What peptide purity really means.
Purity is the share of the peptide-containing material in your sample that is the correct peptide, usually measured as percent peak area on an HPLC chromatogram. It is not the share of the vial's mass that is peptide — water, salts, and counterions sit outside the purity number. That's why 99% purity and peptide content are different figures.
The definition
Purity answers one question: of everything in the sample that shows up in the analysis, how much is the target peptide? The industry standard is reversed-phase HPLC: the sample is separated into peaks, and purity is the percentage of total peak area belonging to the main peak.
What 99% purity tells you
- The dominant species in the sample is your peptide.
- Related impurities — truncated sequences, deletion products, oxidation products — are each small fractions.
- The synthesis and purification were competent.
What 99% purity does not tell you
- How much of the vial's mass is peptide. Water, residual solvents, and counterions like TFA or acetate are invisible to the purity number. A 99%-pure peptide can be 60–80% peptide by mass — the missing concept is peptide content.
- The identity of the other peaks. HPLC shows something is there, not what it is. LC-MS and identity testing answer that.
- Safety. Purity says nothing about endotoxins (endotoxin testing) or sterility (sterility testing).
How purity is actually measured
See HPLC explained for the full walkthrough — column, gradient, detector, chromatogram, area percent. The short version: the analytical result is only as trustworthy as the method, the instrument calibration, and the honesty of the lab behind it, which is why a COA's method and batch details matter as much as the headline number (how to read a COA).
Rules of thumb
- Purity ≠ content. Two different numbers; you need both for accurate concentration math.
- Compare like with like. Purity by HPLC area % is the standard; vendors quoting purity by other measures without saying so are a red flag.
- Higher is better but never perfect. 99%+ is standard for research-grade material; 99.9% claims deserve scrutiny rather than automatic trust.
Questions. answered.
What does 99% purity mean for a peptide?
It means about 99% of the peptide-containing material in the analyzed sample is the target peptide, by HPLC peak area. It says nothing about water, salts, or counterions, and nothing about endotoxin or sterility.
Is purity the same as content?
No. Purity is the composition of the peptide fraction; content is how much of the total mass is peptide at all. A vial can be 99% pure and 70% content.