How to Read a Peptide Analysis Certificate
In a nutshell. A certificate of analysis, or COA, documents what a batch actually contains. Four pieces of data are more important than the rest: HPLC chromatographic purity, identity confirmed by mass spectrometry, net peptide content, and the identity of the issuing laboratory. Without these four elements, the document proves nothing.
What is the purpose of a certificate of analysis?
A vial of peptide is a white powder. To the naked eye, there is no way to distinguish a batch that is 99% pure from a mixture containing 60% truncated chains. The certificate of analysis is the only document that transforms this powder into usable experimental data.
It answers three questions:
- Is this the correct molecule? This is the identity.
- In what proportion, relative to the other species present? This is the purity.
- How much is actually in the vial? This is the net peptide content.
These three questions are distinct. A batch may pass the first and second tests but fail the third, and your actual concentration will be off by 20% without any warning.
Section 1: Batch Identification
At the top of the document, you should find the name of the compound, its sequence or formula, the lot number, the date of analysis, and the quantity analyzed.
The lot number is the critical point. A certificate without a lot number is merely an advertising document: it is not tied to any identifiable production run, and there is no link between the vial you are holding and the analysis you are reading.
A single product must have as many certificates as there are batches produced. A seller who displays a single certificate for all sales over the past two years does not analyze their batches.
Section 2: Chromatographic Purity (HPLC)
This is the number everyone looks at—and the one that’s most often misinterpreted.
Reverse-phase high-performance liquid chromatography, RP-HPLC, separates the species present based on their hydrophobicity. Each species elutes from the column at its own retention time, and a UV detector measures the absorbance at the outlet, typically at 214 nanometers—the wavelength at which the peptide bond absorbs.
Purity is expressed as a percentage of area: the area of the main peak divided by the total area of all detected peaks.
Three things to check, in order:
- The chromatogram must be included. A reported percentage without the graph is unverifiable. The graph shows the main peak, its symmetry, and the secondary peaks.
- The conditions must be specified: column, gradient, mobile phase, wavelength. Without them, the figure is not reproducible.
- The baseline must be clean. A noisy chromatogram or obviously forced integration is no more reassuring than the absence of a measurement.
What HPLC purity does not tell us: It measures only those species that absorb UV light and elute from the column. Water, salts, and counterions are not included. This is precisely why the following section exists.
Section 3: Identity (Mass Spectrometry)
Purity indicates that a single species is dominant. It does not specify which one. Mass spectrometry answers that question.
The certificate must indicate the theoretical mass, calculated from the sequence, and the observed mass, as measured. The two must match within the precision of the method.
| Method | What it provides |
|---|---|
| ESI-MS | Molecular mass, often with multiple charge states |
| MALDI-TOF | Molecular mass, suitable for longer peptides |
| LC-MS | Chromatographic separation and mass in the same analysis |
An unexplained mass discrepancy is a major red flag: it may indicate an incorrect sequence, an unreported modification, or a compound that is simply not the one claimed.
Section 4: Net Peptide Content
This is the section most often missing, and the most important in practice.
A synthetic peptide is never pure peptide by mass. It is purified in the form of a salt, typically trifluoroacetate (TFA) or acetate, and it retains water. The mass of the vial is therefore the sum of the peptide, the counterions, and the residual water.
The net peptide content is the percentage of the total mass that is actually peptide. It typically ranges between 70 and 85% for a TFA salt.
A concrete example makes this clear:
| Vial A | Vial B | |
|---|---|---|
| Labeled mass | 10 mg | 10 mg |
| HPLC purity | 99% | 99% |
| Net peptide content | 82% | not measured |
| Peptide actually present | 8.2 mg | unknown |
Both vials are listed as 99% pure. One allows for the calculation of an accurate molar concentration; the other does not. Purity and net content are two different quantities, and confusing the two will skew all concentrations in an experiment.
Accepted measurement methods include elemental nitrogen analysis, amino acid analysis after hydrolysis, or quantitative NMR.
Section 5: Other Analyses
Depending on the laboratory’s standards, a comprehensive certificate may also include:
- Water content, by Karl Fischer titration
- Counterion content (acetate or TFA), by ion chromatography
- Bacterial endotoxins, by LAL test, expressed in endotoxin units per milligram
- Microbial load, or bioburden
- Heavy metals and elemental impurities, by ICP-MS
- Appearance and solubility
None of these analyses is mandatory for all applications. However, their inclusion indicates the rigor of the upstream quality control process.
Section 6: Who signed the document?
This is the crucial question—and often the only one people forget to ask.
A certificate issued by the seller itself is not independent evidence; it is a declaration. A certificate issued by a third-party laboratory, separate from the seller and the manufacturer, is independent evidence.
What to look for at the bottom of the document:
- The name and address of the laboratory that issued it
- Any accreditation, typically ISO/IEC 17025 for testing laboratories
- The date of analysis and the signature of the person in charge
Red Flags
A certificate with any of the following flaws proves nothing:
- No lot number, or a number that never changes from one lot to the next
- No chromatogram, only a percentage
- No analytical conditions: column, gradient, or wavelength missing
- No laboratory named, or a laboratory that turns out to be the seller
- A purity of 99.9% consistently claimed for all compounds, including those that are the most difficult to synthesize
- A poor-quality image, cropped, with no header or signature
- No net peptide content, even though the seller emphasizes its analytical rigor
Where can you view our certificates?
Every PeptidAs batch is analyzed by an independent third-party laboratory before being offered for sale. Identity is confirmed by LC-MS and purity is measured by HPLC-UV. The full reports are available on the certificates of analysis page, and the certificate for the corresponding batch is included with every order.
To understand what these molecules are before reading the analysis, see What Is a Peptide?. To prepare the vial once you receive it, see how to reconstitute a lyophilized peptide.
Frequently asked questions
What is the difference between purity and net peptide content?
HPLC purity is the proportion of the main peak among the peptide species detected by UV. The net peptide content is the portion of the vial's total mass that is actually peptide, with the remainder consisting of water and counterions. A batch may have 99% purity and 80% net content.
Is a certificate without a chromatogram valid?
No. A purity percentage without the chromatogram and without the analytical conditions is unverifiable. The chromatogram shows the symmetry of the main peak, the secondary peaks, and the quality of the baseline.
Why is the lot number important?
Because it is the only link between the bottle you are holding and the analysis you are reading. A certificate without a lot number—or one whose number never changes—cannot be linked to any identifiable production batch.
What does the TFA content listed on a certificate mean?
Trifluoroacetate is the counterion resulting from HPLC purification. It contributes to the total mass of the vial even though it is not part of the peptide, and it can interfere with certain cellular assays. Its concentration is therefore useful information, not a defect in itself.
Is an ISO 17025-accredited laboratory required?
It is not mandatory, but accreditation certifies that the laboratory has had its methods and competence validated by a third-party organization. What matters most is that the laboratory be independent of the seller and clearly identified on the document.
Sources cited
- International Council for Harmonization. ICH Q6A: Specifications—Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products, 1999.
- ISO/IEC 17025:2017. General requirements for the competence of calibration and testing laboratories.
- Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods in Molecular Biology, 2007;386:3-55.
Compounds mentioned in this article
The research products mentioned above.
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