Net Peptide Content vs Purity: Why a COA Reports Two Different Numbers
For in vitro laboratory research use only. This article explains two analytical results. It makes no claims about the effects of any material.
A certificate of analysis for a peptide often shows two results that sound alike: purity and net content. They answer different questions, so they are rarely the same number. This guide explains what each one measures and why both are reported.
The short answer
- Purity answers: of the components the method can detect, what share is the main peptide?
- Net peptide content answers: of the material in the sample, how much is the peptide itself?
Purity is a ratio between peaks on a chart. Net content is about the whole sample, including things the purity method can’t see.
What purity measures
Peptide purity is usually measured by HPLC-UV. The instrument separates the sample into components, and a UV detector records each one as a peak. Purity is the area of the main peak as a percentage of all integrated peaks. Our HPLC purity explainer covers the number, and our guide to reading an HPLC chromatogram walks through the chart.
The key limit: the detector only sees components that absorb UV light at the chosen wavelength. Water and many salts don’t appear as peaks, so they aren’t part of the purity calculation.
What net peptide content measures
A lyophilized (freeze-dried) peptide powder is not 100% peptide by weight, even when its purity is high. Besides the peptide, the powder can contain:
- Water. Lyophilized material holds on to some moisture.
- Counter-ions. Peptides are commonly made and purified as salts. Trifluoroacetate (TFA) and acetate are common examples. These ions are paired with the peptide and add to the weight of the powder.
- Other non-peptide material, such as residual salts.
Net peptide content looks at how much of the material is the peptide itself. Labs measure it with methods such as amino acid analysis or nitrogen (elemental) analysis, and report it either as a percentage of the powder’s weight or as an amount, for example in milligrams.
A worked example
A simple example with made-up numbers: a sample shows 99% purity by HPLC-UV. Separately, the lab finds that 80% of the powder’s weight is peptide, and the rest is water and counter-ions.
- The 99% says that, among the UV-visible components, the main peptide makes up 99% of the peak area.
- The 80% says that the powder as a whole is 80% peptide by weight.
Both results can be correct at the same time, because they measure different things.
Why the two numbers don’t match
- Different denominators. Purity compares peaks with other peaks. Net content compares the peptide with the whole sample.
- Invisible material. Water and counter-ions lower net content but don’t show up in a purity chromatogram.
- Different methods. Purity and net content come from separate tests, each with its own method details.
Neither result tells you what the main component is. That’s the identity result, usually from LC-MS. See our LC-MS identity explainer.
Reading net content on a certificate
Check how the certificate states it. Some labs report a percentage of the powder’s weight. Others report a measured amount in milligrams. On Peptix certificates, net content is the amount the lab measured in the tested vial, as printed on the certificate. It can differ from the labeled amount stated on the product label.
As with every result on a COA, a certificate reports results for the sample tested. It is not a guarantee for other lots. For definitions of the other terms on a certificate, see our research peptide glossary.
Peptix certificates
The peptide lots with a published certificate on our peptide COA lookup page each have a third-party Certificate of Analysis showing the lot number, HPLC-UV purity and LC-MS identity. Where a certificate reports net content, the measured amount is printed on that certificate. For example, you can view the BPC-157 10 mg product with its lot certificate, or browse all research peptides.
For research use only. Not for human or animal use.
