"99% pure" is the most common claim in research peptides and the least explained. The number comes from high-performance liquid chromatography, and it is a real, useful measurement. It is also a narrow one. This article explains how the number is produced so you can judge what it proves.
How the measurement works
A small volume of the reconstituted peptide is injected into a column packed with fine particles. A solvent gradient pushes the sample through. Different molecules interact with the column differently and exit at different times, which separates the mixture into its components. A detector at the outlet, usually ultraviolet absorbance, records a signal as each component passes. The result is a chromatogram: a plot of signal against time, with one peak per component.
Purity is the area of the main peak divided by the total area of all peaks, expressed as a percentage. A chromatogram with one tall peak and almost nothing else is what a 99%+ result looks like.
Why the wavelength matters
Peptides are usually detected at 214 or 220 nanometers, where the peptide bond absorbs. Some certificates report at 280 nanometers instead, which only detects aromatic residues. A peptide with few aromatic amino acids will show a weak signal at 280, and impurities without aromatics will not show at all. The wavelength should be stated on the certificate, and 214 or 220 is the standard for a purity claim.
What the impurity peaks are
Solid-phase peptide synthesis builds a chain one residue at a time. Each coupling step is slightly less than perfect, so a small fraction of chains miss a residue (deletion sequences) or carry a residue that did not deprotect cleanly (protected sequences). Oxidation of methionine, deamidation of asparagine, and aggregation add more minor peaks. These are the shoulders and small bumps next to the main peak. For a compound like semaglutide with a fatty-acid side chain, incomplete attachment of that chain is another common minor species.
What the number cannot tell you
HPLC purity is about the peptide fraction. It says nothing about:
- Identity. A 99% pure sample of the wrong peptide is still 99% pure. Identity comes from mass spectrometry, covered in our COA guide.
- Net peptide content. Water and counter-ions in the powder are not seen by the detector at all.
- Endotoxin. Bacterial cell-wall fragments do not absorb at the peptide wavelength in any meaningful way.
- Microbial contamination. Living organisms are not detected by the method.
- Heavy metals. Inorganic contaminants require ICP-MS or a similar elemental method.
We wrote about those gaps in the tests a purity number cannot see.
Area percent versus true content
The purity figure assumes every species absorbs light equally at the detection wavelength. That is close to true for peptide-bond absorbance at 214 nm, because every residue contributes one bond, but it is not exact. A truncated impurity with fewer bonds absorbs less and is slightly under-counted. Non-peptide contaminants that do not absorb at all are invisible. So area percent is a good relative measure and a poor absolute one, which is why serious certificates pair it with net peptide content by amino-acid analysis or nitrogen determination.
Reading a chromatogram yourself
If a vendor attaches the trace, three things are worth a look. One main peak, sharp and symmetrical. Small satellite peaks rather than a broad hump, which suggests aggregation or degradation. A flat baseline before and after, which suggests the run was clean. A peak that splits into two of similar size is a reason to ask what the second species is.
Why the same compound can show different purity at different vendors
Synthesis route, purification effort and the wavelength used all move the number. A longer, more complex peptide such as tesamorelin at forty-four residues is harder to purify to the same figure as a short one, so a slightly lower value on a long peptide is not automatically worse quality. Comparing two vendors only works if both report the method and wavelength.
Reverse-phase columns and why peptides suit them
Almost all peptide purity work uses reverse-phase HPLC. The column packing is hydrophobic, usually C18 silica, and the mobile phase starts water-rich and becomes more organic over the run, typically with acetonitrile. A small amount of trifluoroacetic acid is added to sharpen peaks by pairing with charged residues. Peptides elute roughly in order of hydrophobicity, so a lipidated compound such as semaglutide comes off late and a short hydrophilic peptide comes off early. Knowing this helps interpret where the impurities sit: a shoulder just before the main peak is often a deletion sequence, and a peak well after it is often an aggregate or a doubly lipidated species.
Method details worth asking for
- Column chemistry and dimensions.
- Gradient program and run time.
- Detection wavelength.
- Injection amount, since overloading a column broadens peaks and can hide small impurities.
- Whether the reported purity is area percent, which is the norm, or a calibrated assay against a reference standard, which is rarer and more rigorous.
Frequently asked questions
Is 98% purity bad?
Not by itself. The question is what the other 2% is and whether it matters for the model. Deletion sequences are usually inert. Oxidized species can behave differently. The chromatogram and the mass spec together answer that.
Does purity change after reconstitution?
It can. Peptides in solution degrade faster than lyophilized powder, so a purity value applies to the sealed vial at the test date. Our storage guide covers this.
Why do coenzymes like NAD+ show different chromatograms?
NAD+ is not a peptide, so it is analyzed with a different method and wavelength. The same purity principle applies, but the trace looks different and should not be compared with a peptide's.
References
- Mant CT, Hodges RS. 1991. High-Performance Liquid Chromatography of Peptides and Proteins: Separation, Analysis, and Conformation. CRC Press.
- Aguilar MI, ed. 2004. HPLC of Peptides and Proteins: Methods and Protocols. Methods in Molecular Biology.
- D'Hondt M, et al. 2014. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis.
Research use only. This article is analytical background for laboratory researchers. Homegrown Peptides products are not for human or animal use.
