Purity vs. Potency on a Peptide COA: Why 99% Pure Is Not 99% Full
Two numbers on a certificate of analysis are easy to confuse: purity and potency. They answer different questions, and a vial can score well on one while falling short on the other. This guide explains the difference and shows how to read both together.
For laboratory research use only. This guide explains how an analytical report is structured. It is educational and is not a statement about the safety, effects or suitability of any compound for human or animal use.
Two questions, two numbers
| Line on the COA | The question it answers | Typical form |
|---|---|---|
| HPLC purity | Of the peptide-related material the instrument detected, what share is the target molecule? | A percentage, such as 99.8% |
| Potency (content) | How much of the target peptide is actually in the vial, compared with the amount on the label? | Milligrams, and a percentage of the labeled amount |
Purity is about quality of the peptide that is there. Potency is about how much of it there is. Neither number can be worked out from the other.
How the same vial can score well on one and poorly on the other
The table below uses made-up numbers (they are not from any specific lot) to show the combinations a reader can run into.
| Illustrative sample | HPLC purity | Content vs. label | What it shows |
|---|---|---|---|
| A | 99.8% | 101% | Clean peptide, close to the labeled amount. |
| B | 99.8% | 84% | Clean peptide, but less of it than the label states. |
| C | 91.0% | 100% | The labeled amount of the target is present, but more related impurities than a 98% specification would allow. |
Looking at purity alone, A and B look identical. Looking at content alone, A and C look identical. Only the two lines together give the full picture.
How each number is measured
- Purity comes from the HPLC chromatogram. The instrument separates the sample and records a signal for each component. Purity is the main peak’s area as a share of the total detected peptide-related area. Our guide to how purity is tested explains the method.
- Potency is a quantitative assay. The lab determines how many milligrams of the target peptide are in the vial and reports it, along with that figure as a percentage of the labeled content. On the reports we publish it appears as the “HPLC Potency Assay” row, next to a specification such as “90 – 110% of 10 mg”.
A worked example from a published report
This is the BT Labs report for HLX-3RT 10mg, lot RT082026-02 (tested August 30, 2026), published on our COA page:
| Row | Specification | Result |
|---|---|---|
| Mass | As recorded | 58.8 mg |
| HPLC purity | ≥ 98% | 99.8% |
| HPLC potency | 90 – 110% of 10 mg | 9.3 mg (93.2%) |
| Peptide-to-excipient ratio | Lab’s recommended range | 9.3 : 49.5 mg (1:5.3) |
Reading it line by line:
- Purity is 99.8%. Almost all of the detected peptide-related signal is the target molecule.
- Content is 9.3 mg against a 10 mg label. That is 93.2%, inside the report’s 90–110% window but below the labeled amount. It is a different statement from purity.
- The masses agree. 9.3 mg of peptide plus 49.5 mg of everything else equals the 58.8 mg total mass. Only about 16% of the vial contents, by mass, is the peptide itself. See our guide to the peptide-to-excipient ratio.
Why content can differ from the label
A labeled amount is a target, and a laboratory specification normally allows a window around it (here 90–110%). Differences can come from fill variation when the vial was prepared, moisture or salt content in the powder, and the normal uncertainty of the assay itself. A result inside the window is within the lab’s specification, and a result outside it is a reason to ask questions, not an automatic verdict.
Counter-ions such as TFA or acetate are one common source of that salt content; see counter-ions on a peptide COA.
What a complete COA looks like
- Both a purity line and a potency (content) line, each with its method.
- A specification printed beside every result, so you can see what the lab compared it against.
- A mass line and, where reported, a peptide-to-excipient ratio that agree with the potency figure.
- The laboratory’s name, report number, lot number and dates.
Common red flags
- A purity percentage with no content result at all, and no statement of why.
- A potency figure with no specification to compare it against.
- Content well outside the printed window with no comment from the lab or the seller.
- The same purity and content numbers repeated across different products or lots.
- Purity quoted by the seller that does not match the figure on the lab’s report.
Where to go next
Unfamiliar terms are defined in our glossary. Our guide to reading a peptide certificate of analysis walks through every field. Every HLX Peptide certificate is published unedited on the COA page, and the Quality & Testing page describes how we test. For general lab handling of freeze-dried material, see the storage and stability guide.
All products are sold strictly for laboratory research use only. Not for human or veterinary use, and not a drug, food, or cosmetic.