bench_rat2026-08-20
Worth adding for other readers: endotoxin testing and sterility testing are separate analyses, and a certificate covering one says nothing about the other. This is the point that cost me a cell culture run.
What a Peptide COA Does Not Tell You
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Key findings
- A standard peptide certificate reports chromatographic purity and, sometimes, molecular identity. That is the whole of it.
- Sterility, endotoxin load, residual solvents, water content and biological potency are separate analyses on separate instruments.
- A silent certificate does not mean a clean sample. It means the question was never asked.
- A supplier that prints what was not tested is telling you more than one that prints only good numbers.
Certificates of analysis get read for the number at the top and rarely for the shape of the document. The shape is where the information is. A certificate records the tests someone chose to run, and for research peptides that choice is narrow: purity by HPLC, sometimes identity by mass spectrometry, and then the page ends.
Everything below is a test that page does not contain. None of these omissions implies a problem with the material. Each implies that if the answer matters to the work, it has to come from somewhere else.
Sterility
Purity is a chemical measurement. It counts molecules of the target against molecules of everything else, and a bacterium is not a molecule. A lyophilized powder can be 99% one peptide by HPLC and still carry viable organisms, because the instrument that produced the 99% was never looking for them.
Sterility testing is its own procedure, defined under USP General Chapter 71, and it takes days rather than minutes because it works by giving anything present a chance to grow. Research peptides are ordinarily sold non-sterile. That is not a scandal, it is the category. It starts to matter when material is destined for cell culture or an animal model, where an unnoticed organism ruins the experiment rather than the vial.
Endotoxin
Endotoxins are fragments of gram-negative bacterial cell wall, and they are awkward precisely because killing the bacteria does not remove them. A sample can be sterile and still carry enough endotoxin to produce an inflammatory response in an animal model or to distort cell viability in vitro, which surfaces as a confusing measurement rather than an obvious contamination.
The measurement is the LAL assay, reported in endotoxin units. It appears on almost no research-grade certificate. For in vitro work with sensitive lines, and for anything administered in an animal model, its absence is the most consequential silence on the page.
Residual solvents
Synthesis and purification run on organic solvents: acetonitrile, trifluoroacetic acid, methanol, dimethylformamide. Drying removes most of what was used. Traces stay, and TFA in particular tends to remain as a counterion rather than an impurity, which means part of the mass in the vial is not peptide at all.
Gas chromatography detects and quantifies what is left. When a certificate does mention solvents it often reports "ND" with no detection limit attached, which is a weaker statement than it looks: not detected by an instrument whose sensitivity goes unstated.
Water content
Lyophilized peptides pull moisture out of the air, so the powder in the vial is a mixture of peptide, water, and counterion. A certificate reporting purity says nothing about that ratio. The number on the label describes what is in the vial while the peptide fraction of it goes unmentioned.
Karl Fischer titration measures water directly and appears rarely. Reconstitution arithmetic that assumes the vial holds only peptide is arithmetic built on an unverified assumption.
Potency
Chemical purity and biological activity are different properties. A correctly built, highly pure peptide can be inactive because it folded wrong, degraded in storage, or carries a substitution mass spectrometry cannot see. Potency is established by an assay designed around one compound's mechanism, which is expensive, slow, and outside what an analytical chemistry lab offers alongside a purity report. Standard certificates do not carry it, and a purity figure is not a substitute.
Why the honest certificate says what it skipped
Two documents can report the same purity and communicate completely different things. One prints the number and stops. The other prints the number and adds that sterility and endotoxin were not performed. The second is more useful, and it is the harder document to publish, because it names a gap a competitor's certificate leaves unmentioned.
That asymmetry is worth watching for. A supplier willing to write down what it did not test is describing its own limits, and a supplier describing its limits is easier to believe on the parts it did measure. Silence reads as completeness right up until you look at the shape of the page.
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