What Are Endotoxins?

Endotoxins are lipopolysaccharides (LPS) found in the outer membrane of gram-negative bacteria. They are released when bacteria die and fragment. In research settings, endotoxin contamination most commonly occurs during peptide synthesis through contaminated reagents, water, or equipment. Endotoxins are extremely potent immune activators — nanogram-per-kilogram quantities can trigger fever, inflammation, and in high doses, septic shock.

Why HPLC Cannot Detect Them

HPLC separates and quantifies UV-absorbing organic compounds. Endotoxins (LPS) are large lipopolysaccharide aggregates with poor UV absorption at the wavelengths used for peptide analysis. They pass through HPLC completely undetected. A peptide with 99.5% HPLC purity can be heavily contaminated with endotoxins — the purity test simply has no visibility into this class of contaminants. This is not a flaw in HPLC; it was designed to analyze organic molecules, not pyrogenic lipopolysaccharides.

The LAL Assay

The Limulus Amebocyte Lysate (LAL) assay is the gold standard for endotoxin detection. It uses lysate derived from horseshoe crab (Limulus polyphemus) blood cells, which clot in the presence of endotoxins. The reaction rate is proportional to endotoxin concentration, allowing quantification in endotoxin units (EU/mL or EU/mg). The FDA requires LAL testing for all injectable pharmaceutical products. Most peptide vendors do not perform this test because it requires specialized equipment and adds cost.

Why This Matters for Research Integrity

A researcher injecting a peptide contaminated with endotoxins will observe immune responses — fever, lethargy, increased inflammatory markers — that have nothing to do with the peptide being studied. If the researcher does not know the endotoxin status of their compound, these effects will be misattributed to the peptide, generating false conclusions. Endotoxin contamination is one of the most underappreciated sources of confounding error in animal peptide research.

Valence Grade Endotoxin Standard

Every batch we produce is tested for endotoxin content using the LAL assay through our ISO 17025-accredited testing partner. We publish the endotoxin result on the batch COA alongside HPLC and MS data. Our standard is <1 EU/mg per batch — consistent with pharmaceutical injectables and more stringent than any other research peptide vendor we are aware of.