Endotoxin and Heavy Metals Testing Explained

A peptide can post an excellent purity percentage and still carry contaminants that the purity number never sees. That is not a contradiction — it is a reminder that purity and contamination are measured by different methods. This guide covers the two screens that round out a serious research COA.

The gap purity leaves

HPLC purity reports the share of the target among detected organic peaks. Bacterial endotoxins and trace metals are different classes of material and are not what an HPLC purity method is set up to quantify. To know whether they are present, a lab runs dedicated assays.

Endotoxin and the LAL test

Endotoxins are lipopolysaccharide fragments from the outer membrane of gram-negative bacteria. They can be present even when a sample looks clean and are detected by the LAL test (Limulus Amebocyte Lysate), which reacts to endotoxin at very low concentrations. Endotoxin results are reported in endotoxin units, and a low result indicates a low bacterial-residue load in the sample as tested.

Heavy metals and ICP-MS

Heavy metals such as lead, arsenic, cadmium, and mercury can enter a sample from reagents, equipment, or starting materials. The standard method is ICP-MS (inductively coupled plasma mass spectrometry), which atomizes the sample and measures elements at trace concentrations. ICP-MS is sensitive and specific, making it the reference technique for elemental contaminants.

Residual solvents and counter-ions

Synthesis and purification leave traces behind. Residual solvents and counter-ions such as TFA (trifluoroacetic acid) can remain bound to a peptide. These also add mass without adding peptide, which is part of why purity and net peptide content differ — a point covered in what purity % means.

What a complete COA covers

The more of this table a COA fills in, the more complete the picture. For how these results are presented and verified, see how to read a COA and the public batch verification records.