How to Read an HPLC Chromatogram
TL;DR · The short version
To read an HPLC chromatogram, start with the axes and method, then the assignments and integration table. Area percentage describes included detector response. The chart’s shape alone doesn’t establish identity or vial content.
A large peak accompanies a purity result of 98%. Before judging it, find the calculation and method behind that percentage.
The chromatogram and separate numerical example here are made up for explanation. They aren’t Pure Lab Peptides batch results.
1. Read the axes before the peaks
TL;DR: The horizontal axis usually shows time; the vertical axis shows detector response.
Retention time locates a peak along the time axis. Peak height measures the signal above the baseline at the peak maximum. Peak area accounts for the signal across the integrated peak, so height and area are different measurements. Shimadzu’s guide to reading a chromatogram illustrates these definitions.
Read the detector units as well as its name. UV may report absorbance or milli-absorbance; another detector may use a different scale.
2. Ask how the main peak was assigned
TL;DR: A retention-time match can support an assignment, but a peak is not a complete identity certificate.
Retention time comparisons need matching chromatographic conditions. A time from another method isn’t an interchangeable reference.
Also, one visible peak can contain coeluting material. A peptide LC–high-resolution-MS study showed how mass-spectrometric evidence could distinguish and quantify some related impurities even when they coeluted. Its results depended on the peptides and method evaluated.
The peak label gives the assignment, not necessarily its supporting evidence. Find the stated basis of identification in the report.
3. Read the denominator in the purity calculation
TL;DR: Area percentage compares one integrated peak with the total area included in that calculation.
Suppose a hypothetical integration table contains these three entries:
| Included peak | Integrated area |
|---|---|
| Main peak | 9,800 |
| Other peak A | 150 |
| Other peak B | 50 |
| Total included area | 10,000 |
The calculation is 9,800 ÷ 10,000 × 100 = 98%. That is the main peak’s share of the included response.
It does not establish that 98% of everything in the vial is that peptide. Detector response, the peaks included and the reporting basis matter. An amount claim needs a suitable quantitative method and calibration or reference evidence, as the identification and quantitation primer describes.
4. Check what the software counted
TL;DR: The percentage depends on integration choices as well as the recorded signal.
Match integration-table entries to the plotted peaks. Check the included peaks, baseline and any excluded signals or manual changes. Those decisions affect the percentage you’re reading.
A shoulder, a broad peak or a drifting baseline deserves a method-specific explanation. The Waters HPLC troubleshooting guide illustrates why peak shape and baseline problems need investigation. A screenshot alone usually cannot identify their cause.
If cropping hides these details, request the complete report. A high percentage becomes interpretable when its calculation is available.
5. Use the report to answer a specific question
TL;DR: Request the missing evidence by name rather than asking whether the chromatogram “looks good.”
- Identity: How was the assigned peak identified?
- Purity: Which responses were included in the reported percentage?
- Quantity: Was content measured, in what units and on what basis?
- Traceability: Do the sample identifier, lot and test date match the material being reviewed?
Keep the reported result separate from your interpretation. “98% area” doesn’t become “98% of vial mass” without evidence supporting that conversion.
For a comparison of the techniques, continue with HPLC vs LC-MS vs NMR.