Guide · 7 min read
Net peptide content vs purity: how much is really in the vial?
A COA can show 99.5% purity for a vial that's badly underfilled. Purity and quantity are separate measurements, and the second one is the one most COAs skip. Here's how to tell them apart and what to look for.
Three numbers that are easy to mix up
| Gross weight | Everything in the vial: the peptide, its counter-ion salt (acetate or trifluoroacetate), residual water and solvent. |
|---|---|
| Net peptide content | The share of that powder that is peptide, as a percentage. Measured by amino acid analysis, nitrogen (elemental) analysis or UV absorbance. |
| HPLC purity | The share of the UV signal (peak area) in the chromatogram that belongs to the correct molecule, rather than to deletion sequences and other impurities. See HPLC purity explained. |
Bachem, one of the largest peptide manufacturers, puts it plainly: research peptides are typically over 95% pure by HPLC, but the peptide content of the solid may be only 70 to 90%, because peptides carry counter-ions and moisture. Other manufacturers give 60 to 90%, and some as wide as 50 to 90%, depending on the sequence and how it was purified.
How to work out how much peptide you actually have
When a COA gives you all three numbers, multiply them:
Peptide of interest = gross weight × net peptide content × purity
For example, a 5 mg gross fill with 84% net peptide content and 96% purity contains about 5 × 0.84 × 0.96 = 4.0 mg of the target peptide. Two vials labeled "5 mg" can differ by a milligram or more in real content without either being mislabeled, which is why the definition behind the label matters.
What research-peptide COAs usually report
Most COAs in this market don't report a net peptide content percentage. Instead they report a measured amount in milligrams, often labeled "net content" or "quantity", worked out by quantitative HPLC against a reference standard. Look for three things:
- Is it measured or copied from the label? A figure that exactly equals the label, to the decimal, on every report is a warning sign.
- Is it mg of peptide or mg of powder? The method line should make this clear. If it doesn't, ask the lab.
- Is there more than one vial? Some labs test three vials from one lot. The spread between them is real information about fill consistency.
Public reports from Freedom Diagnostics show how much fills vary, in both directions. One 10 mg PT-141 lot measured 9.34, 8.83 and 9.08 mg across three vials. A 100 mg GHK-Cu lot measured 99.76, 98.24 and 105.88 mg. A single 10 mg MOTS-c vial measured 11.60 mg. The label is a target, not a guarantee. By Finnrick's ±5% standard, five of those seven vials fall outside "good". Our more lenient checker would pass most of them, but the lowest PT-141 vial, at 88% of its label, would get a warning, and the MOTS-c vial is a 16% overfill. See how to verify a Freedom Diagnostics COA.
How much variation is acceptable?
There's no single legal standard for research peptides, so here are the two reference points we use:
- Finnrick, which publishes independent test results for many vendors, treats a measured quantity within 5% of the label claim as good.
- The PeptideCOACheck checker is more lenient, because it only flags clear problems:
| Measured vs label | Checker result | What it usually means |
|---|---|---|
| 90% to 120% | Pass | Normal fill variation |
| 80% to under 90% | Warning | Worth asking the supplier about |
| Under 80% | Issue | A large underfill |
| Over 120% | Warning | The COA may be for a different fill size or batch |
The full rules are on the methodology page.
Salt forms change the math
The same peptide sold as an acetate or a trifluoroacetate (TFA) salt has the same measured mass in a mass spectrometer, because the counter-ion separates in the instrument. But the salt adds weight to the powder. A vial labeled "10 mg" of peptide acetate contains less than 10 mg of peptide if the label refers to the powder. A careful COA states the salt form, and a careful label says whether its figure is peptide or powder. Several of our peptide reference pages note the salt forms a compound is commonly sold in.
What to ask a supplier
- Does the COA report a measured quantity, and is it mg of peptide or mg of powder?
- Which method measured it, and against what reference standard?
- Was more than one vial from the lot tested?
- Does the label figure refer to peptide content or gross fill?
A supplier who answers these directly is giving you more than most. Combine the answers with the vendor checklist.
Have the label and measured amount? Enter both in the checker's optional section for an instant grade.
Open the COA checker →Sources
- Bachem: Care and handling of peptides: Research peptides are typically over 95% pure by HPLC while peptide content of the solid ranges from 70 to 90%.
- AmbioPharm: What is net peptide content?: Net peptide content is usually 60 to 90% of gross weight, measured by elemental analysis, amino acid analysis or UV.
- Bio-Synthesis: What does net peptide content mean?: Gives a 50 to 90% range depending on purity, sequence and method.
- AnaSpec peptide FAQ: Worked example combining gross weight, net peptide content and purity.
- Freedom Diagnostics COA, PT-141 10 mg, accession 2604220029 (vendor-hosted copy): Three-vial result: 9.34, 8.83 and 9.08 mg. Verify with the lab's lookup before relying on it.
- Freedom Diagnostics COA, GHK-Cu 100 mg, accession 2604140169 (vendor-hosted copy): Three-vial result: 99.76, 98.24 and 105.88 mg.
- Freedom Diagnostics COA, MOTS-c 10 mg, accession 2608100362 (vendor-hosted copy): Net content 11.60 mg.
- Finnrick testing methodology: Treats a measured quantity within 5% of the label claim as good.
- PeptideCOACheck methodology: How the checker grades measured content against the label.
Last updated . Written and maintained under our editorial policy. Found an error? See corrections.
Frequently asked questions
What is a good net peptide content?
For lyophilized research peptides, net peptide content of the powder is commonly 70 to 90%, and suppliers cite ranges as wide as 50 to 90% depending on the sequence and salt. What matters more for a vial is whether the measured milligrams of peptide match the label.
Does a 99% purity result mean my 10 mg vial holds 10 mg?
No. Purity is the share of the peptide material that is the right molecule. It says nothing about how much powder went into the vial, or how much of that powder is salt and water. Only a quantity or net content result answers that.
Why does my 10 mg vial's COA show 11 mg?
Labs measure the peptide in the vial, and fills vary. A modest overfill is common. Far more than the label, say over 120%, often means the COA is for a different fill size or batch.
How does the PeptideCOACheck checker grade net content?
Measured content of 90 to 120% of the label passes, 80 to 90% gets a warning, and under 80% is flagged as a serious underfill. Over 120% also gets a warning, because it usually means the COA doesn't match the vial.
Educational content only. Not medical advice. Research peptides are sold for laboratory research purposes only.