Every peptide vendor talks about purity. "99% pure." "≥98% by HPLC." "Pharmaceutical grade." Most of these numbers are real. None of them are complete. This guide explains exactly what a purity figure measures, what it doesn't, and how serious researchers think about it.
What HPLC actually measures
High-performance liquid chromatography (HPLC) separates the components of a sample by passing it through a column. Different molecules travel at different speeds based on their interaction with the column packing material. The output is a chromatogram — a graph of detector signal over time, with peaks corresponding to individual compounds.
The "purity" number on a peptide COA is the percentage of the total integrated peak area attributable to the target peptide. If the target peptide produces a peak that represents 99% of the total signal at the chosen detection wavelength, the COA reports 99% purity.
What 99% purity does and doesn't mean
What it means
- 99% of the UV-absorbing material at 220 nm is the target peptide
- The synthesis was clean
- The HPLC purification removed most byproducts
What it doesn't mean
- The vial is sterile
- The vial is endotoxin-free
- The vial is heavy-metal-free
- The peptide is the right peptide (that's identity, confirmed by mass spec)
- The vial contains the labeled mass (that's quantitation, often a separate measurement)
- Storage and shipping have preserved the peptide since the COA was issued
The detection wavelength caveat
Most peptide HPLC purity is reported at 220 nm — a wavelength where the peptide bond itself absorbs. But:
- Impurities that don't absorb at 220 nm are invisible to a 220-nm purity measurement.
- Peptides with aromatic residues (Trp, Tyr, Phe) absorb strongly at 280 nm; running purity at 280 nm gives different numbers and may miss non-aromatic impurities.
- Salt counterions, residual solvents, and water are not detected at all by UV.
A complete COA discloses the wavelength used and ideally provides chromatograms at multiple wavelengths.
The four impurity categories you actually care about
1. Sequence-related impurities
Deletion sequences (peptides missing one or more amino acids), insertion sequences, and racemization products. These are the closest in structure to the target peptide and the hardest to separate. They're what HPLC catches.
2. Modification products
Oxidation (especially of Met, Cys, Trp), deamidation (of Asn, Gln), and disulfide scrambling in cysteine-containing peptides. Some are visible by HPLC; others require specific assays.
3. Process contaminants
Residual solvents (acetonitrile, TFA, DMF), counterions, and trace reagents. Invisible to UV. Detected by NMR, gas chromatography, or specific titrations.
4. Biological contaminants
Endotoxins, sterility, microbial bioburden, mycoplasma. Completely invisible to chemical analysis. Require dedicated microbiological assays.
HPLC purity catches category 1 and parts of 2. Categories 3 and 4 require entirely different tests that most peptide suppliers skip.
This is the central point of our companion article on why third-party testing matters: the gap between "99% pure" and "safe to use in research" is filled by tests that purity numbers don't cover.
What purity threshold should you accept?
The honest answer depends on the application:
- ≥99% — appropriate for any application where impurity profile could confound results. The default for serious research.
- 97–99% — acceptable for screening assays and exploratory work where small impurities are unlikely to confound the readout.
- 95–97% — borderline. Acceptable only if the impurity profile is well-characterized and known to be inert in your assay context.
- <95% — should raise questions. Find out what the impurities are before using.
Beyond the percentage: read the chromatogram
A 99% purity number alongside a clean chromatogram (one dominant peak, minimal noise, well-resolved baseline) is meaningful. The same number alongside a chromatogram with shoulders, multiple medium peaks, or poor resolution is less meaningful. The chromatogram tells you whether the integrated number represents reality or wishful integration.
Reputable suppliers publish the chromatogram itself, not just the derived number. If yours doesn't, ask.
Is 99% purity always better than 98% purity?
For most research applications, the 1% difference is within the noise of the assay you're running. The bigger question is what makes up the impurity 1% and whether the supplier has tested for biological contaminants beyond chemical purity.
Can I trust a purity number without seeing the chromatogram?
You can trust it as a data point, but you can't fully evaluate it. The chromatogram tells you the quality of the underlying analysis. A great supplier publishes both.
What's the difference between purity and potency?
Purity is the percentage of the sample that is the target molecule. Potency is the biological activity per unit mass. A peptide can be 99% pure and have low potency if it's been damaged by mishandling, oxidation, or improper storage. Both matter.
How to read a purity figure like a chemist
- What was the detection wavelength? (220 nm is standard; verify it.)
- What was the column and gradient? (More information signals more rigor.)
- Is the chromatogram included? (Required for full evaluation.)
- What other tests were run on the same batch? (Sterility, endotoxin, heavy metals — purity alone is incomplete.)
- Who issued the COA? (Third-party labs carry significantly more weight than in-house.)
To see what a complete batch certification looks like — purity, identity, sterility, endotoxin, and heavy metals all published together — browse our COA library.
Compliance Notice: American Peptides products are sold strictly for laboratory and academic research purposes only. They are not intended for human or veterinary consumption, diagnosis, treatment, or prevention of any disease. All content on this page is educational in nature and does not constitute medical advice or product claims. Researchers are responsible for handling these compounds in accordance with their institutions safety protocols and applicable laws.
