure describing how much of the weighed powder is peptide rather than counter-ions, residual water and salts.
- Mass spectrometry should report a measured mass that matches the theoretical monoisotopic or average mass of the sequence to within the instrument’s stated tolerance.
- Regulatory frameworks for peptide characterisation are well established: ICH Q6B for specification structure, ICH Q2(R2) for method validation, and FDA guidance setting impurity identification at 0.10% for synthetic peptides.
- All material described here is supplied strictly for in-vitro laboratory research and is not a medicine.
What is a peptide certificate of analysis?
A peptide certificate of analysis is a signed analytical report covering a single manufactured batch, stating the tests performed, the acceptance criterion for each, and the result obtained. It exists to answer one question: is the material in this specific batch what the label says it is, to the stated quality?
The structure is not arbitrary. ICH Q6B, the harmonised guideline on specifications for biotechnological and biological products, frames a specification as a list of tests with references to analytical procedures and appropriate acceptance criteria, and it separates identity, purity and impurities, quantity, and physicochemical characterisation as distinct concerns.1 A well-built peptide COA reflects that structure whether or not it says so.
The important consequence for a researcher is that a COA is not a marketing document and is not transferable between batches. Two vials of the same compound from the same supplier, made months apart, have different certificates. If the batch number on the certificate does not match the batch number on the vial, the certificate tells you nothing about the vial.
GenoPept publishes per-batch third-party certificates covering HPLC purity and mass spectrometry identity at /coa-certificates/, indexed by batch so that a vial can be reconciled to its analytical record.
The anatomy of a COA, field by field
A complete peptide COA carries six blocks: a header identifying product and batch, an identity section, a purity result, a mass confirmation, physical and handling data, and a sign-off. The table below gives each field, what it means, and what a satisfactory entry looks like.
| Field | What it is | What a good entry looks like |
|---|---|---|
| Product name | The compound, using the INN or accepted research name | Unambiguous name, no brand names, consistent with the vial label |
| Batch / lot number | The manufacturing identifier this certificate applies to | Present, specific, and matching the number printed on the vial |
| Date of manufacture / analysis | When the batch was made and tested | Both present; analysis date at or after manufacture |
| Sequence | One-letter or three-letter amino acid sequence, with modifications | Full sequence with N/C-terminal modifications and any acylation stated |
| Molecular formula and MW | Theoretical composition and mass of the target molecule | Formula and MW that reconcile with the stated sequence |
| CAS number | Chemical Abstracts registry identifier, where one exists | Present for registered compounds; absent is acceptable for novel research sequences |
| Appearance | Visual description of the solid | “White to off-white lyophilised powder” or equivalent |
| Purity (RP-HPLC) | Area percentage of the main peak relative to all detected peptide-related peaks | A stated method, a stated wavelength, a numeric result, and the chromatogram itself |
| Identity (MS) | Measured molecular mass compared to theory | Found mass, theoretical mass, ionisation mode, and the spectrum |
| Net peptide content | Fraction of the weighed mass that is peptide backbone | A percentage with the method named (typically amino acid analysis) |
| Water content | Residual moisture in the lyophilisate | Percentage by Karl Fischer or loss on drying |
| Counter-ion | The salt form, commonly trifluoroacetate or acetate | Identified, with content where determined |
| Solubility | Recommended diluent for reconstitution | Specific enough to act on, e.g. “soluble in water” or “use dilute acetic acid” |
| Storage | Recommended conditions and window | Temperature and form, e.g. “−20 °C, sealed, desiccated” |
| Testing laboratory | Who performed the analysis | Named laboratory, ideally independent of the manufacturer |
| Sign-off | Analyst or QC authorisation | Name, role and date |
HPLC purity: what the number actually measures
HPLC purity is the area of the main chromatographic peak expressed as a percentage of the total area of all detected peaks. It is a relative measure of how much of the peptide-related material in the sample is the target sequence — not a measure of how much of the powder in the vial is peptide.
The standard technique is reversed-phase high performance liquid chromatography (RP-HPLC). The sample is injected onto a hydrophobic stationary phase, usually a C18-bonded silica column, and eluted with a gradient of increasing organic solvent — typically acetonitrile against water, both containing a small percentage of an acidic ion-pairing agent such as trifluoroacetic acid. Peptides elute in order of hydrophobicity, and a UV detector set near 214 nm, where the peptide bond itself absorbs, records the eluate.
Two consequences follow directly from that description. First, anything that does not absorb at the detection wavelength is invisible: inorganic salts, most counter-ions and water contribute no peak and therefore do not reduce the purity figure. Second, an impurity that co-elutes with the main peak is counted as main peak. Both are reasons the chromatogram itself matters more than the number extracted from it.
What the impurity peaks usually are
Solid-phase peptide synthesis produces a characteristic impurity profile. Deletion sequences arise when a coupling step fails and the chain continues one residue short; they typically elute close to the main peak. Truncated sequences result from failed capping. Oxidised species, usually methionine sulfoxide, elute earlier because oxidation increases polarity. Isomerised and deamidated forms — isoaspartate from aspartate or asparagine — often appear as shoulders or partially resolved twin peaks. Diketopiperazine and pyroglutamate species from N-terminal chemistry are also well documented degradation and synthesis products.2
Regulatory thresholds put the scale in context. FDA’s guidance on abbreviated applications for certain highly purified synthetic peptide products recommends identifying every peptide-related impurity present at 0.10% of the drug substance or greater, and treats a new specified peptide-related impurity above 0.5% as raising immunogenicity concerns requiring characterisation and justification.3 Research-grade material is not held to that standard, but the numbers show what “well characterised” means analytically.
Why the method details matter
A purity percentage without its method is close to meaningless. A short, shallow gradient can bury impurities under the main peak; a long, well-resolved gradient separates them and reports a lower — and more honest — number. ICH Q2(R2) sets out the validation characteristics an analytical procedure should demonstrate for its intended purpose, including specificity, accuracy, precision, range, linearity and detection and quantitation limits.4 A certificate that names the column, the gradient, the flow rate and the detection wavelength is one you can evaluate; one that reports only “Purity: 99%” is not.
Mass spectrometry: confirming identity
Mass spectrometry on a peptide COA answers a different question from HPLC: not “how clean is it” but “is it the right molecule”. The instrument ionises the peptide, separates ions by mass-to-charge ratio, and reports a measured molecular mass that should match the mass calculated from the stated sequence.
Two ionisation techniques dominate. Electrospray ionisation (ESI) produces a series of multiply charged ions; software deconvolutes that charge-state envelope into a single neutral mass. MALDI-TOF produces predominantly singly charged ions and reads more directly, at somewhat lower mass accuracy for larger molecules. Either is acceptable provided the certificate states which was used.
Reading the result is a comparison. The certificate should give a theoretical mass derived from the molecular formula and a found mass from the instrument. For a peptide of a few thousand daltons on a routine instrument, agreement to within roughly one dalton on the average mass is a normal expectation; high-resolution instruments do considerably better. ICH Q6B lists molecular weight determination among the physicochemical characterisation tests expected for this class of product.1
A mismatch is informative rather than merely disappointing. A found mass 16 daltons above theory suggests a single oxidation. Plus 18 suggests hydrolysis. Minus 17 suggests loss of ammonia or pyroglutamate formation at an N-terminal glutamine. A difference matching one residue’s mass suggests a deletion sequence. These are the same species that appear as impurity peaks on the chromatogram, seen from the other side.
Purity vs net peptide content
This is the single most misread pair of numbers on any peptide certificate. Purity is a chromatographic ratio; net peptide content is a mass fraction. A batch can honestly be 99% pure by HPLC and still be only 80% peptide by weight, because the remaining 20% is counter-ion, adsorbed water and residual salt — none of which absorb UV at 214 nm and none of which therefore appear on the chromatogram at all.
The counter-ion is usually the largest contributor. Peptides purified by RP-HPLC with trifluoroacetic acid as the ion-pairing agent are isolated as trifluoroacetate salts, and the TFA associates with every basic side chain. A peptide with several arginine or lysine residues can carry a substantial trifluoroacetate mass fraction. Residual water in a lyophilisate typically adds several percent more.
Net peptide content is determined by amino acid analysis: the sample is hydrolysed to free amino acids, which are quantified against standards, giving an absolute measure of peptide backbone per unit mass. Where a certificate does not report it, laboratories working to molar accuracy either commission the analysis or account explicitly for the uncertainty.
The practical rule is simple. For work where relative concentration is sufficient — comparing treatments prepared from the same stock — nominal mass is fine. For work where absolute molar concentration matters, use net peptide content.
Appearance, water content, counter-ion and the rest
The remaining fields are less glamorous but describe what you will physically handle. Appearance should read “white to off-white lyophilised powder” for most peptides. Copper-complexed peptides such as GHK-Cu are an intended exception, presenting as blue solids that give blue solutions.
Water content, measured by Karl Fischer titration or loss on drying, matters for two reasons: it is part of the mass you weigh, and residual moisture drives solid-state degradation in the lyophilisate. Peptides rich in aspartate, asparagine, methionine or cysteine are more sensitive to it, because moisture mobilises hydrolysis, deamidation and oxidation in the dry state.2,5
Counter-ion identity is more consequential than it looks. Trifluoroacetate is the default from TFA-based purification, but it is cytotoxic to some cell lines at concentrations that can be reached in culture, and where that matters an acetate or hydrochloride salt exchange is specified. A certificate that names the counter-ion lets you make that judgement; one that does not, does not.
Solubility and storage lines translate the chemistry into bench instructions and should be read alongside the guides on reconstitution and storage.
Third-party testing and batch traceability
Third-party tested peptides are those whose analysis was performed by a laboratory independent of the entity selling them. The distinction matters because a COA is an assertion, and the value of an assertion depends on who is making it and what they have to lose.
In-house certificates are not inherently untrustworthy — a manufacturer with a validated QC laboratory produces perfectly good data. But an independent report removes the conflict of interest, and for research material where no regulator inspects the process, that removal does real work.
- Match the batch. Read the lot number from the vial and find the certificate carrying that exact number. Anything else is a different batch.
- Confirm the identity block reconciles. Does the stated molecular weight follow from the stated sequence and formula? A published CAS number can be checked against a public chemical database in under a minute.
- Look at the chromatogram, not the headline. Is there a trace? Is the main peak well resolved, symmetrical and not obviously a merged doublet? Is the baseline flat?
- Check the mass result against theory. Found and theoretical masses should both be printed, with ionisation mode stated.
- Note who signed it. A named laboratory with a report number and a date is materially stronger evidence than an unsigned PDF with a logo.
- File it with the material. Store the certificate alongside your receipt record so that any result generated from the batch can be traced back to its analysis.
Red flags: how to spot a weak certificate
Most weak certificates fail in predictable ways: they omit the batch link, omit the raw data, or report numbers that cannot be true. The table below separates the signals.
| Red flag | Why it matters | Green flag |
|---|---|---|
| No batch or lot number | The document cannot be tied to your vial | Batch number on the certificate matching the vial label |
| A purity number with no chromatogram | Unverifiable; hides resolution and co-elution problems | Full trace with axes, retention times and integration |
| No mass spectrometry at all | Purity alone does not establish that the molecule is correct | ESI or MALDI spectrum with found and theoretical mass |
| “Purity >99.9%” on every product | Implausible uniformity across different syntheses | Batch-specific figures that vary realistically |
| No method details | A purity figure cannot be evaluated without its gradient and detection conditions | Column, gradient, flow rate, wavelength stated |
| MW inconsistent with the sequence | Suggests a copied template rather than a real analysis | Formula, MW and sequence that reconcile |
| Undated or unsigned | No accountability, no traceable analysis event | Named analyst or laboratory, report number and date |
| Same certificate reused across batches | The data does not describe the material you hold | A new certificate for each production batch |
| Certificate only supplied on request, or not at all | Analytical data should be a default, not a concession | Certificates published openly and indexed by batch |
Batch-tested peptides in the GenoPept store
Frequently asked questions
What does a peptide certificate of analysis actually show?
It shows the analytical results for one specific manufactured batch: the compound’s identity by sequence, formula and molecular weight; its purity by RP-HPLC as an area percentage; confirmation of molecular mass by mass spectrometry; physical data such as appearance, solubility and storage; and the identity of the laboratory that performed the testing. It is evidence about that batch only.
Does 99% purity mean 99% of the vial is peptide?
No. HPLC purity is the main peak’s share of total detected peak area — a ratio among peptide-related species. Counter-ions such as trifluoroacetate, residual water and inorganic salts do not absorb UV at the detection wavelength and are not counted. A 99% pure peptide can be substantially less than 99% peptide by mass; net peptide content is the figure that answers that question.
What is net peptide content and how is it measured?
Net peptide content is the proportion of the weighed powder that is actual peptide, as opposed to counter-ion, water and salts. It is normally determined by amino acid analysis: the sample is hydrolysed to free amino acids which are quantified against standards. Use it whenever an experiment depends on absolute molar concentration rather than relative comparison.
Why does the COA show both a theoretical and a found mass?
The theoretical mass is calculated from the molecular formula implied by the stated sequence; the found mass is what the instrument measured. Comparing them tests identity. Close agreement supports the claimed structure; a difference of plus 16 suggests oxidation, plus 18 hydrolysis, and a gap equal to one residue’s mass suggests a deletion sequence.
What is third-party testing and why does it matter?
Third-party testing means the analysis was carried out by a laboratory independent of the seller. It matters because a certificate is an assertion of quality, and an independent laboratory has no commercial interest in the result. In-house QC data from a competent laboratory can be perfectly sound, but independent analysis removes the conflict of interest entirely.
Is a COA without a chromatogram acceptable?
It is much weaker. A bare purity percentage cannot be evaluated: you cannot see whether the main peak is well resolved or a merged doublet, whether the baseline is flat, or whether the gradient was long enough to separate close-eluting impurities. A certificate that includes the full trace with retention times and integration is materially more credible.
Why do certificates mention trifluoroacetate?
Because most synthetic peptides are purified by reversed-phase HPLC using trifluoroacetic acid as an ion-pairing agent, and the peptide is isolated as its trifluoroacetate salt. TFA associates with basic side chains, so peptides rich in arginine or lysine carry more of it. TFA is cytotoxic to some cell lines, so where cell culture is involved an acetate salt exchange may be specified.
How do I check whether a COA belongs to my vial?
Read the batch or lot number printed on the vial and find the certificate carrying that exact number. If the numbers do not match, the certificate describes different material. Certificates should also carry a date of analysis consistent with the manufacturing date, and a named laboratory with a report reference.
References
- International Council for Harmonisation. ICH Q6B: Specifications — Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. Step 4, 10 March 1999. ICH
- Merck (Sigma-Aldrich). Peptide stability and potential degradation pathways. Technical article. Technical article
- U.S. Food and Drug Administration. ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin. Draft guidance, October 2017. FDA
- International Council for Harmonisation. ICH Q2(R2): Validation of Analytical Procedures. Step 4, 1 November 2023. ICH
- Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharmaceutical Research. 2010;27(4):544–575. PubMed
- Wang W. Lyophilization and development of solid protein pharmaceuticals. International Journal of Pharmaceutics. 2000;203(1–2):1–60. PubMed
Batch-verified research peptides, with the data published
GenoPept supplies lyophilised research peptides with a per-batch third-party certificate of analysis covering HPLC purity and mass spectrometry identity, published openly and indexed by batch, dispatched from the UK, strictly for laboratory research.