What HPLC Purity Percentage Actually Tells You

An HPLC purity percentage tells you one specific thing: of everything the detector saw coming off the column, what fraction was your target compound. That is useful, and it is narrow. It says nothing about how much peptide is in the vial, and nothing about whether the peptide is even the right one. Most people read the purity number as a grade for the whole product. It is a grade for a single test.

Here is what the number is built from, and where it quietly misleads.

What the percentage actually measures

Purity is a ratio of areas, not a count of molecules. High-performance liquid chromatography pushes your dissolved sample through a column that separates compounds by how strongly each one interacts with the packing material. Different compounds exit at different times. Each one registers as a peak on the trace, and the detector records the area under every peak. Divide the area of the target peak by the total area of all peaks, and that ratio is the purity percentage.

So 99.6% means the target peak accounted for 99.6% of the total detected area. The remaining 0.4% is everything else the detector picked up: truncated sequences, deletion products, leftover synthesis reagents, breakdown from age or heat. On one of our BPC-157 lots, BT Labs in Florida reported 99.6% against a specification of at least 98%, with the target peak coming off at 4.055 minutes.

Retention time and wavelength are not decoration

That exit time, the retention time, is a fingerprint for the compound under those exact column conditions. Run the same peptide through the same method next week and it should land at the same spot. A shifted retention time on a method that is supposed to be fixed means something changed, and that is worth noticing.

Wavelength is the other detail people skip. Peptide bonds absorb strongly in the low ultraviolet, so peptides are read at 220 nm, which is what both of our labs use. Read the same sample at 280 nm instead and you would only see residues with aromatic rings, missing most of the molecule. When a certificate does not say what wavelength the purity was measured at, you cannot actually compare it to anything.

The baseline is where the honesty lives

A high number sitting over a noisy baseline is not a clean result. The baseline is the flat line the trace returns to between peaks, and it is where the small stuff shows up: minor humps, a scatter of tiny unlabeled peaks, a line that drifts instead of sitting flat. Those are impurities the software may or may not have counted, depending on where the analyst told it to draw the integration line.

That last part matters more than it sounds. Purity is an integrated area, and moving the baseline a little changes how much of a shoulder or a low hump gets folded into the main peak. A genuinely clean chromatogram is one sharp peak returning to a flat, quiet baseline. That is a different thing from a 99% figure typed above a trace nobody shows you.

Purity, potency, and identity are three separate tests

They measure three different things, and a vial can pass one while failing another.

• Purity is the fraction of the active fraction that is your target compound. That is the HPLC number.
• Potency is how many milligrams of actual peptide are in the vial. A lot can be 99% pure and still underfilled.
• Identity is whether the molecule is the one on the label. That is confirmed by mass spectrometry, which weighs the molecule, not by HPLC.

You can have a 99% pure sample of the wrong peptide, and the purity number would look excellent the whole time. GHK-Cu, for example, has a known mass around 401.94; the only way to confirm a vial actually contains it rather than something with a similar retention time is the mass spec, not the purity figure. We walk through how these tests sit together on a real certificate in [how to read a peptide COA](/research/how-to-read-a-peptide-coa), using two of our own lab reports.

What the number cannot tell you

The purity percentage is blind to anything the UV detector does not see. Counterions, residual water, and salts often do not absorb at 220 nm, so they do not show up as peaks and do not count against purity. A co-eluting impurity, one that happens to leave the column at almost the same time as the target, can hide underneath the main peak and inflate the number. And a percentage printed with no chromatogram behind it is not data. It is a typed claim.

None of this makes the purity number worthless. It makes it one line of evidence that means something only when you can see the trace, the wavelength, the method, and the identity test sitting next to it. Read on its own, "99%+" is the easiest figure on a certificate to write down and the hardest to verify. Ask for the rest.

*All products are supplied as laboratory reagents for research use only. Not for human or veterinary use.*