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HPLC Explained: How Peptide Purity Is Measured
How reversed-phase HPLC separates a peptide from its impurities, how to read a chromatogram, and why the reported purity depends on the method.
Bench card
- Reversed-phase HPLC separates peptides by hydrophobicity using a water-to-acetonitrile gradient.
- Detection at 214–220 nm sees the peptide bond, so nearly all peptide species are detected.
- Purity is the main peak area as a percentage of all integrated peaks.
- Co-eluting impurities, integration choices and wavelength can all change the number.
- A purity figure means little without the chromatogram and the method behind it.
High-performance liquid chromatography (HPLC) is the standard method for measuring peptide purity. Almost every peptide COA reports an HPLC result. Understanding how it works makes it much easier to judge what that number tells you.
How reversed-phase HPLC works
The sample is injected into a stream of solvent pumped through a column packed with small particles coated with hydrophobic chains, usually C18. Peptides stick to this surface. The solvent starts mostly aqueous and becomes steadily richer in an organic solvent, typically acetonitrile, over the run. As it does, peptides release from the column in order of increasing hydrophobicity. A small amount of acid, often 0.1% trifluoroacetic acid or formic acid, sharpens the peaks. Mant and colleagues give a detailed account of peptide HPLC method choices.
Detection
A UV detector measures absorbance as material leaves the column. At 214–220 nm the peptide bond itself absorbs, so nearly all peptide species are detected, roughly in proportion to their number of peptide bonds. At 280 nm only tryptophan and tyrosine absorb strongly, so impurities lacking them may be missed. Kuipers and Gruppen showed how absorbance at 214 nm can be predicted from amino acid composition, a reminder that equal areas do not always mean equal amounts.
Reading a chromatogram
- x-axis: retention time, the minutes after injection.
- y-axis: detector response, usually in milli-absorbance units (mAU).
- Main peak: the target peptide; ideally tall, narrow and symmetrical.
- Small peaks: impurities such as deletion sequences, truncations, incompletely deprotected species, oxidized or deamidated forms.
- Early peaks: often salts, solvent or the injection disturbance; usually excluded from integration.
How purity is calculated
purity (%) = main peak area ÷ total integrated peak area × 100
That definition hides several judgment calls:
| Choice | Effect on reported purity |
|---|---|
| Detection wavelength | 280 nm can miss impurities without aromatic residues |
| Gradient steepness | A steep, fast gradient can merge impurities into the main peak |
| Integration thresholds | Ignoring small peaks raises the figure |
| Run time | A short run may miss late-eluting impurities |
| Column choice | Different selectivity separates different impurities |
Limits of HPLC
- Impurities that co-elute with the main peak are counted as main peak. Mass spectrometry of the main peak can reveal them.
- Non-UV-absorbing contaminants, such as salts and many counterions, are invisible.
- HPLC purity says nothing about how much of the powder is peptide.
Pairing HPLC with mass spectrometry (LC-MS) addresses the first of these. See Mass Spectrometry Explained.
References
- Mant CT, Chen Y, Yan Z, Popa TV, Kovacs JM, Mills JB, Tripet BP, Hodges RS. HPLC analysis and purification of peptides. Methods Mol Biol. 2007;386:3–55. DOI
- USP General Chapter <621> Chromatography. United States Pharmacopeia.
- Kuipers BJH, Gruppen H. Prediction of molar extinction coefficients of proteins and peptides using UV absorption of the constituent amino acids at 214 nm. J Agric Food Chem. 2007;55(14):5445–5451. PubMed
- ICH Q2(R2). Validation of Analytical Procedures. International Council for Harmonisation, 2023. ICH
- D'Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, De Spiegeleer B. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2–30. DOI