A purity percentage without a chromatogram is just a number
Area normalisation at 220 nm can be presented honestly or dishonestly. Here is what to look for in the trace, and why we publish ours.
Every peptide supplier quotes a purity figure and almost all of them quote something in the high nineties. The figures cluster so tightly that they have stopped carrying information. That is not because the material is uniformly excellent — it is because the number is easy to produce and hard to check.
Where the flexibility hides
Area normalisation is a legitimate, standard method. The flexibility is not in the arithmetic; it is in the choices made before the arithmetic starts.
- Gradient steepness. A steep, short gradient co-elutes close impurities with the main peak. They stop being counted as impurities because the integrator can no longer see them as separate.
- Run length. Stop the run at 12 minutes and anything hydrophobic enough to elute at 16 never appears in the total area.
- Integration windows. Excluding an early solvent region is normal practice. Excluding a region that happens to contain a deletion sequence is not, and the difference is invisible in the reported number.
- Detection wavelength. Reporting at 280 nm instead of 220 nm only sees residues with aromatic side chains, so impurities lacking them contribute nothing.
What a trace tells you that a figure cannot
Given the chromatogram, several things become checkable in seconds without any specialist training.
- Peak symmetry. A main peak with a pronounced shoulder or a tail is usually hiding something that co-elutes. A symmetrical peak is a good sign; an asymmetrical one is a question.
- Baseline behaviour. A drifting or noisy baseline makes integration boundaries arbitrary, which makes the resulting percentage soft.
- Where the run ends. If the trace stops shortly after the main peak, nothing late-eluting could have been detected.
- The impurity profile. Several small resolved peaks is a normal, well-characterised synthesis. One large unresolved region is a different situation entirely.
Our position
We publish the chromatogram for every released lot, alongside the method, the impurity table with retention times and areas, and the name of the analyst who released it. Not on request — published, against the lot number, before the vial ships.
It is not a competitive advantage we are especially proud of. It is a low bar, and it should be ordinary. But it is the difference between asking you to trust a number and letting you check one.