Almost every research peptide listing you’ll read quotes a purity figure. 98%. 99%. 99.5%. It’s the number the whole industry competes on.
It’s also not the number that tells you how much peptide is in the vial.
Two different questions
Purity is a ratio. Of the peptide-related material in the sample, what percentage is the target compound versus related impurities — truncated sequences, deletion products, oxidation byproducts? A chromatogram with one tall peak and almost nothing else gives you a high purity number.
Net peptide content is a mass. Of the total powder sitting in the vial, how much of it is actually peptide?
Those are not the same question, and the second one is the one that determines what you’re working with.
Where the rest of the powder goes
Lyophilized peptide isn’t pure peptide by mass, and it never is. What else is in there:
- Counter-ions. Most synthetic peptides come off the purification column as a salt, commonly trifluoroacetate or acetate. That salt is bound to the peptide and it has mass. Depending on the sequence and how many basic residues it carries, this can account for a meaningful fraction of the powder on its own.
- Residual water. Lyophilized material is hygroscopic. Even well-dried powder retains some moisture, and it picks up more every time a vial is opened in humid air.
- Excipients, where used. Some products include a bulking agent such as mannitol to produce a stable cake. That’s legitimate formulation, but it’s mass in the vial that isn’t peptide.
None of this shows up in a purity figure. Purity is measured on the peptide fraction. Salt and water aren’t impurities of the peptide — they’re the other stuff in the tube. A vial can be 99% pure by HPLC and still be well under 99% peptide by weight, and both statements are true at once.
Why this gets quietly exploited
Because purity is the flattering number, and net content is the honest one.
A vendor who fills to a target of “10 mg” without specifying whether that’s 10 mg of powder or 10 mg of peptide has given themselves room. If the powder is 10 mg and the peptide fraction is lower, the label is defensible and the customer got less than they thought. Nothing was technically claimed that wasn’t true.
Two vials from two vendors, both labeled 10 mg, both advertised at 99% purity, can contain measurably different amounts of peptide. If you’re comparing prices across suppliers on a per-milligram basis, you may not be comparing the same thing at all.
What we publish
Both numbers, per lot, on the certificate.
| Lot | Compound | Label | Purity | Net content |
|---|---|---|---|---|
| ML-001 | ML-RT | 10 mg | 99.84% | 12.11 mg |
| ML-003 | GHK-Cu | 50 mg | 99.54% | 53.65 mg |
Both lots came in above their label figure. We publish it either way — the point of putting the number on the certificate is that we don’t get to choose which lots you see it for.
If a lot ever comes back under label, it goes on the certificate exactly the same way, and it doesn’t ship.
What to ask a supplier
Three questions, and the answers are more revealing than any purity percentage:
- Does your certificate report net peptide content, or only purity?
- Is the milligram figure on the label peptide mass or total powder mass?
- Can I see the certificate for the specific lot I’d receive, rather than a representative one?
A supplier who can answer all three without hedging is doing the work. One who can only quote a purity percentage is telling you what they measure — and what they don’t.
All Molecula Labs products are supplied strictly for laboratory research use. Not for human or animal consumption. Not drugs, foods, cosmetics, supplements, or medical devices. Not evaluated by the FDA.