Endotoxin and Sterility Checks on Research Peptides
Endotoxin is a frequent and easily missed reason that peptide experiments produce unreliable data, above all in cell culture. HPLC can rate a peptide 99% pure while it still holds enough endotoxin to distort a result.
This guide covers what endotoxins are, why they matter, how laboratories measure them, and how to interpret the endotoxin and sterility lines you will see on a certificate.
On this page
Endotoxins in brief
Endotoxins are lipopolysaccharides (LPS), large molecules found in the outer membrane of Gram-negative bacteria. They are set free as the bacteria die and break up. The key point is that endotoxins are heat-stable and chemically robust. Killing the bacteria, or filtering them out, leaves the endotoxin behind.
That is how a sample can be sterile, with nothing living in it, and still carry endotoxin. These are two different problems, and each one needs its own test.
Why they matter in the lab
Endotoxins strongly activate the immune system. During cell-culture and immunology work, tiny amounts can set off cytokine release, switch on signalling pathways and move experimental readouts. A contaminated peptide therefore brings in a variable unrelated to the compound under study, and nobody can see it without a test. In sensitive work, ruling out endotoxin ranks among the most important checks you can make.
Measuring endotoxin: the LAL assay
The standard method is the LAL assay (Limulus Amebocyte Lysate). Its reagent reacts when endotoxin is present. Results are given in endotoxin units per milligram (EU/mg). Lower values are better. An animal-free version based on recombinant Factor C is gaining ground; it measures the same thing.
Sterility testing
Sterility testing is a different check. It looks for viable bacteria and fungi and is normally reported as a pass or a fail. Read side by side, the sterility and endotoxin results describe microbial quality far better than either does alone.
Finding the results on a CoA
On the certificate, look for two lines: an endotoxin value in EU/mg and a sterility pass or fail. A smaller EU/mg value means cleaner material. A report that carries both protects your data better than one that lists purity and nothing else. For the rest of the document, see our guide to reading a peptide Certificate of Analysis.
Endotoxin data at NordPeps
Certificates from an independent third-party laboratory are published per batch on our certificates page as they become available. Where a certificate includes endotoxin or sterility results, you will find them there, listed with the batch they cover.
Common questions
Sterility testing and endotoxin testing: how do they differ?
A sterility test looks for living bacteria and fungi. An endotoxin test measures bacterial toxins (lipopolysaccharides), which survive heat and can persist after the bacteria are dead. A sterile sample may still contain endotoxin, so both tests are needed.
What is EU/mg?
Endotoxin units per milligram. It is the figure produced by the LAL assay and quantifies endotoxin contamination. A lower number means cleaner material.
If a peptide is 99% pure, why worry about endotoxin?
HPLC purity looks at the peptide fraction. It does not detect microbial contaminants. A 99%-pure peptide can still carry endotoxin, and that endotoxin alone can distort cell-culture and immunology experiments.