What endotoxin is
Endotoxin is lipopolysaccharide (LPS), a structural component of the outer membrane of gram-negative bacteria. It is shed during bacterial growth and death and is remarkably stable — it survives many sterilization steps that kill the bacteria themselves. Because it can be introduced through water, glassware, or handling anywhere in synthesis and fill, endotoxin is tracked as its own contamination parameter rather than assumed away.
How it is measured: the LAL assay
The standard method is the Limulus Amebocyte Lysate (LAL) assay, which uses a reagent derived from horseshoe crab blood that reacts specifically to endotoxin. The result is reported as a concentration in endotoxin units per milligram (EU/mg) or per milliliter (EU/mL). A lower number means less bacterial contamination; a stated acceptance threshold on the Certificate of Analysis is what makes the number meaningful.
Why HPLC and mass spectrometry miss it
This is the key point most buyers overlook: purity and identity assays do not see endotoxin. HPLC reports what fraction of the sample is the target compound; mass spectrometry confirms the molecular weight matches the intended peptide. A reagent can be 99% pure, correctly identified, and still carry an endotoxin load that neither method registers. Only a dedicated LAL test on the panel reveals it.
| Assay | What it answers | Catches endotoxin? |
|---|---|---|
| HPLC | How pure is the sample? | No |
| Mass spectrometry | Is it the right molecule? | No |
| LAL (endotoxin) | Is there gram-negative bacterial contamination? | Yes |
| Heavy metals | Are metal residues within limits? | No |
| Sterility | Is there viable microbial growth? | No (different question) |
Why it matters for research
Endotoxin is a potent activator of immune and inflammatory signaling. In cell culture it can trigger cytokine release, activate macrophages and Toll-like receptor pathways, and shift baseline behavior of the very systems under study. In vivo it can provoke inflammatory responses unrelated to the compound being investigated. The practical consequence is irreproducibility: uncontrolled endotoxin is a well-documented hidden variable behind experiments that will not replicate. Screening for it removes one of the most common confounders before it reaches the bench.
Where it sits on the Ethos Bio panel
Every Ethos Bio lot is independently verified by MZ Biolabs, a third-party analytical laboratory in Arizona, on a five-point panel: RP-HPLC purity, HPLC-MS identity, endotoxin (LAL), heavy metals, and sterility. Endotoxin is not an optional add-on — it is one of the five points, and the result travels on the Certificate of Analysis that ships with the vial.
See the full panel
Read how the five-point methodology works end to end, then browse the public COA library to see endotoxin results reported per lot.
Frequently asked questions
What does endotoxin testing detect?
It detects lipopolysaccharide (LPS) from the outer membrane of gram-negative bacteria. It is a contamination measure, not a purity or identity measure — HPLC and mass spectrometry cannot see endotoxin, which is why a complete panel tests for it separately.
How is endotoxin measured?
The standard method is the Limulus Amebocyte Lysate (LAL) assay, which reacts to endotoxin and yields a concentration reported in endotoxin units per milligram (EU/mg) or per milliliter (EU/mL). Lower values indicate less bacterial contamination.
Why does endotoxin matter in research?
Endotoxin is a potent activator of immune and inflammatory pathways in vitro and in vivo. Even trace amounts can trigger cytokine release and confound experimental results, so uncontrolled endotoxin is a common hidden source of irreproducible data.
Do HPLC and mass spectrometry catch endotoxin?
No. HPLC measures purity and mass spectrometry confirms identity, but neither detects bacterial endotoxin. A peptide can be high-purity and correctly identified yet still carry endotoxin — only a dedicated LAL test on the panel would reveal it.