HPLC purity, as printed on a certificate of analysis, is almost always a peak-area percentage: the area under the target compound's peak divided by the total area under every peak in the chromatogram. A figure of ≥99% means that calculation returned 99 or higher, not that independent, orthogonal testing confirmed 99% chemical purity by mass — and the difference between those two statements matters more than most COAs make clear. Every batch of Semax sold on this site is verified with this method alongside mass spectrometry, for exactly the reasons set out below.
How does HPLC separate and quantify a sample?
High-performance liquid chromatography works by pumping a sample through a column packed with a stationary phase, typically a silica-based material with a bonded organic layer. Different compounds in the sample interact with that stationary phase to different degrees and therefore travel through the column at different rates, exiting — or "eluting" — at different times. A detector, most often measuring ultraviolet absorbance, records each eluting compound as a peak on a chromatogram, with peak position (retention time) indicating identity by comparison to a reference standard, and peak area indicating relative quantity.
What does "area normalisation" purity actually mean?
Area normalisation purity takes the area of the target peptide's peak and divides it by the sum of all peak areas in the trace, then expresses that ratio as a percentage. This is the method behind the great majority of ≥99% purity claims in the research peptide market, and it is a legitimate, widely used analytical technique — but it carries a specific assumption: that every compound present responds to the detector in proportion to how much of it is actually there. That assumption does not always hold.
Why can two 99%-purity COAs describe different things?
UV detectors respond to how strongly a compound absorbs light at the wavelength being monitored, and different compounds — including different impurities — do not absorb UV light equally. A minor impurity that absorbs weakly at the monitored wavelength can be present in a meaningful quantity by mass while contributing very little peak area, which means the area-normalisation percentage can overstate true mass purity. Analytical chemistry literature on peptide purification specifically flags this as a source of discrepancy: impurities that complicate quantification exist regardless of which single detection method is used, which is why a robust purity assessment combines chromatographic separation with an orthogonal quantification method rather than relying on UV peak area alone. In practice, this means two vials both labelled ≥99% by HPLC can carry meaningfully different actual impurity profiles depending on what detector, wavelength and reference standard were used.
What HPLC purity does not tell you
An HPLC trace alone does not confirm that the major peak is the intended sequence — it confirms that most of the detectable material elutes as a single peak at a given retention time, and a wrong or truncated peptide with similar chromatographic behaviour could, in principle, produce a similar trace. It also does not measure residual solvent, counter-ion content, or moisture, all of which affect the practical mass of active peptide in a vial even when the chromatographic purity figure is high. This is precisely why a complete COA pairs the HPLC purity figure with an independent identity check, as covered in how to read a peptide COA.
Why mass spectrometry is a necessary complement
Mass spectrometry addresses the gap HPLC leaves open: it measures the mass-to-charge ratio of ionised molecules directly, via electrospray ionisation (ESI) or matrix-assisted laser desorption/ionisation (MALDI), and confirms whether the dominant species in the sample matches the expected molecular weight for the labelled sequence. A batch of Semax or Selank with a clean 99% HPLC trace but a mass spectrometry result that does not match the reference molecular weight is a red flag that HPLC data alone would not have caught. The two techniques are complementary rather than redundant, and a COA presenting only one of them is presenting half the picture. Once a compound's purity and identity are confirmed, how it is stored afterwards materially affects how long that purity figure remains accurate, which is the subject of the peptide storage temperature guide.
What does a typical HPLC run for a peptide like Semax look like?
A standard reversed-phase HPLC method for a short peptide such as Semax uses a C18 silica column, an acidic aqueous mobile phase (commonly water with a small percentage of trifluoroacetic acid) paired with an acetonitrile gradient, and UV detection at 214 nm, a wavelength that captures the peptide backbone's amide bond absorbance rather than relying solely on aromatic side chains. Over the course of a run, the gradient increases the proportion of acetonitrile, and the target peptide elutes at a characteristic retention time determined by its hydrophobicity. A well-resolved trace shows the target peak clearly separated from any neighbouring peaks — truncated sequences, deletion products, or diastereomers formed during synthesis — each of which would otherwise co-elute and be invisible to the area-normalisation calculation.
How does gradient and column choice affect the reported figure?
Two laboratories running the same peptide sample on different columns, gradients or detection wavelengths can report different purity figures for material that is, at the molecular level, identical — because resolution between the target peak and closely related impurities depends heavily on those method parameters. A shallow gradient run over a longer time typically resolves more closely related species than a fast, steep gradient, which is one reason methodology sections matter as much as the final percentage: a COA that states only "HPLC purity: 99.2%" without describing the column, gradient and detection wavelength used gives a researcher no way to judge how rigorous that separation actually was. This variability across methods reinforces why a fixed reference standard and a consistent method, ideally the same one across every batch of a given compound, are more useful to a research programme than a single isolated percentage.
Understanding what a purity percentage does and does not represent is a research use only exercise in reading laboratory documentation critically, not a substitute for independent verification where a study requires it. Batch-specific HPLC and mass spectrometry data for every compound in the catalogue is available on request, and the guide to peptide reconstitution and storage covers what happens to that purity figure once a vial is opened.
