Peptide purity: what the percentage on a COA actually means
Short answer: the purity figure on a peptide COA is usually the share of the sample that shows up as the target peptide in an HPLC test. "98% purity" means the main peak takes up 98% of the detected peak area. It does not mean 98% of the powder in the vial is that peptide.
How is peptide purity tested?
The standard method is HPLC (high-performance liquid chromatography). The lab dissolves a sample and runs it through a column that separates the molecules in it. A UV detector draws a trace called the chromatogram, with one peak for each substance it picks up. Purity is the main peak's area as a percentage of all detected peak areas.
HPLC on its own doesn't confirm which molecule the main peak is. That's why a complete COA pairs it with mass spectrometry (MS), which checks that the molecular mass matches the expected compound. See how to read a COA line by line.
What a purity percentage leaves out
- Things the detector can't see. Water, salts and other material that don't absorb UV light don't appear as peaks, so they don't lower the purity figure.
- Net peptide content. Lyophilized peptide powder also contains bound water and counter-ions. How much of the powder's mass is peptide is a separate measurement, often reported as net peptide content, and it's normally lower than the HPLC purity. A COA that reports only purity hasn't told you this.
- The wrong compound. A sample can be "pure" by HPLC and still be the wrong molecule if identity wasn't checked by mass spectrometry.
- Other lots. A purity figure describes the lot that was tested, and nothing else. See why "same batch" isn't proof.
What purity should you look for?
Suppliers commonly advertise 98% or 99%. The number matters less than whether you can check it:
- the chromatogram is included, not just the percentage
- an identity test (MS) sits alongside it
- the lot number matches your vial
- the lab is independent and can confirm the report
A verified 97% tells you more than an unverifiable 99.9%.
How to test peptides for purity
To check a supplier's claim yourself, send a sealed sample to an independent analytical lab and ask for HPLC purity with the chromatogram, plus MS identity. Ask whether the method is within the lab's ISO/IEC 17025 accredited scope. Expect to provide the compound name and, if you have it, the expected molecular mass.
Where to get peptides tested for purity
Look for an independent lab, with no ties to the supplier, that runs HPLC and MS on peptides and can confirm its own reports. Before you rely on a lab, check its accreditation certificate and scope with the accreditation body that issued it. The supplier checklist lists the questions to ask whoever does the testing.