A certificate of analysis is the only document standing between a claim about what is in a vial and the actual contents of that vial. Every reputable supplier of research compounds provides one. Very few buyers know how to read it, which is unfortunate, because a COA is where the difference between a well-characterized material and an expensive unknown becomes visible.
This article explains what each section of a peptide COA measures, what it deliberately does not measure, and how the figures relate to one another. The recurring theme is that no single number on the document is sufficient on its own — purity, identity, and content are three separate questions answered by three separate methods, and a certificate reporting only one of them has left the other two open.
What a Certificate of Analysis Is
A COA is a document recording the analytical testing performed on a specific manufactured batch. The operative word is batch. A COA describes one production lot, tested once, and is valid only for material from that lot. A certificate with no batch or lot number is not a certificate of analysis — it is marketing material formatted to resemble one.
A complete COA identifies the compound, the batch, the date of analysis, the testing laboratory, the methods used, the specification for each test, and the result obtained. Where the analysis was performed by an independent laboratory rather than the manufacturer, the document typically carries that laboratory letterhead and a report number traceable to their own records.
HPLC Purity: What It Measures and What It Excludes
Reversed-phase high-performance liquid chromatography is the standard method for assessing peptide purity, and the number it produces — "98.7%", "99.2%" — is the figure most prominently displayed on most certificates.
The method separates components of a sample by hydrophobicity as they pass through a column, producing a chromatogram in which each component appears as a peak. The purity figure is the area of the main peak expressed as a percentage of total peak area, usually with ultraviolet detection at 214 nm (where the peptide bond absorbs) or 280 nm (where aromatic residues absorb) [2].
Understanding what that percentage excludes is more important than understanding what it includes.
It is a relative measure, not an absolute one. The figure says the main peak represents 98.7% of the material the detector saw. Anything that does not absorb at the detection wavelength is invisible to the calculation. Salts, most counterions, and water contribute no peak and are therefore excluded from the denominator entirely.
It measures homogeneity, not identity. A 99% pure sample of the wrong peptide is 99% pure. HPLC establishes that the material is predominantly one substance; it does not establish which substance. This is why identity confirmation is a separate test.
It depends on the method. Purity is measured under specific gradient, column, and detection conditions. A poorly resolved method can co-elute an impurity underneath the main peak and report a higher purity than a well-resolved method would on the same material. This is why a COA should state the method, and why an unusually high purity figure reported without method details deserves more skepticism than a slightly lower figure reported with them.
Reading the chromatogram itself, when one is provided, is worth the effort. A clean profile shows a sharp, symmetric main peak with a flat baseline. Shoulders on the main peak suggest closely related impurities — often deletion sequences missing a residue, or oxidized variants. A cluster of small peaks eluting early typically indicates truncated fragments or residual synthesis reagents.
Mass Spectrometry: Identity, Not Purity
Mass spectrometry answers the question HPLC cannot: is this the intended molecule?
The instrument ionizes the sample and measures mass-to-charge ratios, producing an observed molecular weight compared against the theoretical weight calculated from the intended sequence. Agreement within the method tolerance confirms that the mass of the material matches the target.
Two limitations are worth knowing. First, mass confirms composition but not necessarily sequence order — two peptides with the same amino acids in different arrangements share a molecular weight. In practice, for a peptide made by a defined stepwise synthesis, mass agreement is strong evidence of identity, but tandem MS or sequencing is the rigorous confirmation where it matters. Second, an observed mass differing from theoretical by a characteristic increment often indicates a specific modification rather than a wrong compound: +16 Da suggests oxidation, typically of methionine; +1 Da suggests deamidation.
A COA reporting HPLC purity but no mass spectrometry has documented that the material is homogeneous without documenting what it is. For structurally similar compounds this gap is significant. Tirzepatide and Retatrutide are both 39-residue peptides built on GIP-derived backbones with fatty acid modifications — similar enough that a purity figure alone does not distinguish them.
Net Peptide Content: The Number That Changes Your Math
If HPLC purity is the most displayed figure, net peptide content is the most consequential one — and it is frequently absent.
Net peptide content is the percentage of the lyophilized solid that is actually peptide. The remainder consists of counterion salt, residual water, and any excipients. It is determined by quantitative amino acid analysis, in which the sample is hydrolyzed to free amino acids and quantified against standards, or by quantitative nitrogen analysis [1].
The reason this matters: peptides purified by reversed-phase HPLC typically use trifluoroacetic acid as the ion-pairing agent, and the peptide is isolated as its TFA salt. Each basic residue — lysine, arginine, histidine, plus the free N-terminus — can carry an associated counterion. For a sequence with several basic residues, TFA salt commonly accounts for a substantial fraction of total mass, with net peptide content for TFA-salt peptides often falling in the 70–90% range depending on composition. Acetate-salt peptides generally run higher, because acetate is a smaller counterion.
The practical consequence is direct. A vial labeled 5 mg with a net peptide content of 80% contains approximately 4 mg of peptide. Every concentration calculated from the label weight is 20% high, and that error propagates silently through every downstream measurement. Two vials from different suppliers, both labeled 5 mg and both reporting 99% HPLC purity, can differ by 20% in actual peptide delivered — and nothing on a purity-only certificate would reveal it.
This is the single most useful thing to look for on a COA, and its absence is the most useful thing to notice.
Water Content, Counterion, and Residual Solvents
Water content is measured by Karl Fischer titration. Lyophilized peptides are hygroscopic, and residual moisture both adds mass and accelerates degradation in storage [3,4]. Typical specifications fall in the low single-digit percentages.
Counterion content identifies and quantifies the salt form — TFA or acetate. Beyond its effect on net peptide content, the counterion is relevant to certain experimental systems: residual TFA has been reported to inhibit proliferation in osteoblast and chondrocyte cultures at low concentrations, which is why some cell-based applications specify acetate-exchanged material [5].
Residual solvents from synthesis and purification — acetonitrile, DMF, methanol, diethyl ether — are quantified by gas chromatography where the application of the material warrants it.
Appearance is a simple visual specification, typically "white to off-white lyophilized powder." It is worth reading because it gives you something to check against. Material that does not match its own appearance specification on arrival is a question worth raising before anything else is done with it.
Sterility and endotoxin testing appears where relevant to the application. For general research material these are often not performed, and their absence is not by itself a red flag — but a certificate claiming sterility without a corresponding test result is.
Third-Party Versus In-House Testing
Manufacturers test their own output as a matter of routine process control, and in-house data from a competent manufacturer is real data. The reason independent third-party testing carries additional weight is structural rather than technical: the laboratory performing the analysis has no commercial stake in the result.
A third-party report should identify the testing laboratory by name, carry its own report identifier, state the date the sample was received and analyzed, and describe the methods used. Reports meeting that description can be verified independently. Reports that cannot be traced back to an identifiable laboratory cannot.
Red Flags
The following patterns are common and each indicates a specific problem.
- No batch or lot number. The certificate cannot be tied to the material in hand. This is disqualifying.
- No date. An undated analysis could predate the batch it purports to describe.
- The same certificate across multiple products. A COA describes one compound and one lot. Reuse indicates the document is decorative.
- Purity with no identity test. Homogeneous material of unconfirmed identity.
- No net peptide content. Concentration calculations rest on label weight alone.
- No named testing laboratory. The analysis is unattributable and unverifiable.
- A results column with no specification column. A number with no acceptance criterion cannot pass or fail.
- Implausibly perfect figures. Purity above 99.5% across an entire product range is unusual in ordinary manufacturing and warrants method details.
- A low-resolution image with no chromatogram. The summary table is the conclusion; the chromatogram is the evidence.
- Results that do not match the stated specification. This occurs more often than expected, and it indicates that nobody is reading these documents — which is generally the assumption they are produced under.
Matching a Certificate to Your Vial
The document is only useful if it corresponds to the material you have.
Find the lot or batch number printed on the vial and confirm it matches the certificate. Confirm the compound name and the stated quantity. Confirm that the analysis date on the certificate is consistent with the manufacture date of the batch. Then check the material against the appearance specification.
Where a supplier publishes certificates by batch — as we do on our certificates of analysis page — this check takes under a minute. Where a supplier provides a certificate only on request, or provides a single generic document per product rather than per lot, the check cannot be completed at all, and that is itself the finding.
Why This Is Worth Learning
Peptide research depends on knowing what is in the vial. A well-designed experiment interrogating the wrong compound, or the right compound at 20% below the intended concentration, produces data that looks legitimate and means nothing. The failure is invisible in the results — it is only visible in the documentation, and only to someone reading it.
Learning to read a COA takes an afternoon. It is the highest-leverage skill in sourcing research materials, and it is the one that suppliers with weak documentation are relying on buyers not to acquire.
Related reading: reconstituting lyophilized peptides covers how net peptide content feeds into concentration calculations, and our introduction to peptide structure covers the chemistry behind the degradation pathways these tests are designed to detect.
References
- Bachem AG. Quality control of amino acids and peptides: analytical methods, purity determination, and net peptide content. Bachem Knowledge Center.
- Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods in Molecular Biology. 2007;386:3–55.
- Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharmaceutical Research. 2010;27(4):544–575.
- Wang W. Lyophilization and development of solid protein pharmaceuticals. International Journal of Pharmaceutics. 2000;203(1–2):1–60.
- Cornish J, Callon KE, Lin CQ, et al. Trifluoroacetate, a contaminant in purified proteins, inhibits proliferation of osteoblasts and chondrocytes. American Journal of Physiology — Endocrinology and Metabolism. 1999;277(5):E779–E783.
- Lau JL, Dunn MK. Therapeutic peptides: historical perspectives, current development trends, and future directions. Bioorganic & Medicinal Chemistry. 2018;26(10):2700–2707.
- Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. Journal of the American Chemical Society. 1963;85(14):2149–2154.
All compounds referenced are intended strictly for laboratory research use only. They are not for human or veterinary consumption, diagnostic use, or therapeutic application.