A purity figure like "≥98% by HPLC" printed on a vial label is really a summary of an entire chromatography run — and understanding how that run works makes it possible to evaluate whether the number means anything.
How the separation actually happens
High-Performance Liquid Chromatography forces a liquid sample through a column packed with a solid stationary phase — for peptides, almost always a reverse-phase C18 silica column — under high pressure. A mobile phase, typically a gradient of water and acetonitrile with a small amount of trifluoroacetic acid as an ion-pairing modifier, carries the sample through the column. Different molecules interact with the stationary phase to different degrees based on size, charge, and hydrophobicity, so they travel through the column at different speeds and exit — elute — at different times. Each compound's characteristic exit time is its retention time under those specific run conditions.
Turning a chromatogram into a percentage
A detector — usually a UV absorbance detector set around 214–220 nm, where the peptide bond itself absorbs — records a signal as material exits the column, producing a chromatogram: a series of peaks over time. Each peak's area is proportional to how much of that compound was present in the sample. The purity percentage reported on a Certificate of Analysis is typically the target peptide's peak area divided by the total peak area of everything detected in the run. A result of ≥98% means impurities — truncated synthesis sequences, deletion peptides, oxidized variants, or residual solvents — account for roughly 2% or less of the detected material.
What HPLC purity does not tell you
HPLC purity is a relative measurement of how clean the sample is, not a confirmation of what the main peak actually is. Two different peptides could, in principle, produce a similarly clean single peak under a given set of run conditions. That's why purity and identity are answered by two different tests: HPLC addresses "how much of the sample is a single compound?" while mass spectrometry addresses "is that compound the right one?" by measuring molecular mass directly. See our guide on amino acid sequences for how that identity is defined in the first place.
Reading the number in context
A bare percentage with no supporting detail is a marketing claim. A genuine result names the method, and is tied to a specific production batch on a Certificate of Analysis — ideally one generated by an independent, third-party laboratory rather than the seller's own internal process. When comparing suppliers, the documentation behind the number is worth more scrutiny than the number itself; see our supplier checklist for the full set of criteria worth weighing.
Typical purity ranges in practice
Across the research peptide market, ≥98% is a common threshold for material marketed as research-grade, with some suppliers reporting figures above 99% on select batches. Purity below that range isn't automatically unusable for every application, but it does mean a larger fraction of the sample is something other than the target compound — which matters more for some experimental designs than others. Knowing the actual figure for your batch, rather than assuming a generic "high purity" claim applies uniformly across a product line, is the difference between a documented material and an assumed one.
Research Use Only. Supplied strictly for laboratory research and development — not for human or veterinary use, consumption, or any therapeutic or diagnostic purpose. This article is research education, not usage guidance.
See a third-party–tested compound: BPC-157 (batch COA available) →