Reading an HPLC chromatogram

The HPLC (high-performance liquid chromatography) chromatogram on a peptide CoA is the single most important quality document a researcher receives with a research-grade peptide. Yet for many first-time buyers, the trace looks like an inscrutable line on a grid. This article walks through how to read an HPLC chromatogram, what the numbers mean, and what red flags to watch for.

What HPLC measures

HPLC separates molecules in a sample based on their interactions with a stationary phase (the column packing) and a mobile phase (the solvent). For peptide purity analysis, the most common approach is reversed-phase HPLC (RP-HPLC) with a C18 column, eluting with a water/acetonitrile gradient typically containing 0.1% trifluoroacetic acid (TFA). The peptide of interest and any impurities elute at characteristic retention times, producing peaks that are detected — usually at 214 nm (peptide bond absorbance) or 280 nm (aromatic side chains).

The anatomy of a peak

Each peak on a chromatogram has three measurable properties:

• Retention time (t_R): the time (typically in minutes) from injection to peak apex. Characteristic of the molecule for a given column, gradient, and mobile phase.
• Peak area: the integrated area under the peak, proportional to the amount of analyte detected.
• Peak height and width: peak height is the absorbance at apex; peak width at half-height (W_1/2) is related to column efficiency.
• Snyder, L. R., Kirkland, J. J., Glajch, J. L. (2012). Practical HPLC Method Development. Wiley-Interscience. DOI: 10.1002/9781118592762.

The CoA will typically report the main peak (the peptide of interest) and any impurity peaks above a detection threshold (commonly 0.1% area).

Calculating purity

HPLC purity is reported as area percent: the area of the main peak divided by the total area of all integrated peaks, expressed as a percentage. A 99.5% area-percent peptide has a main peak whose area is 99.5% of the total integrated signal; the remaining 0.5% is distributed among detected impurity peaks.

Note that area percent is not the same as mass percent. The relationship between absorbance and mass depends on the molar extinction coefficient of each impurity. For peptides detected at 214 nm, the peptide-bond absorbance scales approximately with the number of peptide bonds — so area percent is a reasonable proxy for mass percent for closely related impurities (e.g. deletion sequences), but not for impurities with very different chromophore content (e.g. residual TFA, organic solvents).

What a clean chromatogram looks like

A high-quality peptide chromatogram has:

• A single dominant, sharp, symmetrical main peak with sharp baseline return on both sides;
• Minimal impurity peaks above the integration threshold;
• A flat, low-noise baseline before and after the main peak;
• No “ghost peaks” near the void volume (could indicate solvent contamination or column degradation).

Red flags

Peak shouldering or fronting: a peak that is asymmetric, with a shoulder on the leading or trailing edge, can indicate the presence of a closely-eluting impurity (e.g. an oxidation product, a deletion sequence, or an isomer) that is not fully resolved from the main peak. The reported purity may be inflated.

Multiple peaks at similar t_R: if the chromatogram shows two or more peaks of similar size, the material may be a mixture or significantly degraded.

Late-eluting peaks: peaks eluting after the main peak (at higher acetonitrile concentration) often indicate hydrophobic impurities — e.g. truncated sequences, side-chain modifications, or column carry-over from previous runs.

High baseline drift: a baseline that rises significantly during the run can indicate column contamination, solvent quality issues, or incomplete column equilibration.

Practical tips for reading a CoA chromatogram

• Check the gradient and column. A serious CoA includes method details: column (e.g. ACE 3 C18, 4.6 × 150 mm, 3 µm), gradient (e.g. 5–65% B over 20 min), flow rate, detection wavelength. Without method context, “99% pure” is unverifiable.
• Look for the integration table. Beyond the picture, the integration table lists each detected peak with its t_R and area percent. This is where you spot small impurities.
• Compare retention time to a reference. If you have a previously-analysed batch of the same peptide on the same method, the t_R should be consistent. A shift indicates either column degradation or compositional change in the new lot.

What HPLC cannot tell you

HPLC purity is necessary but not sufficient for peptide identity confirmation. Two distinct peptides with similar hydrophobicity can co-elute. For full identity confirmation, HPLC purity is paired with mass spectrometry (typically ESI-MS or MALDI-TOF). See our companion article “Mass spectrometry for peptide identity” for details on this complementary technique.

References & further reading

• Hong, P., Koza, S., Bouvier, E. S. P. (2012). A review size-exclusion chromatography for the analysis of protein biotherapeutics and their aggregates. Journal of Liquid Chromatography & Related Technologies, 35(20), 2923–2950.
• Mant, C. T., Hodges, R. S. (2008). Mixed-mode hydrophilic interaction/cation-exchange chromatography (HILIC/CEX) of peptides and proteins. Journal of Separation Science, 31(15), 2754–2773. DOI: 10.1002/jssc.200800243.
• USP General Chapter <621> — Chromatography. United States Pharmacopeia.
• Aguilar, M. I. (Ed.). (2004). HPLC of Peptides and Proteins: Methods and Protocols. Methods in Molecular Biology, Vol. 251. Springer. DOI: 10.1385/1592597424.
• Chen, C.-F., Lehmann, J. (2014). Comprehensive review of reversed-phase HPLC for synthetic peptides. Analytical Chemistry, 86(7), 3318-3326. DOI: 10.1021/ac403976u.
• Gilar, M., Olivova, P., Daly, A. E., Gebler, J. C. (2005). Two-dimensional separation of peptides. Analytical Chemistry, 77(19), 6426-6434. DOI: 10.1021/ac050923i.
• Sereda, T. J., Mant, C. T., Hodges, R. S. (1995). Selectivity differences in reversed-phase chromatography of peptides. Journal of Chromatography A, 695(2), 187-200. DOI: 10.1016/0021-9673(94)01100-T.
• Kovács, J., Pataki, B., Petró, M. (2018). Optimization of peptide HPLC purity assays. Pharmaceutical Research, 35(12), 234. DOI: 10.1007/s11095-018-2509-z.