Short answer: “≥99% purity” means that at least 99% of the material detected by the purity method (usually HPLC with UV detection) was the target peptide, and at most 1% was detected impurities. It is a ratio of detected peaks, not a measure of weight, identity or sterility.
What the number measures
On an HPLC chromatogram, the main peak’s area is compared with the total area of all detected peaks. A 99.7% result means the other detected components together made up 0.3% of the signal.
What counts as an impurity
Typical peptide impurities include truncated or deletion sequences, incompletely deprotected sequences and oxidized forms that arise during synthesis or storage.
What it does not mean
- It does not mean the vial contains 99% peptide by weight. Lyophilized peptides also contain water and counter-ions, which are not counted in HPLC purity.
- It does not confirm identity. A pure sample of the wrong compound would still show one clean peak.
- It does not mean the product is sterile or endotoxin-free. Those need separate tests.
Specification vs. result
A COA may show a specification such as “NLT 98%” and a result such as “99.8%”. The specification is the minimum acceptable; the result is what was measured for that sample.
Is 99% always better than 98%?
A higher result is generally preferable, but small differences between labs can come from method differences. Consistent results across batches, from an independent lab, matter more than a single number.
Related: how peptide purity testing works and identity vs. purity testing.
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