What does HPLC measure?
High-performance liquid chromatography pushes a dissolved sample through a column packed with a separation medium. Different components travel at different speeds and emerge — elute — at different times, producing a chromatogram of peaks. The target compound forms one peak; related impurities form others.
Purity is calculated from the area of the target peak relative to the total area of all peaks. Reported as a percentage, this is the figure that appears as the purity result on a Certificate of Analysis.
What does net peptide content measure?
Net peptide content describes how much of a vial's dry mass is actually peptide. A lyophilized powder also carries water and counter-ions from the salt form the peptide was isolated as, and those add mass without being peptide.
Quantifying net peptide content gives the peptide mass per vial in milligrams, read against the amount stated on the label. It answers a different question from purity: not what fraction of the sample is the target, but how much peptide the vial actually holds.
How do purity and net peptide content differ?
The two measurements answer different questions and are read differently. The table summarizes the distinction.
| Purity (HPLC) | Net peptide content | |
|---|---|---|
| Question answered | How pure is the sample? | How much peptide is in the vial? |
| Property measured | Relative amount of each component | Peptide mass as a share of dry weight |
| Typical readout | Purity as a percentage (peak area) | Peptide mass in milligrams per vial |
| What it detects | Related impurities and their proportion | Water, counter-ions, and salt in the dry mass |
Why do both measurements matter?
A sample can be highly pure yet contain only a small mass of peptide, and a vial can hold ample mass yet carry more impurities. Purity without net content, or net content without purity, leaves one of those questions open.
Reporting both closes the gap: the purity percentage establishes that the sample is predominantly one component, and the net peptide content establishes how much peptide the vial actually contains. This is why every Certificate of Analysis reports both an HPLC purity result and the net peptide content.