Quality · 6 min read
Endotoxin Testing (LAL) Explained for Research Peptide Buyers
What bacterial endotoxin is, how LAL and recombinant factor C tests work, how results are reported in EU/mg, and why endotoxin matters for in-vitro assays.
Endotoxins are lipopolysaccharides from the outer membrane of Gram-negative bacteria. They survive lyophilization and ordinary heat, and even trace amounts can trigger strong responses in immune-cell and other in-vitro assays. For a research reagent, an unreported endotoxin level is an uncontrolled variable.
How endotoxin gets into peptides
The usual sources are water, buffers, glassware, and equipment used during synthesis and purification. Controlled manufacturing and depyrogenated containers keep levels low; testing confirms it.
The LAL test
The Limulus amebocyte lysate (LAL) test uses a clotting cascade from horseshoe crab blood cells that reacts to endotoxin. Three formats are common:
- Gel-clot: a pass/fail limit test based on whether a firm clot forms
- Turbidimetric: measures the rise in turbidity over time
- Chromogenic: measures color released from a synthetic substrate
Recombinant factor C (rFC) assays reproduce the first step of the cascade without animal-derived lysate and are increasingly accepted as an alternative method.
How results are reported
Results are expressed in endotoxin units (EU), normalized to mass, for example EU/mg. A useful certificate states the method, the result, and the specification limit side by side. "Pass" without a number or limit is not enough to assess suitability for a sensitive assay.
Interference and controls
Some peptides enhance or inhibit the LAL reaction. Labs validate each product with a positive product control, a spike of known endotoxin, to confirm the result is not masked.
Endotoxin is one of the eight assays on the North Wall release panel. It is especially relevant for peptides used in immune-cell assays such as Thymosin Alpha-1 and LL-37. Also see how to read a COA.
Educational content for laboratory professionals. North Wall products are for in-vitro research only and are not for human or veterinary use.