"99% purity" on a peptide vial means that in a high-performance liquid chromatography (HPLC) run, 99% of the total detected peak area belonged to the target compound. It is a chromatographic percentage, not a statement about the mass of the vial's contents. Understanding what the number does and does not cover is the difference between reading a Certificate of Analysis and just looking at it.
What HPLC actually measures
In reversed-phase HPLC, the sample is dissolved and pushed through a column packed with a stationary phase. Different molecules travel through at different speeds, so they leave the column — elute — at different times. A UV detector records what comes off, usually at 214 nm (the peptide bond absorbs there) or 220 nm.
The output is a chromatogram: a baseline with peaks. The software integrates the area under each peak. Purity is reported as:
purity % = (area of the main peak ÷ total area of all peaks) × 100
So 99.2% means the main peak accounted for 99.2% of everything the detector saw. Three consequences follow immediately:
- It is relative, not absolute. Anything the UV detector cannot see is invisible to the calculation — including water, residual salts and most inorganic content.
- It says nothing about identity. A pure peak of the wrong molecule reports as high purity.
- The detection wavelength matters. A report that does not state the wavelength is less informative than one that does.
Why mass spectrometry has to be there too
Mass spectrometry (MS) ionises the sample and measures the mass-to-charge ratio of the resulting ions. For a peptide, the observed molecular mass is compared against the theoretical mass calculated from the amino acid sequence. A match within a small tolerance confirms the compound is what the label says.
This is the identity check that HPLC cannot perform. A complete Certificate of Analysis carries both: HPLC for purity, MS for identity. If a supplier's document shows only one, half the question is unanswered. Our guide to reading a COA walks through both sections.
Purity is not the same as peptide content
This is the most common misreading. A vial labelled 99% purity and 100 mg does not necessarily contain 100 mg of peptide. Lyophilized peptides typically carry counter-ions (usually acetate or trifluoroacetate), bound water and residual salts. Those add mass.
Two separate figures describe this:
| Figure | What it answers | Method |
|---|---|---|
| Chromatographic purity | Of the peptide present, what fraction is the target? | HPLC, area % |
| Net peptide content | Of the total powder mass, what fraction is peptide? | Amino acid analysis or nitrogen determination |
Net peptide content for a standard lyophilized preparation is commonly in the 70–90% range even when chromatographic purity is above 98%. This is normal and expected, not a sign of adulteration. It is covered separately in net peptide content vs gross weight.
What a credible purity claim looks like
A purity figure carries weight only when it is attached to a specific batch. The report should state:
- The compound name and, ideally, the CAS number
- A batch or lot identifier that matches the vial
- The date of analysis
- The testing laboratory
- The HPLC method: column, gradient, detection wavelength
- The observed and theoretical molecular mass from MS
A purity percentage with none of that attached is marketing text, not analytical data.
Common ways purity numbers get inflated
- Reporting a supplier's COA as your own. The batch tested may not be the batch shipped.
- Cropping the chromatogram. Impurity peaks near the solvent front or at long retention times can be excluded by narrowing the integration window.
- Quoting a specification instead of a result. "≥99%" is a target. "99.4%" with a chromatogram is a measurement.
- Omitting the wavelength. Impurities that absorb weakly at the chosen wavelength are under-counted.
Summary
Purity percentage answers one narrow question well: what proportion of the detected material is the target compound. It does not confirm identity — that is MS. It does not tell you how much peptide is in the vial — that is net peptide content. All three figures together describe the material; any one of them alone does not.
Batch documentation is published on every product page in the HolyPeps catalog.
All products are intended for laboratory and research use only. They are not medicines, are not for human or veterinary use, and are not for diagnostic or therapeutic application.