Reading a Purity Number
A 98% purity claim sounds precise. It describes one specific measurement and says less than people assume.
Stop 2 of 7 on the Bench Line.
A certificate of analysis usually lists a purity percentage and a mass spectrum. Each answers a different question.
What HPLC purity means
High-performance liquid chromatography (HPLC) pushes the sample through a column that separates molecules by how they interact with it. A detector, often measuring UV absorbance near 214 nm where peptide bonds absorb, records a trace of peaks. Purity is typically reported as the area of the main peak divided by the total area of all peaks.
Things this number does not tell you:
- It only counts what the detector sees. Salts, water and some impurities are invisible at that wavelength.
- Impurities that leave the column at the same time as the product are hidden inside the main peak.
- Different methods can give different values for the same sample.
Identity is a separate test
Mass spectrometry measures the molecular weight. If the observed mass matches the expected mass of the target sequence, that supports identity. It does not prove the full sequence is right, because two different arrangements of the same amino acids can have the same mass.
Purity versus content
A vial of freeze-dried peptide also holds water and counter-ions, such as trifluoroacetate left from synthesis. Net peptide content is the true fraction of the powder that is peptide, and it is often around 70% to 85% even when HPLC purity is above 95%. Both numbers can be correct at once.
What to look for
- The method, column and wavelength are stated.
- Chromatogram and spectrum are included, not only a number.
- The batch number matches the vial.
- An independent laboratory produced it.
A purity figure from an unnamed laboratory with no raw data is a claim, not a result.
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Education only, not medical advice. See the service notice.