Net peptide content, explained
How much peptide is really in the vial, and why it is a different number from purity.
Net peptide content is the actual mass of peptide in a vial, not counting water, counter-ions such as acetate or TFA, and other non-peptide material. It is measured on its own, apart from HPLC purity, and a COA usually reports it in milligrams next to the label claim.
11.08 mg measured against a 10 mg label
Is net peptide content the same as purity?
No. They answer two different questions about the same vial.
| Chromatographic purity | Net peptide content | |
|---|---|---|
| Question it answers | How clean is the peptide that is there? | How much peptide is there? |
| How it is reported | Percentage of detected peak area | Milligrams per vial, or percent of powder weight |
| Typical method | HPLC-UV | Quantitative HPLC, amino acid analysis or nitrogen analysis |
| What it ignores | Water, salts and counter-ions | Which impurities are present |
A powder can be 99% pure by HPLC and still carry part of its weight as water and counter-ions. That is why purity and net content sit on separate lines of a COA.
What lowers net peptide content?
Counter-ions
Peptides are usually supplied as a salt, paired with acetate or TFA from synthesis and purification. The counter-ion is weighed with the powder but is not peptide. Counter-ion, defined.
Water
Freeze-dried peptides still hold some water, and most pull more from the air if a vial is left open. Why peptides are hygroscopic.
Filling and weighing
Small differences in filling each vial move the real figure slightly above or below the label claim.
How do you read net content on a COA?
- 1
Find the label claim
The amount printed on the vial, such as 10 mg. It is the target the vial was filled to.
- 2
Find the measured figure
The net content line shows what the lab actually measured, usually in milligrams.
On a real report: 11.08 mg measured, against a 10 mg label claim (lot SK10). - 3
Check the method
The COA should name how net content was measured, for example quantitative HPLC. A number with no method is hard to compare.
On a real report: net content by HPLC-UV. - 4
Read it against its specification
Some reports set a criterion, such as “at or above label claim”. Others simply report the figure. Either way, the number should be on the page.
Published lots show measured net content wherever the lab reports it. Browse the COA library or read a full COA in 5 checks.
How is net peptide content measured?
Quantitative HPLC
The sample’s main peak is compared with a reference standard of known amount, which converts peak area into milligrams.
Amino acid analysis
The peptide is broken into its amino acids, which are counted to work back to the amount of peptide.
Elemental nitrogen analysis
The nitrogen in the sample is measured and converted to peptide mass using the sequence.
Each has its own strengths. What matters on a COA is that the method is named and the figure is reported as measured. See the full peptide glossary for the other terms on a report.
Questions about net peptide content
What is net peptide content?
Net peptide content is the actual mass of peptide in a vial or sample, not counting water, counter-ions such as acetate or TFA, and other non-peptide material. A COA usually reports it in milligrams next to the label claim, or as a percentage of the powder weight.
Is net peptide content the same as purity?
No. Purity, measured by HPLC, is the share of detected material that is the target peptide. Net peptide content is how much peptide is actually in the vial. A sample can be 99% pure by HPLC and still carry part of its weight as water and counter-ions.
Why is net peptide content different from the label claim?
The label claim is the amount the vial is filled to hold. Net peptide content is what a lab measured after the fact. Filling, weighing and the salt and water carried by the powder all move the real figure slightly above or below the label.
How is net peptide content measured?
Common methods are quantitative HPLC against a reference standard, amino acid analysis and elemental nitrogen analysis. The COA should name the method used next to the result.
Can you calculate net peptide content from purity?
No. Multiplying HPLC purity by the label weight does not give net peptide content, because purity ignores water and counter-ions. Net content has to be measured directly.