What “≥99% purity” means, and what it doesn't
“≥99% purity” is the headline number on most research peptide listings. Here is what it actually measures.
How the number is calculated
Purity is almost always measured by HPLC. It is the area of the target peptide's peak as a share of the total area of all peaks the detector sees. A result of 99.3% means 99.3% of the detected peptide-related material is the target sequence.
What the other 1% usually is
Peptides are built one amino acid at a time, and each step is not perfectly efficient. Common impurities are:
- Deletion sequences: the target with one amino acid missing.
- Truncated sequences: chains that stopped growing early.
- Incompletely deprotected chains: the right sequence with a leftover chemical protecting group.
- Oxidized or deamidated forms that appear during handling or storage.
Purity is not net peptide content
A vial labeled 10 mg contains 10 mg of lyophilized powder, and that powder also holds residual water and counter-ions (such as acetate or trifluoroacetate) from purification. Net peptide content, the share of the powder's weight that is actually peptide, is a separate measurement. A 99% pure peptide can have a net peptide content well below 99%.
Why it matters
In research, impurities are uncontrolled variables. Closely related sequences can behave differently from the target in an assay, and the impurity profile can vary from batch to batch. Higher purity, documented by an independent lab and tied to a lot number, makes results easier to reproduce.