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An HPLC chromatogram is the record a detector draws as separated components leave the column: signal on the vertical axis, time on the horizontal. Each peak is a component. Its position (retention time) helps identify it, and its area, converted through a calibration curve, gives amount. A number read off a chromatogram is only trustworthy after the resolution, tailing and repeatability checks described below.
Short answer
Read a chromatogram in a fixed order: identify the peak by retention time against a reference run under the same conditions, check how well it is separated from its neighbour, check its shape, check that replicate injections agree, and only then convert peak area to concentration. Skipping to the last step is the most common way to report a number that the data does not support.
Definition and scope
In HPLC the components of a sample are separated on the stationary phase and pass through the detector in succession, and a chromatogram is recorded (Hong Kong Chinese Medicine Regulatory Office, HKCMMS Appendix IV(B)). This page covers how to read that record. How the separation is built is covered in the HPLC guide and in stepwise HPLC method development; column choice is covered in the GC and HPLC column selection guide.
How a chromatogram is used in practice
Identify: retention time against a reference
Peaks are identified by comparing retention times with chemical reference substances run under the same liquid chromatography conditions. A second approach is to spike a reference substance into the sample and check that the suspected peak grows rather than a new peak appearing (HKCMMS Appendix IV(B)). The FDA guidance on analytical procedures asks that chromatographic methods document retention times (RTs) for identification on a reference-standard comparison basis, and relative retention times (RRTs) with acceptable ranges for known and unknown impurities (FDA, Analytical Procedures and Methods Validation for Drugs and Biologics, July 2015).
Quantify: peak area through a calibration curve
HKCMMS describes quantitation from a five-point calibration curve of peak area against concentration, with the slope, y-intercept and r2 recorded. With a diode-array detector, concentration is (A − I) / m, where A is the sample peak area, I the intercept and m the slope. Area is therefore a signal, not an amount, until the curve converts it.
Check the system: suitability
The FDA guidance says system suitability criteria such as peak tailing, precision and resolution acceptance criteria may be required as applicable, and points to its Validation of Chromatographic Methods reviewer guidance and USP General Chapter <621> for chromatographic systems. The values that make a run acceptable come from the method or monograph, not from the chromatogram itself.
The parameters read from a chromatogram
Four measurements are taken directly from peak positions and widths. All formulas and definitions below are from HKCMMS Appendix IV(B).
| Parameter | Formula | Variables | What it tells you |
|---|---|---|---|
| Plate number (n) | n = 5.54 (tR / Wh/2)2 | tR = retention time of the peak; Wh/2 = peak width at half height | A measure of column efficiency |
| Resolution (R) | R = 2(tR2 − tR1) / (W1 + W2) | tR1, tR2 = retention times of adjacent peaks; W1, W2 = their widths | For accurate quantitation, R between the analyte peak and the adjacent peak must be larger than 1.5 unless otherwise specified |
| Tailing factor (T) | T = W0.05h / 2d1 | W0.05h = width at 5% of peak height; d1 = distance from the perpendicular through the peak maximum to the leading edge at 5% height | Peak symmetry; worth inspecting especially when quantitating by peak height |
| Repeatability (RSD) | RSD of at least five replicate injections | Peak area and retention time | Must meet the monograph requirement |
A read-it-in-this-order checklist
- Identify the peak. Does its retention time match the reference substance run under the same conditions, or does it grow when the reference is spiked in?
- Check resolution. Is R against the adjacent peak larger than 1.5, or whatever the method specifies?
- Check tailing. Is T inspected, particularly if the method quantifies by peak height?
- Check repeatability. Do at least five replicate injections give area and retention time RSDs that meet the monograph?
- Convert area to amount. Use the calibration curve, with its slope, intercept and r2, not the raw area.
This order matters because each step protects the next: a misidentified or merged peak makes a precise area meaningless.
Where it matters for labs and quality systems
For regulated work, the chromatogram is the evidence behind three documented decisions: which peak is the analyte (RT or RRT against a reference standard), whether the system was fit to run (resolution, tailing, precision) and what the amount is (calibration). The FDA guidance expects the acceptable RT and RRT ranges and sample result reporting criteria to be written into the method, so a lab reading a chromatogram is checking it against limits that already exist in the procedure. Unexpected peaks are covered in ghost peaks in HPLC; routine upkeep is covered in the HPLC preventive maintenance schedule.
Common misunderstandings
Retention time is not a fixed property of a compound
It is specific to the conditions of the run, which is why identification compares against a reference substance run under the same conditions, and why relative retention times are documented alongside absolute ones.
Peak area is not concentration
Area becomes concentration only through a calibration curve built for that method: (A − I) / m.
A peak at the expected time is not proof of the compound
Matching retention time is a comparison with a reference. Spiking the reference into the sample is a further check that the peak is the intended one.
A good-looking peak can still fail the run
Resolution, tailing and replicate precision are separate checks, and acceptance limits come from the method or monograph rather than from how the trace looks.
Frequently asked questions
What does a chromatogram show in HPLC?
The detector response over time as separated components pass through it in succession. Each component appears as a peak.
How do you identify a peak on a chromatogram?
Compare its retention time with a chemical reference substance run under the same conditions, or spike a reference into the sample and confirm the peak.
What resolution is needed between two peaks?
For accurate quantitation, HKCMMS requires R larger than 1.5 between the analyte peak and the adjacent peak unless the method states otherwise.
How many injections are needed to assess repeatability?
HKCMMS estimates repeatability as the RSD of at least five replicate injections, applied to peak area and retention time.
How is concentration calculated from a peak?
From a calibration curve of peak area against concentration: concentration = (A − I) / m, with A the peak area, I the intercept and m the slope.
Sources
- U.S. FDA (CDER/CBER), Analytical Procedures and Methods Validation for Drugs and Biologics, Guidance for Industry, July 2015. fda.gov. Read 2026-10-29.
- Chinese Medicine Regulatory Office, Department of Health, Hong Kong, HKCMMS Appendix IV(B), Chromatography: HPLC / UHPLC. cmro.gov.hk. Read 2026-10-29.
