How to Read a Chromatogram

What do the two axes show?

The horizontal axis shows how long each component takes to reach the detector after injection. Components that interact more strongly with the column usually appear later. Retention time can support a comparison with a reference analyzed under the same conditions, but it does not prove identity: different compounds can elute at the same time, and method changes can shift a peak.

The vertical axis shows detector response. In peptide methods using ultraviolet detection, it reflects absorbance at the selected wavelength. Together, the two axes show detector response over time.

RelatedHow HPLC separates molecules

What are the baseline and the peaks?

The baseline is the detector signal recorded when no distinct sample component is passing through the detector. An ideal baseline would be flat and stable, but real chromatograms contain some noise and may also show drift. Noise is a rapid fluctuation in the signal. It can affect how confidently a very small peak is distinguished from the baseline. Drift is a gradual movement of the baseline upward or downward during the run. In gradient HPLC, baseline drift may occur because the mobile-phase composition changes over time. The solvents and additives used in the method can contribute differently to ultraviolet absorbance as their proportions change.

For example, a chromatogram may appear highly impure, with peaks throughout the trace. Troubleshooting may reveal that the problem lies in the chromatographic system rather than in the peptide. Before concluding that a sample is impure, the laboratory should confirm whether the signal originates from the sample or the instrument.

A peak is the rise and fall in detector response as a band of material passes through the detector. Its retention time is normally assigned at the maximum response. Peak width also matters. A narrow peak occupies a smaller time interval than a broad peak. When two components have similar retention times, narrower peaks can make them easier to distinguish. Broadening increases the overlap between close components and can make a minor impurity more difficult to resolve.

Peak shape should therefore be evaluated together with retention time and area. A large peak is not automatically a well-resolved or analytically specific peak.

Why does purity use peak area, not height?

Because area captures the detector response across the whole peak. Height measures only the highest point. A short, broad peak can therefore represent as much material as a tall, narrow one. Pharmacopeial methods may use either measure for specific purposes, but the purity percentage is based on peak area.

Same area, different heights

On a peptide certificate, purity is usually reported as area percent: divide the main peak area by the total area of all peaks included in the calculation, then multiply by 100. The harmonized pharmacopeial chapter excludes peaks from solvents and the mobile phase, as well as peaks below the reporting threshold, generally 0.05%. One limitation is that the calculation assumes the included compounds respond similarly at the selected wavelength. This is only approximate: Kuipers and Gruppen found that tryptophan absorbs about 30 times more strongly than a peptide bond at 214 nm.

Integration is the step in which the software decides where a peak begins and ends and where the baseline runs beneath it. The analyst defines the settings, so changing them can change the calculated areas, and therefore the reported purity. FDA data-integrity guidance notes that changing the baseline and reprocessing data can make peaks appear smaller or larger. In a worked example by Blanc and colleagues, an impurity with a true value of 0.100% was reported between 0.028% and 0.104%, depending on how it was separated from the main peak. In practice, it’s essential to establish the integration rules during method development and then apply them consistently. The reported purity can change when integration settings are modified, especially around shoulders and partially resolved peaks.

What do side peaks, shoulders and early bumps mean?

A side peak is a separate detector response appearing before or after the main peak. In a synthetic peptide sample, side peaks may represent peptide-related impurities, degradation products, process-related components, or another substance that responds at the selected wavelength. The chromatogram alone normally cannot establish which explanation is correct.

A shoulder is a distortion on the side of a larger peak. It may indicate a second component that is only partially separated from the main component, although column problems, sample-solvent effects, excessive loading, and other chromatographic conditions can also alter peak shape. If an impurity co-elutes with the target peptide, both responses may appear within one peak. Improving chromatographic resolution may separate the hidden component, while an orthogonal method such as mass spectrometry can help confirm its identity.

Early spikes or dips may arise from the injection, sample solvent, or material that is not retained by the column. These signals occur around the system’s hold-up time. Depending on the method, solvent and mobile-phase disturbances may be excluded from purity calculations.

The practical point is that peak position alone does not explain peak origin. The method, blank, sample preparation, system suitability, peak shape, and any orthogonal data all contribute to the interpretation.

RelatedWhat makes up the rest of a purity figure

What does the trace on a real certificate show?

The certificate of analysis library includes one for a batch of BPC-157, and it prints its chromatogram with the method line "Method: RP-HPLC | Column: C18 | Detection: DAD @ 214 nm".

The readings below are taken by eye from the printed axes, so they're approximate. The vertical axis is labeled mAU and runs from 0 to 3,500. The horizontal axis is labeled Retention Time (min) and runs from 0 to a little past 30. In the first two minutes, the trace dips, spikes to roughly 550 mAU, shows a smaller second spike and dips again. Then it settles. A single tall, narrow peak labeled BPC-157 rises just past 14 minutes to about 2,700 mAU. A few blips, too small to measure by eye, sit near 4, 10, 20 and 26 minutes, and after the main peak the baseline creeps gently upward to the end of the run.

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Swipe sideways to see the whole trace.

• 1The vertical axis: mAU, from 0 to 3,500.
• 2The horizontal axis: retention time in minutes, from 0 to a little past 30.
• 3The first two minutes: a dip, a spike to roughly 550 mAU, a smaller second spike and another dip.
• 4One tall, narrow peak labeled BPC-157, just past 14 minutes, at about 2,700 mAU.
• 5Blips too small to measure by eye, near 4, 10, 20 and 26 minutes (the arrowheads).
• 6After the main peak, the baseline creeps gently upward to the end of the run.

What the printout doesn't include is a peak table: no retention-time figures and no areas. You can see the shape behind the certificate's purity result of 99.621%, but you can't recompute that number from the picture. how to read a certificate of analysis walks through the rest of the document.

BPC-157 on this siteBPC-157 ReferenceBPC-157 ResearchRegeneration Peptides

What can't a chromatogram tell you?

A chromatogram does not establish identity on its own. A peak at the expected retention time shows that a detected component reached the detector at that point under that method. It does not prove that the component has the expected molecular mass or sequence. Identity requires additional evidence, commonly including mass spectrometry.

A chromatogram also cannot show a component that the detector does not adequately detect. Detector response depends on the wavelength and chemical structure. A component with little response under the selected conditions may be underrepresented.

Finally, a chromatogram does not measure net peptide content. Water, counter-ions, residual solvents, and other contributions to powder weight are not represented in the HPLC area percentage in the same way as resolved peptide-related impurities.

A chromatogram should therefore be interpreted together with the HPLC result, integration information, identity data, net peptide content, and method conditions. No single trace provides a complete assessment of a peptide sample.

RelatedHow mass spectrometry confirms identity