Endotoxin testing and research peptides
What endotoxin is, how LAL measures it, when it matters for laboratory work, and why Alphex does not put an endotoxin line on its COA.
Endotoxin appears on some research peptide COAs and not on others, and the presence or absence of a line item is often read as a quality signal in itself. It is not that simple. This note explains what endotoxin is, how it is measured, when the result is relevant to laboratory work, why Alphex does not include an endotoxin assay on its standard COA, and what to do if your assay needs the figure. All Alphex products are for laboratory and in-vitro research only.
What endotoxin is
Endotoxins are lipopolysaccharides (LPS) from the outer membrane of Gram-negative bacteria. They are released when the bacteria die and lyse, they are heat-stable, they survive filter sterilisation and most chemical treatments, and they are pyrogenic and immunostimulatory at very low concentrations. Their relevance to a research peptide is indirect: the peptide itself does not contain endotoxin, but the water, reagents and equipment used in synthesis, purification and lyophilisation can introduce it, and it stays with the product.
How it is measured
The Limulus amebocyte lysate (LAL) assay, in gel-clot, turbidimetric or chromogenic formats, detects LPS by its activation of a clotting cascade from horseshoe crab blood; recombinant Factor C assays do the same without the animal source. Results are reported in endotoxin units (EU) per mg of peptide or per mL of solution. The assays are sensitive to interference from the sample itself (some peptides inhibit or enhance the reaction), so a validated method for each peptide is needed for a reliable number.
When it matters
- Cell-based assays using immune or endothelial cells, where LPS at picogram levels can trigger cytokine responses that swamp a peptide's effect.
- Any in-vitro system with Toll-like receptor 4 signalling.
- Animal-model work in licensed research settings where pyrogen load must be controlled.
When it does not matter: analytical work, receptor-binding assays without live immune cells, enzymatic assays, formulation and stability studies, and most chemistry. For a large fraction of research peptide use, endotoxin is irrelevant to the result.
What Alphex reports, and does not
The Alphex Certificate of Analysis reports HPLC purity at 220 nm against a reference standard, with the chromatogram, batch number and issuing laboratory. It does not include an endotoxin (LAL) assay or a water-content (Karl Fischer) determination, and Alphex does not claim to run either. We say this plainly because some suppliers list "endotoxin: passes" with no number, method or laboratory, and a line item that cannot be verified is worse than an honest omission. See what 99+% HPLC purity actually tells you for what the standard COA does and does not cover.
If your assay needs an endotoxin figure
- Ask us before ordering via the COA request form: where the testing laboratory has run LAL on a batch we will tell you, and where it has not we will say so.
- Test in-house or via a contract laboratory: chromogenic LAL kits are inexpensive and a validated method for a specific peptide is a half-day's work.
- Include the controls that make the question moot: a vehicle control processed identically, and where possible an LPS-neutralising control (polymyxin B) to show a peptide effect is not an endotoxin effect.
- Handle for low endotoxin: endotoxin-free water and tubes, since your own reagents can contribute more than the peptide.
Frequently asked questions
Does bacteriostatic water remove endotoxin?
No. Benzyl alcohol inhibits bacterial growth; it does not destroy LPS already present. See benzyl alcohol explained.
Does 99+% HPLC purity imply low endotoxin?
No. LPS does not absorb at 220 nm in a way HPLC purity would report. They are independent measurements.
Is a peptide with no endotoxin figure unsafe to use in cell culture?
It is untested for endotoxin, which is different. Test it, or design controls, if the cells are LPS-sensitive.
Sources and further reading
- ICH Quality Guidelines (Q1A stability, Q3A/Q3B impurities, Q6 specifications): the reference framework for purity, impurity and stability testing
Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.
Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.
Published 12 February 2026, last reviewed 16 August 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.