High-Performance Liquid Chromatography (HPLC) is the industry-standard analytical method for assessing peptide purity. Understanding what an HPLC report actually measures — and what it does not — is essential for interpreting any Certificate of Analysis.
What HPLC Actually Measures
HPLC separates the components of a sample by passing the dissolved sample through a column packed with a stationary phase, using a moving liquid mobile phase. Different compounds interact differently with the stationary phase and emerge at different times — their “retention time.”
The detector at the column outlet records a signal (typically UV absorbance) over time, producing a chromatogram with peaks. Each peak represents a distinct compound. The area under each peak is proportional to the amount of that compound in the sample.
How Purity Is Calculated
For peptides, purity is most commonly reported as Area Percent (Area %):
If the main peak (your target peptide) represents 98.5% of the total detected area, the sample is reported as 98.5% pure by HPLC.
Important Subtleties
- Area % is detector-dependent. If an impurity does not absorb UV at the wavelength used (commonly 220 nm for the peptide bond), it will not contribute to the area total. Purity figures are always relative to what the detector can see.
- Co-eluting impurities are invisible. If an impurity has a retention time identical to the target peak, the two will overlap and be counted as one peak. This is why method development matters.
- Solvent and salt content are typically not in the chromatogram. Net peptide content (after accounting for water, counter-ions, and residual solvents) is a separate measurement — often reported by elemental analysis or amino acid analysis.
Reading an HPLC Chromatogram
A typical chromatogram has time on the x-axis and detector signal on the y-axis. You will see:
- The main peak — your target peptide, usually the largest peak.
- Smaller adjacent peaks — typically synthesis-related impurities (deletion sequences, oxidation products, incomplete deprotection products).
- The solvent front — an early signal where unretained material elutes. This is normally excluded from the purity calculation.
- The baseline — the detector signal in the absence of any eluting compound. A clean, flat baseline indicates good method conditions.
Common Method Variations
| Method | Use Case |
|---|---|
| Reverse-Phase HPLC (RP-HPLC) | Most common for peptide purity. Separates by hydrophobicity. |
| Size-Exclusion HPLC (SEC) | Detects aggregation, dimer/trimer formation, and large impurities. |
| Ion-Exchange HPLC (IEX) | Separates by net charge. Useful for characterizing charge variants. |
| LC-MS (HPLC + Mass Spec) | Confirms identity by molecular mass in addition to purity. |
What a Good COA Should Show
A credible Certificate of Analysis includes:
- The product name and sequence.
- The lot or batch number.
- The HPLC method used (column, mobile phase, gradient, detector wavelength).
- The chromatogram image, not just the final purity number.
- Mass spectrometry confirmation of identity (molecular weight match).
- The analytical laboratory’s name and the date of analysis.
Why Independent Testing Matters
A manufacturer testing its own product is doing in-house quality control. Independent third-party testing — where a separately incorporated, accredited laboratory analyzes a sample with no commercial relationship to the outcome — is the gold standard for transparency. The same chromatographic method can be run by both parties, and the agreement (or disagreement) between the two reports is itself a meaningful data point.
Limitations to Be Aware Of
- HPLC purity is not the same as net peptide content. A sample can be 99% pure by HPLC but contain only 80% peptide by mass once water, salts, and counter-ions are accounted for.
- Different methods can yield slightly different purity numbers for the same sample.
- Purity is meaningful only relative to a stated method.
Understanding what HPLC does and does not measure is the foundation of intelligent COA interpretation. When in doubt, look at the chromatogram itself — the shape, the baseline, the impurity peaks — not just the headline percentage.