What a certificate is and is not
A certificate of analysis records the tests a laboratory performed on one batch and the results it obtained. It is evidence about that batch, on those properties, at that time. It is not a general statement about the product, the supplier or any other lot.
Two things follow. A certificate whose lot number does not match your vial tells you about a batch you do not have. And a property the certificate did not test is not established by the certificate, however good the properties it did test look.
The fields that carry the most information
Identity comes from mass spectrometry: an observed mass matching the theoretical mass says the molecule is the right weight. Purity comes from HPLC as an area percentage, usually at 214 nanometres. Together they answer the two central questions, is this the right thing and how much of it is something else.
Peptide content, where present, is the absolute figure: how much of the powder is peptide once counterions and water are excluded. Water content by Karl Fischer and residual solvent by gas chromatography fill in the rest of the mass balance.
- •Lot or batch number: ties the document to your vial and nothing else
- •Identity by mass spectrometry: observed against theoretical mass
- •Purity by HPLC: area percent, with the wavelength and method stated
- •Peptide content: absolute, usually by amino acid analysis
- •Water content: Karl Fischer titration
- •Appearance, sequence, molecular formula and test date
Reading the chromatogram, not just the number
A purity percentage compresses a chromatogram into one figure and discards everything else. The trace itself shows whether the baseline is clean, whether the main peak is symmetric, and whether the gradient ran long enough for a late-eluting species to appear.
A shoulder on the main peak is the most consequential thing a number hides, because a partly co-eluting impurity is counted inside the main peak and inflates the result. A certificate that includes the chromatogram is a substantially stronger document than one that reports only the percentage.
Who performed the test
A certificate from an independent accredited laboratory carries more weight than one produced in-house by the seller, for the obvious reason. Third-party certificates name the laboratory, carry a report identifier and can usually be verified directly with that laboratory.
That verification step is worth taking. A certificate is a document, and a document is the easiest thing about a peptide to fabricate.
How to work through a certificate field by field
A reading order that surfaces the disqualifying problems first, so a document that cannot be tied to your vial is set aside before its numbers are read.
- Match the lot number. Against the vial in front of you. A certificate for a different lot is a document about someone else's material, however impressive its figures.
- Check identity before purity. The observed mass should match the theoretical mass for the stated sequence. Purity is meaningless if the main peak is the wrong molecule.
- Read purity with its method. The percentage, the wavelength and the column. Purity at 280 nm is a weaker claim than the same figure at 214 nm, because it cannot see impurities without aromatic residues.
- Look for the absolute content figures. Peptide content, water and residual solvent. These are what convert a purity ratio into a statement about how much peptide is in the vial.
- Check who tested it and when. An independent accredited laboratory, a report identifier and a test date. A certificate with no laboratory named and no date is not verifiable.
- Ask for the raw traces. The chromatogram and the mass spectrum contain what the summary discards: peak shape, baseline quality and the isotope pattern.
What this method cannot tell you
- •A certificate describes one batch. It says nothing about the next one, or about the vial you were sent if the lot numbers differ.
- •It only reports what was tested. Sterility, endotoxin and elemental impurities are absent from most peptide certificates.
- •A summary certificate is a claim about a trace nobody has shown you. Peak shape and baseline quality are not recoverable from a percentage.
- •Documents are forgeable. Verification with the named laboratory is the only step that addresses that.
Where the numbers come from
How to read a peptide COA: frequently asked questions
A document recording the analytical tests performed on one specific batch of a product and the results obtained.
It is evidence about that batch, on the properties tested, at the time of testing. It is not a general statement about the product line.
At minimum:
- •Product name, sequence and molecular formula
- •Lot or batch number and the test date
- •Identity confirmation by mass spectrometry
- •Purity by HPLC, with the wavelength and method stated
- •Appearance
- •The name of the testing laboratory
Peptide content, water content and residual solvent are the next tier and are less consistently present.
Because it is what ties the document to your vial. A certificate for a different lot describes material you do not have.
It is the single most common gap. A supplier sending the same certificate with every order of a product is not sending you evidence about your batch.
That the molecule has the expected mass. The observed value should be within a few parts per million of the theoretical one on a high-resolution instrument.
It is strong evidence of identity and not proof: isomers and rearranged molecules share the same mass.
214 nm, where the peptide bond absorbs, so all peptide-related species are detected.
At 280 nm only tryptophan and tyrosine absorb, so impurities without them are invisible and the reported purity is inflated.
Yes. The percentage discards peak shape, baseline quality and whether the gradient ran long enough for late-eluting species to appear.
A shoulder on the main peak is the most important thing a number hides, because a partly co-eluting impurity is counted as main peak.
The absolute fraction of the powder that is peptide, after counterions and water. It is usually determined by amino acid analysis, which is a separate and more expensive test.
Its absence is why the purity and net peptide content calculators exist: they estimate it from the fields that are present.
Yes, for the obvious reason that the party performing the test has no interest in its outcome.
A third-party certificate names the laboratory, carries a report identifier and can usually be verified with that laboratory directly.
Contact the testing laboratory with the report identifier. Many accredited laboratories operate a lookup for exactly this.
Formatting quality is not evidence of authenticity. A certificate is a document, and documents are the easiest part of a peptide to fabricate.
No. It reports the tests that were performed, and a typical peptide certificate covers identity and purity only.
Sterility, bacterial endotoxin and elemental impurities are separate tests that most peptide certificates do not include.
Purity is a ratio among the species detected by the chromatographic method. Peptide content is an absolute fraction of the total weight.
A vial can be 99 percent pure and 78 percent peptide, because counterions and water do not appear in a chromatogram.
Usually little, but a mismatch is worth noticing. Most lyophilised peptides are white to off-white powders or cakes.
Discolouration, or a certificate describing a cake when your vial contains loose powder, indicates that something differs between the document and the material.
The tests describe the batch at the date they were performed. A certificate from three years ago says nothing about how the material has stored since.
For a peptide stored properly the identity does not change, but purity can drift as degradation products accumulate.
The mass calculated from the stated sequence and modifications, which the observed mass is compared against.
It is worth checking the theoretical figure yourself: a certificate stating a theoretical mass inconsistent with its own sequence has a serious internal problem.
Yes, and its absence is a real gap. The salt form is the largest single correction between labelled weight and actual peptide.
Without it, the difference between an acetate and a TFA salt leaves ten percentage points or more of peptide content unknown.
Treat that as decisive. A supplier who cannot produce batch-specific analytical documentation is asking you to take the identity and purity on trust.
The COA red flag checker lists the other warning signs that tend to appear alongside this one.
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