The powder in a vial is not 100% peptide by weight, even when its HPLC purity is 99%. During purification and freeze-drying the peptide retains counter-ions (charged molecules that neutralise its charged groups) and some water. Those components do not show up in the ultraviolet chromatogram, but they do add weight.
That is why the peptide content of good-quality material is usually around 70%-90%, depending on the peptide. It is determined with techniques such as amino acid analysis or elemental nitrogen analysis.
The difference between purity and content is easy to see with an example: a material with 99% purity and 80% peptide content has very few peptide impurities, but every 10 mg of powder supplies about 8 mg of peptide.
In a laboratory that needs exact concentrations, ignoring peptide content leads to overestimating the real amount of peptide. It is the parameter that links the vial label to the concentration calculation.
Related terms
Learn it in depth at the Peptide University
- Peptide content, water, counterion and reading the COA · Module 7
- Related impurities and counterions · Module 7
Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).