Most buyers know to look for an HPLC purity figure. Far fewer ask about endotoxins — even though an endotoxin result answers a question that purity simply can’t. This post explains what endotoxins are, how they’re measured, how they appear (or don’t) in the documentation of our top-ranked shops, and exactly what to ask for.
What endotoxins are
Endotoxins are fragments of the outer membrane of certain bacteria — chemically, lipopolysaccharides (LPS). They are released when those bacteria die or break apart, and they are remarkably stable: they survive many of the steps that would remove the bacteria themselves.
That stability is the reason they matter in a lab. Endotoxin contamination can interfere with experiments — particularly cell-based work — even when a material is otherwise very pure. A peptide can show 99% on HPLC and still carry endotoxin, because HPLC isn’t built to see it.
How endotoxins are measured
Results are reported in endotoxin units (EU), usually per milligram of material (EU/mg). The common methods:
| Method | How it works | Notes |
|---|---|---|
| LAL gel-clot | A reagent from horseshoe-crab blood forms a gel when endotoxin is present above a threshold | Simple pass/fail against a limit |
| LAL kinetic turbidimetric | Measures how quickly the reaction mixture turns cloudy | Gives a number, not just pass/fail |
| LAL kinetic chromogenic | Measures a colour change over time | Gives a number; widely used |
| Recombinant factor C (rFC) | Uses a lab-made version of the reacting protein | No animal-derived reagent |
For a buyer, the details of the chemistry matter less than three facts: which method was used, what the result was, and what limit it was compared against. A certificate that only says “endotoxin: pass” without a number or limit tells you little.
Endotoxin vs. sterility vs. purity
These three are often confused, so it’s worth separating them:
- Purity (HPLC): how much of the detected material is the target compound.
- Sterility: whether living microorganisms are present.
- Endotoxin: whether bacterial membrane fragments are present — whether or not any bacteria are still alive.
A sterile sample can still contain endotoxin. A pure sample can still contain endotoxin. Each needs its own test.
What the top three publish today
We read the public documentation of all three:
- Nexo Research: no lab reports published; endotoxin testing not mentioned.
- Europa-Peptide: no public lab reports found; endotoxin testing not mentioned.
- Peptid24: states purity of at least 98% for its lyophilized products; no lab reports found; endotoxin testing not mentioned.
On our spec sheets, all three “Lab testing” fields show a dash — “not published”. That doesn’t mean the shops don’t test; it means they don’t say. Even a stated test panel is still a claim until you see the certificate.
What to ask a shop for
If endotoxin data matters for your work, here’s the five-point request we’d send to any shop:
- “Is there an endotoxin result for batch [number]?” Always tie it to the batch you’ll receive.
- “Which method — gel-clot, kinetic LAL or rFC?” This tells you whether you’ll get a number or just a pass/fail.
- “What is the result in EU/mg, and what limit was applied?” A number plus a stated limit is the useful form.
- “Which lab ran it, and can I verify the certificate?” Independent labs sometimes provide a lookup key.
- “When was it tested?” A recent date on the same batch is what you want.
Write the answers down. If a shop can’t answer, that’s data too — and it’s exactly the kind of gap our spec sheets are designed to show.
How this affects our scores
Stated endotoxin testing, and even more so a published batch-level endotoxin result, counts toward the purity & testing criterion — the heaviest in our formula at 30%. Today none of the three publish endotoxin data, so their purity estimates (8.6 for Nexo Research and Europa-Peptide, 8.3 for Peptid24) are based on the information they do publish. If one of them starts publishing endotoxin results, we’ll update its spec sheet and re-estimate.
As with everything on VialScore: the products discussed are sold for laboratory research use only. This post explains a laboratory test and how to request documentation; it is not medical advice.
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