What HPLC purity does—and doesn’t—tell you
HPLC purity is the most quoted number on a peptide COA. Here is what it actually measures, why it depends on the method, and what it can’t tell you about a vial.
“≥98% purity (HPLC)” is the most common number in peptide catalogs. It is genuinely useful, but it answers a narrower question than most people assume. Understanding that gap is the difference between reading a COA and trusting a headline.
What HPLC purity measures
High-performance liquid chromatography separates the components of a sample as they pass through a column. For peptides this is usually reversed-phase HPLC: components separate mainly by hydrophobicity and leave the column at different times. A UV detector, typically at 214–220 nm where peptide bonds absorb, records each component as a peak.
Reported purity is the area of the main peak as a percentage of the total peak area. So “98.6%” means: under this method, at this wavelength, 98.6% of the detected UV-absorbing signal came from the main peak.
Why the method matters
The same sample can produce different purity numbers on different methods. Results depend on:
- The column and gradient. A shallow gradient can separate impurities that a fast one merges into the main peak.
- The wavelength. Components absorb differently at different wavelengths.
- Integration settings. How the software draws baselines and splits overlapping peaks changes the areas.
That is why a purity value without its method and chromatogram is hard to evaluate, and why numbers from different labs aren’t always directly comparable.
What HPLC purity does not tell you
Identity
A single clean peak shows that the sample is chromatographically uniform, not that it is the intended peptide. Identity needs mass spectrometry, ideally LC-MS, which pairs the separation with a mass for each peak.
How much peptide is in the vial
HPLC purity is a relative measure among UV-absorbing components. Water, counterions such as TFA or acetate, and salts are largely invisible to it. A powder can be 99% pure by HPLC and still be only around 70–85% peptide by weight. That is net peptide content, a separate measurement.
Impurities the detector can’t see or separate
- Components that absorb weakly at the chosen wavelength.
- Impurities that co-elute with the main peak, such as some deletion sequences or closely related variants.
- Stereochemical impurities (for example D-amino acid variants), which standard reversed-phase methods often don’t resolve.
Biological contaminants
Endotoxin and microbial contamination need their own tests.
Condition after shipping and storage
A COA describes the lot when it was tested. Degradation from heat, moisture or repeated handling afterwards isn’t captured.
Reading a purity claim well
- Check the result is for your lot, not the product in general.
- Look for the chromatogram: one dominant peak and small, well-separated minor peaks.
- Check the method and wavelength are stated.
- Look for a paired identity result (MS) on the same lot.
- If you need accurate concentrations, look for net peptide content.
- Treat a catalog “purity target” as a specification, not a measured result.
In short
HPLC purity tells you how uniform a sample looked under one method. Identity, quantity and several kinds of impurity need other tests. A good COA reports purity alongside them, for the specific lot you hold. For a full walkthrough, see How to read a peptide Certificate of Analysis.
For Laboratory Research Use Only — Not for Human or Animal Consumption. Educational content only; not instructions for use outside a laboratory.