What HPLC purity measures, and what it does not
A purity percentage describes the proportion of a sample that is the target compound. It is a composition measurement, not a verdict on quality, potency or fitness for use — and knowing the difference is most of what makes a certificate readable.
Last updated 2026-09-08
High-performance liquid chromatography separates a sample by pushing it through a column. Different molecules move through at different speeds, so they arrive separated — a series of peaks, one per component, each peak's area proportional to how much of that component was present.
Purity is the area of the peak belonging to the target compound, expressed as a percentage of the total peak area. That is the entire definition. It is a statement about composition — what proportion of this sample is the thing it claims to be — and it carries no information about anything else.
Measured purity across the 14 published lots in this archive runs from 99.01% to 99.99%. That spread is what real measurement looks like. A catalogue where every lot reports the same round number is not reporting measurements.
Three numbers that are easy to confuse.
Purity is routinely quoted as though it answered all three questions. It answers one.
| Assay | What it establishes | What a figure of 98% would tell you |
|---|---|---|
| Purity (HPLC) | What fraction of the sample is the target compound | That 98% of the sample is the named peptide and 2% is something else |
| Identity (HPLC + MS) | Whether the material is the compound named on the label | Nothing. Identity is a yes or no — 98% pure material can still be the wrong molecule |
| Net peptide content | How much peptide is in the vial once salts and water are excluded | That 98% of the labelled mass is peptide. A different 98% from the one above, measured on a different basis |
What the number cannot tell you
HPLC reports what it can separate and detect. Something that co-elutes with the target compound — travels through the column at the same speed — is not distinguished from it, which is one reason identity is confirmed by mass spectrometry alongside rather than inferred from the purity figure. Material the detector does not respond to does not appear at all.
None of that makes the number unreliable — it makes it specific. The mistake worth avoiding is reading a narrow answer as a broad one.
Common questions
Does a higher purity percentage mean a better product?
It means a smaller proportion of the sample was something other than the target compound. That is a composition measurement and nothing more — it does not describe potency, suitability or safety, and a vendor presenting it as an overall quality score is reading more into the number than the method supports.
What is the difference between 99% and 99.9% purity?
An order of magnitude in the impurity fraction: one part in a hundred versus one part in a thousand. Whether that difference matters depends entirely on what the material is being used for, which is a question about a study rather than about a certificate.
Why does HPLC purity not add up to 100% with the other assays?
Because the assays measure different things on different bases. Purity is a proportion of the chromatographic peak area; net peptide content is a mass. They are not components of one total and cannot be summed.
For laboratory and research use only · Not for human or veterinary consumption · Not evaluated by the FDA