Guides & comparisons
How to Read a Peptide COA: Every Field, Line by Line
A peptide COA states who tested which lot, when, by which methods and with what result. Read the header, the lot and date fields, HPLC purity, mass-spec identity and net peptide content, in that order, and it tells you far more than the purity figure alone.
A certificate of analysis is a dense, unglamorous document, and most people read exactly two things on it: the compound name and the purity figure. Everything else gets skimmed or assumed. That’s unfortunate, because several of the skipped fields carry more information than the one everybody looks at.
What follows is a walk through a typical peptide COA, field by field — what each line establishes and what it quietly leaves open. This is document handling for research-use-only laboratory materials, nothing more.
Who issued the certificate, and for whom?
Top of the page: issuing laboratory name and address, the client the report was prepared for, a report or certificate number, and an issue date.
The certificate number is the handle you need if you ever want to ask the laboratory whether the document is theirs — the most decisive check available and the one almost nobody performs. The client field tells you the report’s position in the chain: a report prepared for a manufacturer, handed to a reseller, handed to you is still real, but it has travelled, and travel is where certificates and vials come apart.
If the issuing laboratory and the seller are the same entity, you’re reading self-attestation. That isn’t automatically dishonest — plenty of manufacturers run genuine in-house QC — but independence is a structural property. It either exists or it doesn’t.
The strongest answer to both questions — who tested it, and for whom — is a report commissioned for your own batch. Every new Peppies group order includes independent testing of a sample from that order’s batch at no extra cost, with a batch COA for each peptide type, linked to the sampled batch.
Does the product on the certificate match the vial?
Before any of this there is the vial itself, and most people judge it on sight. What a vial’s colour and cake can and cannot tell you covers that first look, and why it never replaces the lot match below.
Compound name, sometimes a CAS number, molecular formula, theoretical molecular weight, and on better certificates the amino acid sequence in one- or three-letter code.
The sequence deserves ten seconds because it is checkable: the stated molecular weight should be consistent with the sequence written above it, and the formula consistent with both. A mismatch is either a typo or a sign the document was assembled from parts of other documents.
Which lot, tested when, and valid until when?
The lot (or batch) number is the only field that connects the paper to the physical vial. It must appear on the vial label and on the certificate, character for character. A product-name match is not a match. And a vial with no lot number cannot be matched to any certificate at all — at that point the document describes some material somewhere, and the analysis stops there.
Dates come in several flavours and they are not interchangeable:
| Field | What it means |
|---|---|
| Manufacture / synthesis date | When the batch was made |
| Test / analysis date | When the sample was actually run — the date the data describes |
| Report / issue date | When the document was produced |
| Retest or expiry date | When re-analysis is suggested, or a stated shelf limit |
Test date is the one that matters: everything on the certificate is a statement about the material as it was on that day. Check the order makes sense, too. An analysis date before the manufacture date, or an issue date before the analysis, is a document that was built rather than generated.
What should the appearance line say?
Usually one line — “white to off-white lyophilised powder”, or similar. Trivial to check against what’s in front of you, and genuinely diagnostic: a description that doesn’t match the vial is a discrepancy you can find without any equipment at all.
What does HPLC purity actually mean?
Method (reversed-phase HPLC), column, gradient, flow rate, detection wavelength — then a percentage. 95% is the common floor for research-grade material and 98%+ is preferred. Detection is typically at 214–220 nm; 214 nm targets the peptide bond itself, which makes it the more honest wavelength for a peptide, because essentially every peptidic species responds rather than only those carrying aromatic residues.
The critical thing about the number: it is an area-under-curve ratio at one wavelength. It tells you what proportion of the detected signal belongs to the main peak, and nothing about what the rest is. A 98% pure peptide whose remaining 2% is a closely related deletion sequence is a different object from one whose 2% is residual solvent, and the percentage cannot distinguish them. The chromatogram partly can, which is why reading the trace itself is worth the effort.
How does mass spectrometry confirm identity?
Two numbers: the theoretical mass calculated from the sequence, and the observed mass measured by the instrument. Agreement means the material is probably the thing on the label.
MALDI-TOF and ESI are both legitimate — they differ in how the sample is ionised, not in credibility. The stated tolerance matters more. Modern instruments routinely reach accuracy in the low parts-per-million, so a certificate quoting ±0.1% on a peptide of roughly 3000 Da is permitting about ±3 Da of disagreement, and a single amino acid substitution can shift the mass by less than that. A loose tolerance turns a confirmatory test into a formality.
What is net peptide content, and why do most people skip it?
Read this one twice, because it is genuinely underexplained.
Purity is a proportion: of the peptidic material present, how much is the target. Content is a quantity: of the powder in the vial, how much is peptide at all. The two are independent and both can be true at once.
Lyophilised peptide is not pure compound in a physical sense. It carries bound water and counterions left from purification. A vial can be 99% pure by HPLC and still be, say, 70% peptide by weight, the balance being water and salt. Gross weight is what the vial contains; net peptide content is how much of that is the compound.
The reference method for content is amino acid analysis (AAA), which hydrolyses the sample and quantifies the released amino acids. Separate experiment, higher cost, frequently absent. Where a certificate states a milligram figure with no net-content line, that figure is gross weight and should be read as gross weight.
What does a COA not tell you?
Sterility, bacterial endotoxin, heavy metals and residual solvents are separate tests with separate methods, and on a standard purity-and-identity certificate they are usually simply missing. Missing is not “failed” — it is unknown, and that difference matters more than almost anything else on the document. What a COA cannot tell you works through the boundary properly.
In what order should you read a COA?
- Lot number on the vial, then the same string on the certificate.
- Dates in a coherent sequence, with a test date recent enough to mean something.
- Sequence, formula and molecular weight consistent with each other.
- Purity method stated in full — wavelength, column, gradient — not just a number.
- Mass spec tolerance tight enough to be confirmatory rather than decorative.
- Net peptide content stated, or explicitly noted as not determined.
- Issuing laboratory independent of the seller, and verifiably real.
None of this needs a chemistry degree. It needs reading the whole page instead of one line of it. The harder problem is practical — keeping the right certificate attached to the right vial six months later is record-keeping, not analysis, and it’s why Peppies stores lot numbers and certificates against individual shelf items rather than in an email folder. See also how a genuine certificate ends up on the wrong vial and why verification belongs in inventory.
Sources
- ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories — iso.org
- ILAC signatory search: which accreditation bodies are internationally recognised — ilac.org
- Accredited-laboratory registers: A2LA — customer.a2la.org; DAkkS (Germany) — dakks.de
- ICH Q2(R2), Validation of analytical procedures — what a validated HPLC purity method has to demonstrate — ich.org
- European Pharmacopoeia (EDQM), the reference for pharmacopoeial purity and assay definitions — edqm.eu
- Janoshik Analytical, public report verification — public.janoshik.com
A certificate describes a sample of a batch on a particular day. That is all it claims, and all we claim for it. Research use only.
Questions this guide answers
- What is the difference between purity and net peptide content on a COA?
- Purity is a proportion: of the peptide material present, how much is the target sequence. Net peptide content is a quantity: how much of the powder in the vial is peptide at all, the rest being water, counter-ions and salts. A 99 % purity line says nothing about content, so both lines have to be read.
- Which field on a peptide COA should you check first?
- The certificate number and the issuing laboratory, because they let you ask the lab whether the document is theirs — the one check that settles a certificate, and the one almost nobody performs. Then the lot number against the vial, then purity, identity and content.
- What does it mean when the seller and the testing laboratory are the same company?
- The certificate is self-attestation. That is not automatically dishonest — many manufacturers run genuine in-house QC — but independence is a structural property of a certificate: it either exists or it does not.
- Can you check a COA's sequence and molecular weight yourself?
- Yes. The stated molecular weight should be consistent with the amino-acid sequence written on the certificate, and the formula with both. A mismatch is either a typo or a sign the document was assembled from parts of other documents.