What ≥99% Peptide Purity Actually Means on a COA

The number on the label

"≥99% purity" is the most common claim on research peptide product pages. It comes from a specific test — reversed-phase HPLC with UV detection at 214 nm — and it means one specific thing: the area under the main peak on the chromatogram is at least 99% of the total integrated area across all detected peaks.

What it measures

HPLC-UV at 214 nm detects the amide bond, so anything with peptide bonds shows up: the target peptide, truncated sequences (missing amino acids), oxidized forms, and side-chain byproducts. A 99.2% number means all impurities combined account for less than 0.8% of the detected material.

What it misses

HPLC-UV purity is not a total assay. It doesn't measure:

• Counterion mass. Peptides are typically shipped as TFA or acetate salts. The salt can account for 5–20% of the powder mass. A vial labeled "10 mg peptide" at 99% HPLC purity may contain 8–9 mg of net peptide.
• Water. Karl Fischer moisture content is reported separately.
• Non-peptide impurities that don't absorb at 214 nm — most solvents, some salts.
• Identity. HPLC tells you the material is pure. Mass spec tells you it's the right molecule.

That's why a complete COA reports HPLC purity, net peptide content, moisture, and mass spec identity — not just one number.

Reading the chromatogram

Look at the shape:

• Sharp, single main peak: clean manufacturing and purification.
• Main peak with a shoulder: possible diastereomer or deamidation product.
• Multiple peaks close in retention time: truncated sequences that co-elute; the 99% number may be optimistic if integration is loose.

Practical purity floors by application

• Structural / analytical work: ≥99%.
• Cell culture: ≥98% with low endotoxin.
• Early screening: ≥95% is often acceptable.

PX1's standard is HPLC ≥99% with LC-MS identity confirmation on every batch. See per-lot reports at /purity-reports.