Guide · 6 min read
What HPLC purity means on a peptide COA
"HPLC purity 99.1%" is the most quoted number on any peptide COA, and the most misunderstood. Here's what the measurement actually is, how to read the chromatogram behind it, and where its limits are.
How HPLC works, in plain English
High-performance liquid chromatography pushes a dissolved sample through a narrow column packed with tiny particles. For peptides, labs almost always use reversed-phase HPLC: the column is coated with a water-repelling (usually C18) material, and the solvent gradually changes from mostly water to mostly acetonitrile. Different molecules stick to the column more or less strongly, so they come out at different times.
A UV detector at the end of the column records how much material leaves at each moment. Peptide bonds absorb UV light strongly around 210–220 nm, so that is the usual detection wavelength. The result is a chromatogram: time on the horizontal axis, detector signal on the vertical axis.
How the purity number is calculated
Each component makes a peak. The software measures the area under every peak, and purity is simply:
Purity (%) = area of main peak ÷ total area of all peaks × 100
So a 99.1% result means 0.9% of the detected signal came from other peaks. In synthetic peptides, those are usually deletion sequences (missing one amino acid), truncated chains, incompletely deprotected residues, or oxidized forms.
Reading the chromatogram
What a good chromatogram shows:
- One tall, sharp main peak with only small peaks around it.
- Labeled axes: retention time in minutes and signal in mAU.
- A peak table listing each peak's retention time, area and area %.
- Method details: column type, wavelength (e.g. 214 nm or 220 nm), gradient and flow rate.
What should make you suspicious: a perfectly smooth peak with zero baseline noise, no axes or units, a chromatogram that looks identical across different products, or a purity number that doesn't match the peak table.
What purity level is "good"?
| HPLC purity | Interpretation |
|---|---|
| ≥ 99% | Excellent. Typical of a carefully purified research peptide. |
| 98 – 99% | Good. The benchmark most reputable suppliers advertise. |
| 95 – 98% | Fair. 2–5% is something else. Ask why. |
| < 95% | Poor for a research-grade peptide. |
| 100.0% | Unusual. Real samples almost always show trace impurities. |
Context matters. A three-residue peptide like GHK-Cu or the four-residue Epitalon is easy to make cleanly, so 96% there is more worrying than 96% for a 39-residue lipidated peptide like tirzepatide. For a fuller breakdown of each band, including label claims and content, see what is a good HPLC purity for peptides.
What HPLC purity can't tell you
- Identity. HPLC can't tell you which molecule the main peak is. The wrong peptide can be 99% pure. Always pair HPLC with mass spectrometry.
- Quantity. Purity is a ratio, not an amount. It doesn't confirm a vial holds 10 mg.
- Non-UV contaminants. Salts, water and some contaminants don't show up at 214 nm. Endotoxin needs a separate test.
- Your batch. A purity figure only describes the batch tested. Check that the batch number matches your vial.
HPLC purity vs net peptide content
These get confused constantly. HPLC purity is the share of peptide-related material that is the target peptide. Net peptide content is the share of the powder's total weight that is peptide at all. Freeze-dried peptides contain counter-ions (acetate, TFA) and bound water, so net content is often 70–90%, even when HPLC purity is above 99%. Both numbers are legitimate. They answer different questions.
Check the purity and mass on your COA in seconds.
Use the free COA checker →Sources
- U.S. FDA: Common deficiencies in comparative peptide impurity profile studies (presentation): How regulators treat individual peptide impurities.
- PubChem (U.S. National Library of Medicine): Reference formulas and masses.
- PeptideCOACheck methodology: The exact rules the checker applies.
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Frequently asked questions
What does 99% HPLC purity mean?
It means 99% of the UV-absorbing signal detected in the chromatogram came from the main peak, which is presumed to be the target peptide. The other 1% comes from impurities such as truncated or deletion sequences.
Is 98% purity good for a peptide?
Yes. 98% is the benchmark most reputable research-peptide suppliers use. 99% or higher is excellent, and anything below 95% is a warning sign.
Does HPLC purity confirm the peptide's identity?
No. HPLC shows how pure a sample is, not what it is. A completely different peptide can show 99% purity. Identity is confirmed by mass spectrometry, ideally on the same COA.
Is HPLC purity the same as peptide content?
No. HPLC purity is the share of the peptide-related signal belonging to the target. Net peptide content is the percentage of the powder's weight that is actually peptide (the rest is counter-ions such as acetate or TFA, plus water). A 99% pure peptide can have 80% net peptide content.
Educational content only. Not medical advice. Research peptides are sold for laboratory research purposes only.