Nearly every research peptide is graded by HPLC purity. Understanding what that number describes, and what it does not, helps you compare suppliers honestly.
How HPLC works
High-performance liquid chromatography separates a sample into its components as they pass through a column. Each component appears as a peak. Purity is calculated from the area under the target peak relative to the total area of all peaks — so a purity of 99% means the target peptide accounts for 99% of the detected material.
Purity is not content
Purity and content answer different questions. Purity asks how much of what is present is the right molecule. Content asks how many milligrams are in the vial. A vial can be highly pure but under-filled, or correctly filled but impure. A complete COA reports both.
Reading the impurity profile
The remaining fraction is the impurity profile: related peptides, synthesis by-products, or residual solvents. For reproducible research, a tight and well-characterised impurity profile matters as much as the headline number.