How to read a certificate of analysis.
What each field on a peptide COA means, what an HPLC purity figure actually measures, what a certificate cannot tell anyone, and how to verify one independently.
A certificate of analysis is the single document standing between a buyer and a claim about what is in a vial. Almost every vendor in this market publishes one, and almost none explain how to read it — which makes the document easy to produce and hard to check.
This guide covers what each field means, what the purity number does and does not measure, the limits of the document itself, and how to confirm a certificate against the issuing laboratory rather than taking a vendor's image on trust.
The fields on a real report
A certificate identifies a specific batch tested on a specific date by a named laboratory. Every one of those elements matters, and a certificate missing any of them is describing nothing in particular.
Batch or lot number
Ties the certificate to one synthesis run. A certificate without a batch number cannot be matched to the vial in hand, which makes it decorative.
Test date
Establishes when the material was analysed. A purity figure describes the material on the day it was tested, not indefinitely afterwards.
Laboratory name
The party actually making the measurement. In-house testing is not third-party testing, and a report that does not name a laboratory is in-house testing by another name.
Task or report ID
The laboratory's own reference for the analysis. This is what makes independent verification possible, because it is the key the lab can look up.
Purity figure
Normally a percentage from HPLC. What it measures is narrower than it looks — see the next section.
Identity confirmation
Usually mass spectrometry, reporting observed molecular weight against the theoretical value for the sequence. This answers whether the right molecule is present, which is a different question from how much of it there is.
What an HPLC purity figure measures
High-performance liquid chromatography separates the components of a sample and detects them as they elute, producing a trace of peaks. Purity is calculated as the area of the target peak as a percentage of the total detected peak area, usually with ultraviolet detection in the region where the peptide bond absorbs.
That definition carries a consequence most vendors leave unstated: the figure is a proportion of what the detector saw, not a proportion of the vial's mass. Anything the detector does not respond to is absent from the denominator.
Lyophilised peptide is not pure peptide by weight. It carries residual water and a counterion left over from purification, commonly trifluoroacetate or acetate. A vial can be 99% pure by chromatographic area and still contain meaningfully less peptide by mass than the label figure implies. The measurement that resolves this is net peptide content, typically by amino acid analysis or nitrogen determination, and it appears on far fewer certificates than an HPLC number does.
Purity is relative, not absolute
It compares the target peak to other detected peaks. It is not a statement about how much of the powder is peptide.
Detection wavelength matters
Impurities that absorb weakly at the chosen wavelength contribute little peak area, so they depress the purity figure less than their actual quantity would suggest.
Gross weight is not net peptide
Counterion and residual water are part of the powder and are not peptide. A certificate reporting only chromatographic purity says nothing about that fraction.
Method conditions belong on the report
Column, gradient and wavelength determine what the run could resolve. A bare percentage with no method is an assertion rather than a measurement.
What a certificate cannot tell anyone
The most useful thing to understand about a COA is the boundary of its claim. It reports what one laboratory measured on one sample on one date. Several things people commonly read into a certificate are simply outside that scope.
Whether the tested batch is the batch received
The certificate describes the batch it names. Matching the batch number on the certificate to the batch on the vial is the buyer's step, not the laboratory's.
Sterility and endotoxin
HPLC and mass spectrometry are chemical analyses. Neither detects microbial contamination or bacterial endotoxin, which require separate tests that most peptide certificates do not include.
Stability after testing
A certificate is a snapshot. It says nothing about how the material was stored or handled between the test date and delivery.
Anything about the compound's effects
A certificate is an identity and purity document. It carries no information whatsoever about biological activity, safety, or whether published research supports any particular use.
Seven things that should prompt a second look
None of the following proves a certificate is fabricated. Each of them makes it unverifiable, which for practical purposes is a similar problem.
- 01
No laboratory named
A report that does not identify who performed the analysis cannot be checked with anyone.
- 02
No batch number, or a batch number that does not match the vial
The certificate then describes some material, but not demonstrably the material in hand.
- 03
No test date
An undated measurement cannot be placed relative to the synthesis or the shipment.
- 04
One certificate reused across many products
Each synthesis run is a separate batch. A single certificate covering a whole catalogue is describing at most one of them.
- 05
Purity quoted to implausible precision
Figures such as 99.98% imply a resolution that routine chromatographic integration does not deliver. Reports normally carry one or two decimal places at most.
- 06
An image with no verification route
A picture of a report is easy to alter. A report that can be looked up at the issuing laboratory is not.
- 07
Testing described as in-house
Self-testing may be honest and competent, but it is not independent, and it should not be presented as third-party verification.
Verifying a certificate independently
The point of a report ID is that it can be checked with the laboratory that issued it, without going through the vendor. Analytical labs serving this market commonly operate a lookup where a report or task number returns the original record.
Every batch we ship carries its certificate in the box, and the reports on this site are published with the issuing laboratory named alongside the batch and, where the lab provides one, its verification reference. Our analyses are performed by Janoshik Analytical and the Brown Institute of Biomolecular Research.
The step that matters is the one a vendor cannot do for a buyer: take the identifier from the certificate to the laboratory rather than accepting the vendor's copy of it.
- 01
Match the batch on the vial to the batch on the certificate
If they differ, the certificate is describing different material, whatever else it says.
- 02
Confirm the laboratory is a third party
Look up the named laboratory independently rather than through a link supplied by the seller.
- 03
Look the report up at source
Use the task or report identifier at the laboratory's own verification route, where one exists.
- 04
Read what was actually measured
Check whether the report covers identity as well as purity, and whether the chromatographic conditions are stated.
Common questions.
- Does a higher purity percentage always mean a better product?
Not necessarily, because the figure is a proportion of detected peak area rather than a proportion of the vial's mass. Two certificates reporting the same percentage can describe powders with different net peptide content, depending on residual water and counterion. A purity figure is most informative when the report also states the chromatographic conditions used to produce it.
- What is the difference between HPLC purity and net peptide content?
HPLC purity compares the target peak to the total detected peak area and describes how much of what was detected is the intended compound. Net peptide content describes what fraction of the powder by mass is peptide, with the remainder being counterion and residual water. They answer different questions, and a certificate reporting only the former says nothing about the latter.
- Can a certificate of analysis confirm a vial is sterile?
No. HPLC and mass spectrometry are chemical analyses that identify and quantify compounds. Neither detects microbial contamination or bacterial endotoxin, both of which require separate microbiological testing that peptide certificates do not normally include.
- Why do some certificates report a molecular weight that differs slightly from the theoretical value?
Mass spectrometry reports an observed mass that is compared against the value calculated from the sequence. Small differences arise from the instrument's mass accuracy and from which isotopic peak is being reported. A result within the method's stated tolerance is consistent with the correct molecule; a substantially different mass is not.
- Is in-house testing the same as third-party testing?
No. Third-party testing means the analysis was performed by a laboratory independent of the seller, and the report names that laboratory. In-house testing may be competent and honestly reported, but the party making the claim and the party checking it are the same, so it provides no independent confirmation.
- What does a report or task identifier on a certificate do?
It is the issuing laboratory's own reference for that analysis, and it is what makes independent confirmation possible. Where a laboratory operates a lookup, that identifier returns the original record directly from the lab rather than from the vendor's copy of it.