Purity and net content answer two different questions. HPLC purity tells you what share of the peptide material is the right sequence. Net peptide content tells you what share of the powder in the vial is peptide at all. A vial can be 99% pure and still hold less peptide than its label, because the powder also contains water, residual solvents and counter-ions.
Notice: Research peptides are sold for in-vitro laboratory research only. They are not FDA-approved, not for human or animal consumption, and this page is not medical advice.
Our HPLC purity guide covers how the purity number is made and what it misses. This page takes the next step: how to think about the milligrams in a vial, and how the "measured content" line on vendor COAs fits in.
Three different "mg" numbers
When you look at a research peptide vial and its paperwork, you can meet three different amounts. They are easy to mix up.
| Number | What it is | Where it comes from |
|---|---|---|
| Label mg | The amount the vendor says is in the vial | Product label |
| Gross powder weight | Everything in the vial: peptide, water, counter-ions, salts | Weighing |
| Net peptide content | The share of the gross weight that is peptide | Amino acid analysis, elemental analysis or UV spectrophotometry |
| Measured content (on vendor COAs) | A lab's estimate of mg of compound in the tested vial | Usually HPLC, per the certificates we opened |
Peptide maker AAPPTEC puts it plainly: the gross weight of a dry peptide "doesn't consist of peptide only, but includes non-peptide components such as water, absorbed solvents, counter ions and salts." AmbioPharm lists the usual methods for measuring net content as "elemental analysis, amino acid analysis (AAA) or UV spectrophotometry." HPLC purity isn't one of them.
Why the powder isn't all peptide
Counter-ions
Most synthetic peptides carry a positive charge on basic residues (Arg, Lys, His) and the N-terminus. Each charge pairs with a negative ion. AAPPTEC notes that peptides purified by HPLC "are usually obtained as TFA salts," with trifluoroacetate counter-ions. Those ions have weight, and they count toward the powder mass.
A 2025 study in Pharmaceuticals (Erckes et al.) measured about 0.333 mg of trifluoroacetate per mg of peptide salt in its test peptides before exchange, and reported counter-ion content reaching up to 35% of total weight. That's one-third of the powder that isn't peptide. For more on salt forms, see TFA vs acetate peptide salts.
Water and solvents
Lyophilized powders keep some residual water and can absorb more from air. Peptide supplier Bachem says the peptide content of a solid "may range from 70 to 90%" because of counter-ions and residual moisture. AAPPTEC gives a wider 50-90%; AmbioPharm says "usually 60-90%."
A worked example
AAPPTEC gives a formula for the theoretical net peptide content of a TFA salt: divide the peptide's molecular weight by the sum of that weight plus the number of TFA counter-ions times 114, the counter-ion's molecular weight.
Take a hypothetical peptide with a molecular weight of 1,000 and three positively charged sites:
theoretical net content = 1000 / (1000 + 3 x 114)
= 1000 / 1342
= about 74.5%
Before counting any water, about a quarter of the powder is counter-ion. If that vial holds 10 mg of powder, it holds about 7.45 mg of peptide, even at 99% HPLC purity. Add a few percent of water and the figure drops further.
This is arithmetic on a made-up peptide. Real peptides differ in charge count and molecular weight, so the only reliable figure for a specific vial is a measured one.
How vendor COAs report content
Some research-peptide certificates now include a measured amount alongside purity. These examples come from certificates cited on our best BPC-157 vendors page, each for BPC-157:
| Vendor | Label | Purity | Measured content | vs label | Lab |
|---|---|---|---|---|---|
| PSPeptides | 5 mg | 99.95% | 4.88 mg | 97.6% | North American Diagnostics (COA) |
| Peptide Plugs | 20 mg | 99.56% | 20.86 mg | 104.3% | Freedom Diagnostics (product page) |
| Alpha Pro Peptides | 10 mg | 99.24% | 12.99 mg net | 129.9% | Freedom Diagnostics (COA) |
The "vs label" column is our arithmetic: measured mg divided by label mg. Vendors are listed here in the order they appear in our editors' ranking, which isn't sorted by score.
Three things stand out.
- High purity, different amounts. All three are above 99% pure. Their content ranges from about 98% to 130% of label. Purity told you nothing about that spread.
- Overfill is common too. Our other compound pages show the same pattern, such as Peptide Plugs' TB-500 10 mg at 12.65 mg measured (best TB-500 vendors) and its GHK-Cu 100 mg at 116.79 mg (best GHK-Cu vendors).
- The basis isn't stated. The PSPeptides and Alpha Pro certificates we opened on September 26, 2026 describe the method as HPLC with UV detection coupled with mass spectrometry. Neither mentions salt form, counter-ions or water, so we can't tell whether "measured content" means peptide as free base, peptide as salt, or something else. That matters when you compare numbers across labs.
Independent tester Finnrick grades "how much material is in the container compared with the vendor's claim," with tolerances such as ±25% for its standard rating. Its methodology page doesn't describe a salt correction either.
What this means for lab math
Concentration is mass divided by volume. The question is which mass.
concentration (mg/mL) = peptide mass (mg) / diluent volume (mL)
Using the Alpha Pro example: 10 mg label ÷ 2 mL gives 5.0 mg/mL. Using the measured 12.99 mg ÷ 2 mL gives about 6.5 mg/mL. That is a 30% gap in every downstream calculation.
Practical steps:
- Match the lot first. A measured content only applies to the lot that was tested. Check the lot on your vial against the certificate.
- Use the measured figure when the lot matches. Enter it in our peptide reconstitution calculator instead of the label mg, and note it in your records.
- Ask what the number includes. If you need exact molar amounts, email the lab and ask whether the reported mg is corrected for counter-ions and water.
- Don't compare purity across labs as if it were content. A 99.9% result from one lab and 99.2% from another says nothing about which vial holds more peptide.
Our COA checklist includes a line for measured content, and the COA reading guide covers the other fields.
When net content isn't reported at all
Many research-peptide certificates show purity and identity only. In that case you have the label mg and nothing to check it against. The label figure may be the gross powder weight, the peptide weight, or a target fill; the certificate won't say.
When choosing a supplier, a lot-matched certificate with a measured amount is more useful than a purity number alone. Our best peptide vendors in the USA ranking scores whether COAs are public, lot-specific and dated, and each review notes which vendors show measured content. Third-party tested peptides compares how each vendor documents testing. Terms such as counter-ion and amino acid analysis are defined in the research peptide glossary.
Updated September 26, 2026. Sources are linked inline. Vendor certificate figures were checked on the dates shown in each linked page.