How to read a peptide
Certificate of Analysis
A Certificate of Analysis is the most important document a peptide vendor hands you, and the easiest one to fake. This guide teaches you to read the chromatograms, separate purity from net peptide content, verify the laboratory, and recognise a forged report on sight.
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A purity number on its own is marketing. A Certificate of Analysis becomes evidence only when it names a batch, a laboratory, a method, and a date, and when you can independently confirm every one of them.
In the research-peptide market there is no prescriber checking identity, no pharmacovigilance, and no regulator standing behind the label. The certificate is the only technical record that travels with the material. Learning to read one properly is the single highest-leverage skill a careful buyer can develop, because it converts a glossy claim into something you can test.
This guide walks through every critical section of a real certificate, reads an HPLC chromatogram and a mass spectrum line by line, explains why a peptide can be 99 percent pure and still mostly salt and water, shows how forged certificates are made and caught, and ends with a checklist you can apply to any report in a few minutes.
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A specification, tested against one batch
Under the pharmaceutical framework that defines these documents, a specification is a list of tests, references to the analytical procedures used, and acceptance criteria, which are the numerical limits or ranges a result must fall within to be acceptable. A Certificate of Analysis is the record showing that a specific batch was tested against that specification and what the results were.
That framing matters because it tells you what a real certificate must contain. Not a headline percentage, but a set of named tests, each tied to a method, each producing a result, each measured against a stated limit, all for a single identified lot. A document that gives you a number and nothing else is not a certificate of analysis. It is a claim wearing the costume of one.
The four load-bearing fields
Lot number, laboratory, method, and date. If any one of the four is missing, the remaining fields cannot be verified, and the document stops functioning as evidence.
An annotated certificate, field by field
Below is the structure of a complete certificate with the values a careful reader checks. Green fields are the ones that should be present and specific. Read them from the top, and stop the moment one is missing rather than skimming to the purity line.
Certificate of Analysis · batch record
Notice how much of the certificate has nothing to do with the purity headline. The lot number, the named lab, the accreditation reference, the method fields, and the verification key are what let an outsider confirm the number is real. A certificate that is all headline and no scaffolding is the pattern to distrust.
Reading an HPLC chromatogram
High-performance liquid chromatography separates the target peptide from its impurities. In the reversed-phase mode used for peptides, the sample travels through a hydrophobic C18 column while a gradient of acetonitrile in water carries the components through. More hydrophobic species hold on longer and emerge later. A detector reads ultraviolet absorbance at 214 nanometres, the wavelength of the peptide bond, so every peptide species is seen.
The x-axis is time. The y-axis is absorbance. Each peak is a species leaving the column, and the time it leaves at is its retention time. The purity figure is arithmetic on the areas under the peaks: the main peak area divided by the total peak area, usually expressed as a percentage. A stated purity of 98.9 percent by HPLC means that 98.9 percent of the absorbing material eluting in the run falls under the target peak.
What good looks like
- One dominant, symmetrical main peak.
- A stable, flat baseline that returns to zero.
- Few and small related-substance peaks.
- A retention time consistent with the compound.
- The solvent front early in the run, excluded from the figure.
What looks suspicious
- A purity number with no chromatogram behind it.
- A drifting or noisy baseline that hides small peaks.
- Broad, split, or badly tailing main peaks.
- Large early or late shoulders on the target peak.
- Axes with no units, no scale, or no retention labels.
The small peaks matter because they have names. Incomplete coupling during synthesis leaves deletion and truncated sequences, missing one or more amino acids. Storage and handling produce oxidation of methionine, deamidation of asparagine and glutamine, and aspartimide formation. These are the related substances a good chromatogram shows as minor peaks and a poor one hides in a noisy baseline. Impurity thresholds in the pharmaceutical framework are strict for exactly this reason: reporting thresholds sit as low as a tenth of a percent, because small peaks can carry real biological consequences.
Reading the mass spectrum
HPLC tells you how pure the sample is. It does not tell you what the molecule is. A perfectly pure wrong peptide produces a beautiful chromatogram. That is why a complete certificate pairs purity with a mass measurement, and why a purity figure with no identity confirmation is only half a certificate.
In electrospray ionisation the peptide picks up several protons and appears as a family of multiply-charged ions, such as the doubly and triply charged forms. The instrument software deconvolutes that charge-state envelope into a single intact mass. That measured mass is then compared with the theoretical mass calculated from the amino acid sequence. A match within a few daltons confirms identity. A mass that is off by the weight of one residue points to a deletion or the wrong sequence entirely.
Read the two methods together. HPLC answers how much of the sample is the target. Mass spectrometry answers whether the target is the molecule you ordered. A certificate that reports only one of them leaves the other question open, and in this market both questions have been answered wrong often enough that neither can be assumed.
Why 99 percent pure is not 99 percent peptide
HPLC area-percent purity describes the proportion of the ultraviolet-absorbing material at the detector that is the target peptide. It says nothing about the mass of the vial. Synthetic peptides are usually supplied as trifluoroacetate or acetate salts and carry residual water, so the counterion and moisture take up weight the purity figure never sees.
The practical result is that a peptide can read 99 percent by HPLC and still be only roughly 70 to 90 percent peptide by weight. The rest is salt and water. That gap is not fraud. It is chemistry, and it is exactly why net peptide content exists as a separate measurement, determined by quantitative amino acid analysis, with water content measured by Karl Fischer titration.
HPLC purity
Area percent. How much of the detected material is the target versus its related substances. A quality signal, not a mass.
Net peptide content
Mass percent. How much of the vial is actually peptide, once salt and water are removed. A different number, measured a different way.
When a listing quotes a single high percentage with no label, assume it is HPLC purity and treat the mass question as unanswered. A certificate that reports both, and keeps them distinct, is the mark of a laboratory that understands what it is measuring.
Forgery patterns, and how to break them
Anyone can build a document that looks like a laboratory report. Fabricated purity figures, borrowed chromatograms, and altered dates cost nothing. What a forger cannot easily do is create a matching record inside the testing laboratory’s own system. That asymmetry is the whole basis of verification: trust the record you can reach independently, not the file you were handed.
The copy-paste certificate
The same chromatogram, date, and numbers appear across several different products, with only the name changed. One trace cannot describe many batches.
The borrowed report
A genuine laboratory report that belongs to a different client or a different batch, presented as if it covers your vial.
The stale result
A real report that is months or years old, reused for new stock. The test date is the tell.
Impossible perfection
An exact 100 percent, or a purity with no decimals. Real purification leaves a trace of related substances; clean round numbers suggest editing.
The headline with no data
A purity percentage with no chromatogram, no method, and no mass. There is nothing to check, which is the point.
The dead verification link
A report identifier that returns nothing in the laboratory’s database, or a laboratory that cannot be reached through details you found yourself.
The defence against all six is the same. Take the lot number and the report identifier off the certificate and confirm them in a place the seller does not control: the laboratory’s public verification portal, or a direct enquiry to the lab using contact details you sourced yourself. If the record does not resolve there, the certificate stands alone, and a certificate that stands alone cannot be trusted.
Verifying the testing laboratory
An independent result is only as good as the independence and competence of the laboratory that produced it. ISO/IEC 17025:2017 is the international standard for the competence of testing laboratories, and it is the credential to look for. Two details are easy to miss. Accreditation is granted for a defined scope, so a laboratory can be accredited for some methods and not the one on your certificate. And accreditation lapses, so a badge is only meaningful while it is current.
The laboratory is named, with a reachable address, not just a logo.
An accreditation reference is given, and the accrediting body lists the lab.
The accredited scope covers the method used, not merely the lab in general.
The result is independent of the seller, not an in-house self-test.
A report identifier resolves in the laboratory's own public database.
Contact details you sourced yourself reach the same laboratory named on the report.
An in-house result is not worthless, but it carries no independent check. When the party selling the material is also the party vouching for it, the certificate loses the one property that made it evidence. Independence is the point of the whole exercise.
The verification checklist
Read it in order. Stop the moment a line fails, because a failure early in the list makes the rest unverifiable.
- 01
Lot match
Your vial's lot number appears on the certificate, exactly.
- 02
Named laboratory
A specific, reachable, independent lab, not a bare logo.
- 03
Current accreditation
ISO 17025, in scope for the method, and still valid.
- 04
HPLC purity + chromatogram
A purity figure with the trace behind it, axes labelled.
- 05
Mass / identity
An intact mass reported against the theoretical value.
- 06
Net peptide content
Reported separately from purity, not implied by it.
- 07
Reasonable numbers
A trace of impurities present; no exact 100 percent.
- 08
Test date
Recent and specific, tied to this batch.
- 09
Verification key
A report identifier that resolves in the lab's own records.
- 10
Independent cross-check
Confirmed somewhere the seller does not control.
Common questions
What is a Certificate of Analysis for a peptide?
A Certificate of Analysis is a testing document that reports the identity and purity of one specific batch of material. For a peptide it should carry a lot number, a named laboratory, the analytical methods used (typically reversed-phase HPLC for purity and mass spectrometry for identity), the numerical results, and the date of testing.
Does 99 percent HPLC purity mean the vial is 99 percent peptide?
No. HPLC area-percent purity describes how much of the ultraviolet-absorbing material at the detector is the target peptide. It does not measure water, salts, or the counterion. A vial can read 99 percent by HPLC and still be only 70 to 90 percent peptide by weight. Net peptide content is a separate measurement, determined by amino acid analysis, with water measured by Karl Fischer titration.
What does ISO 17025 accreditation mean on a COA?
ISO/IEC 17025:2017 sets the requirements for the competence of testing laboratories. Accreditation is granted only for a defined scope of specific tests, so a lab can be accredited for some methods and not others. It signals that an independent body has assessed the laboratory, but you should still confirm the accreditation is current and covers the method on the certificate.
How can I tell if a peptide COA is fake?
Check whether your exact lot number appears on the certificate, whether the laboratory is named and independently reachable, and whether a verification key or report identifier resolves in the laboratory's own public database. Reused chromatograms across different products, an exact 100 percent purity with no decimals, a missing test date, and a purity figure with no underlying chromatogram or method are all common signs of a fabricated or altered report.
Why does the chromatogram matter more than the purity number?
A purity percentage is a claim until you can see the data behind it. The chromatogram shows the peaks, their retention times, and their relative areas, so you can confirm that a single dominant peak really does account for the stated figure and that the impurity peaks are small and few.
Primary references
- ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories.ISO/IEC 17025:2017View source →
- ICH Q6A — Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products (Chemical Substances).ICH Q6A / FDA GuidanceView source →
- ICH Q3A(R2) / Q3B(R2) — Impurities in New Drug Substances and Products. Establishes reporting, identification, and qualification thresholds.ICH Q3A(R2) / Q3B(R2)View source →
- FDA — ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin (guidance on peptide-related impurities).View source →
- FDA — Bulk Drug Substances Used in Compounding Under Section 503A (a valid certificate of analysis is a stated condition).View source →
- United States Pharmacopeia — general chapters on chromatography and biotechnology-derived articles; reversed-phase HPLC as the primary method for peptide identity, purity, and assay.USP <621> / <1503>View source →
Research use only
This guide is educational. It explains how to read a testing document; it does not endorse any vendor, laboratory, or compound, and it is not medical advice. Most peptides sold as research chemicals have no approval for human use. Reading a certificate well tells you what is in a vial. It does not make the contents safe, legal, or appropriate for any purpose.
FREE · FILLABLE PDF
Get the COA Verification Checklist.
A fillable PDF with 46 checks and four hard stops. It is free.