How to Read a Peptide Certificate of Analysis
A field-by-field walkthrough of a peptide COA — what each number establishes, which fields are load-bearing, and what a missing batch number tells you about the rest of the document.
Every vial of research peptide arrives with a document that looks authoritative: letterhead, a chromatogram, a percentage with a decimal place, a signature. Most researchers glance at the purity figure and file it.
The purity figure is not the most useful thing on the page, and on a poorly-constructed certificate it is often the only thing that has been thought about. This is a walkthrough of what each field is doing, in rough order of how much weight it can actually bear.
1. Batch or lot number — the field everything else depends on
A Certificate of Analysis is a record of tests performed on one specific batch of material. Not on the product line, not on a representative sample from last year, not on the compound in general. On a batch.
The batch number is what ties the document to the vial in your hand. If the number on the certificate does not appear on the vial label, the certificate is describing some other material, and nothing else on it is evidence about yours.
This is the single most common defect in research peptide documentation, and it is not usually subtle. A "COA" downloaded from a product page with no batch number, or with a batch number that changes nothing when the stock does, is a marketing asset formatted as a laboratory document.
What to check: the batch number exists, it appears on the vial, and the two match.
2. Product name and sequence
The certificate should state what was supposed to be made, unambiguously. For a peptide, that means the sequence in single- or three-letter code, not only a trade name.
Sequence matters here because names in this market are unreliable. "TB-500" is applied to both full-length thymosin β4 (43 residues, ~4,963 Da) and to the Ac-LKKTETQ fragment (7 residues, ~889 Da). "CJC-1295" is applied to both the DAC and non-DAC forms, which differ by roughly 280 Da and behave differently. A sequence removes the ambiguity that a name preserves.
What to check: a sequence is stated, and it is the sequence you ordered.
3. HPLC purity — what the percentage actually measures
High-performance liquid chromatography separates the components of a sample and a detector records them as they elute. The output is a trace of peaks against retention time. Purity is reported as:
the area of the main peak, divided by the total integrated area of all peaks, expressed as a percentage.
Three things follow from that definition, and all three are routinely misunderstood.
It is relative, not absolute. 99% means the main peak accounts for 99% of everything the detector saw. It does not mean the vial is 99% peptide by mass — water and counter-ion salt are invisible to a UV detector and sit entirely outside the figure. See net peptide content versus purity.
It depends on the detection wavelength. Peptide work typically detects around 214 nm, where the peptide bond absorbs. A method run at 280 nm sees only aromatic residues, and any impurity lacking them becomes invisible — which flatters the number.
It depends on where the integration boundaries were drawn. A deletion sequence eluting close to the main peak can be integrated separately or absorbed into it, and the difference between those two choices can be a full percentage point. This is why the chromatogram is worth more than the number extracted from it.
What to check: the wavelength and the method are stated, and the chromatogram is included rather than described.
4. Mass spectrometry — the field that answers a different question
HPLC answers how much of this material is one thing. Mass spectrometry answers what that thing is. They are separate questions and a certificate that addresses only the first has done half the job.
The instrument ionises the sample, measures mass-to-charge ratios, and the observed molecular mass is compared against the theoretical mass calculated from the intended sequence. A match within tolerance establishes identity. A mismatch of one residue's worth — anywhere from 57 Da for glycine to 186 Da for tryptophan — is a deletion sequence, and those are precisely the impurities chromatography is worst at separating.
A certificate should state both the theoretical and the observed mass. Only one of them is a result; the other is arithmetic. Showing both is what lets a reader check the comparison rather than take it on trust.
What to check: theoretical mass, observed mass, and that they agree.
5. Appearance
Usually dismissed as boilerplate — "white lyophilised powder" — and usually it is. But it is the one field a reader can verify without an instrument, and there are compounds where it carries real information.
GHK-Cu should be blue. It is a copper(II) complex and the colour comes from the coordinated metal. White powder sold as GHK-Cu is either the uncomplexed tripeptide or something else.
For everything else, appearance is a cake-condition report. A collapsed, shrunken or discoloured cake indicates a lyophilisation cycle that went wrong or a temperature excursion in storage — either of which has already affected the material regardless of what the purity figure says. See water content and lyophilisation quality.
6. Date of analysis
Two dates matter and certificates often show only one. The date of manufacture tells you how old the material is. The date of analysis tells you how old the data is. A certificate dated two years before the vial was filled describes the material as it was, not as it is.
Peptides degrade slowly when stored correctly, which is exactly what makes a stale certificate plausible enough to pass unnoticed.
7. What is not on a peptide COA
Worth being explicit, because absence is often read as a pass.
A standard peptide certificate says nothing about sterility, nothing about endotoxin content, and nothing about residual solvents unless those tests are listed on it. These are separate analyses using entirely different methods.
Nor does it make any statement about safety, suitability, or behaviour in any biological system. It is a chemistry document describing a chemical.
A practical order of operations
- Batch number on the certificate matches the vial. If not, stop.
- Sequence stated and correct.
- Observed mass matches theoretical mass.
- Purity figure accompanied by a chromatogram, a wavelength and a method.
- Appearance consistent with what you received.
- Analysis date close to the manufacture date.
Anything that fails at step one makes the rest moot, which is why it is step one.
What this catches, and what it does not
This process catches missing documentation, mismatched material, substituted compounds, stale data and the fabricated certificates that are common enough to be worth assuming — what forged COAs get wrong covers the tells.
It does not catch a competent forgery of a real document, and it does not substitute for an independent result from a laboratory with no interest in the sale. That is what third-party verification is for, and why it is worth understanding what the phrase does and does not mean when a vendor uses it.