The short answer
Read a certificate in four passes: confirm the laboratory is independent and accredited, check the lot number matches the vial, read each analyte against its specification rather than the headline purity figure, and verify the document with the issuing laboratory. A certificate you cannot verify is not evidence.
Start with who issued it, not what it says
The first question about a certificate is not the purity figure — it is who produced it. A result issued by the seller, or by a laboratory the seller owns, is a self-assessment. Independence is what turns a number into evidence, because the laboratory has nothing to gain from the result.
Accreditation to ISO/IEC 17025 is the meaningful marker. It is not a rubber stamp on a result; it is an audit of the laboratory itself — its methods, calibration, competence and record-keeping. A laboratory without it may still be careful, but nothing external has confirmed that.
Look for the laboratory's own name, address and contact details on the document. A certificate that names no laboratory is unattributable, and an unattributable result cannot be followed up by anyone.
Match the lot before you read the numbers
Every synthesis run is analysed separately, so a certificate describes one batch and only that batch. The lot or batch number on the document must match the one printed on the vial in front of you. This is the single most common gap between a genuine certificate and a meaningful one.
A supplier can hold an authentic certificate for a well-made batch and ship material from a different run entirely. The certificate is not lying; it simply is not about your vial. Checking the lot takes seconds and removes that entire class of doubt.
Dates matter for the same reason. A certificate issued long before the material you received suggests either a very large batch or a mismatch worth asking about.
Read the analytes, not the headline
Purity by RP-HPLC is usually reported as area normalisation at 214 nm: the target peak as a percentage of all peptide-related peaks. It answers "how much of the peptide content is the right peptide" — not "how much of this powder is peptide". Both are useful; they are not the same claim.
Net peptide content answers the second question, and it is frequently lower than the labelled weight. Synthetic peptides are typically isolated as salts, so part of the powder's mass is counterion and residual water. A certificate that reports net peptide content is disclosing more, not performing worse.
Identity — by mass spectrometry or retention-time comparison against a reference standard — is a separate test from purity, and the more fundamental one. A 99% purity figure for the wrong molecule is a precise measurement of the wrong thing.
Below those sit the contamination controls: elemental impurities by ICP-MS under USP ⟨233⟩, bacterial endotoxin under USP ⟨85⟩, and sterility. Each rules out a specific way a result could be confounded. A purity-only certificate is silent on all of them.
Verify it, or treat it as unverified
A PDF is a file. It can be edited, reissued or fabricated in minutes, and a scanned certificate hosted on a seller's own website carries exactly as much authority as the seller. This is the weakest link in most supply chains, and the easiest to close.
The strong form is verification at source: a code or QR on the document that resolves to the issuing laboratory's own portal, showing the same result from the laboratory's records. At that point you are no longer trusting the seller's copy of the document — you are reading the laboratory's.
The practical rule is blunt. If a certificate cannot be checked with the laboratory that supposedly produced it, treat it as no certificate at all — not as weak evidence, but as absent evidence.
Questions researchers ask
What is a certificate of analysis for a peptide?
A document from an analytical laboratory reporting the tests run on one specific batch of material and the result of each against a stated specification. It typically covers identity, purity, and contamination controls such as heavy metals, endotoxin and sterility, and it applies only to the lot it names.
What purity should research-grade peptides be?
Research-grade material is commonly specified at ≥95% by RP-HPLC, with many suppliers reporting ≥98–99%. The figure only means something alongside the method, the wavelength and the specification it was measured against — a percentage quoted with no method attached is not a result.
Is in-house testing as good as third-party testing?
No. When the organisation selling the material also certifies it, the analysis and the commercial interest sit in the same place. Independent testing removes that conflict, and ISO/IEC 17025 accreditation adds an external audit of the laboratory's methods and competence.
How do I verify a certificate of analysis is genuine?
Check it against the issuing laboratory rather than the seller. Reputable laboratories provide a verification code or QR that resolves to their own portal, showing the result from their records. If no such route exists, contact the named laboratory directly — and treat an unverifiable document as absent evidence.
Why does net peptide content differ from the labelled weight?
Synthetic peptides are usually isolated as salts, so a share of the powder's mass is counterion (commonly TFA) plus residual water rather than peptide itself. Net peptide content quantifies the actual peptide mass, which is why it can read below the labelled figure.
At a glance
- Independence
- Issued by a laboratory with no commercial interest in the result
- Accreditation
- ISO/IEC 17025 — audits the laboratory's methods and competence
- Specificity
- Bound to a single lot; the lot must match the vial
- Verifiability
- Confirmable at the issuing laboratory, not only as a vendor-hosted PDF
- Scope
- Identity, purity, elemental impurities, endotoxin, sterility
Sources
Written from primary literature and published standards — never from other suppliers' marketing copy.
Written by Ivory Science. Scientific review: pending appointment of a named reviewer. Until then this page cites its sources directly so every claim can be checked at origin.
Related notes
For research use only. Not for human or veterinary use, ingestion or administration. Nothing here is medical advice, and no statement on this page has been evaluated by the FDA. Certificates of analysis is not an approved medicine.