An HPLC chromatogram is a graph of a measured response during a separation. It can look like a simple series of spikes, but its meaning depends on the axis labels, detector channel and way the graph has been displayed. Begin with those features before reading the largest peak as the whole story.
Read both axes and the detector channel
IUPAC defines a chromatogram as a presentation of detector response or another concentration-related quantity against time or effluent volume. In an ordinary time-based HPLC plot, moving right means moving later in the run. Moving up means a greater value of the displayed response, according to its own units.IUPAC Gold Book — Chromatogram (opens in a new tab)
| Label or feature | What to read from it |
|---|---|
| Time, min | Elapsed time on the horizontal axis |
| Absorbance, mAU | UV response in milli-absorbance units |
| 214 nm | The wavelength selected for this UV channel |
| A separate pressure trace | A system measurement, not the peptide's UV signal |
| An offset in an overlay | A display displacement that may separate traces visually |
A page can contain several plots from the same run. Do not transfer units from one panel to another. A UV chromatogram and an ion-current chromatogram can share a time axis while recording different physical responses. Their vertical numbers are not directly interchangeable.
Separate the baseline, apex and integrated area
The baseline is the underlying signal against which a peak is assessed. The apex is the top of the feature. Height measures its vertical extent above the relevant baseline; area accounts for response over its time span. These are different descriptions of the same plotted feature.
For an original geometric example, imagine two triangular signals above a flat zero baseline. One is 100 response units high and 0.2 minutes wide; the other is 50 units high and 0.4 minutes wide. Both have an area of 10 response-unit minutes. The taller drawing therefore need not represent the larger integrated signal.
Check whether the display has changed
Imagine the same recorded trace plotted once from 0 to 1,000 mAU and again from 0 to 100 mAU. A 10 mAU feature occupies one hundredth of the first vertical range but one tenth of the second. It appears much more prominent without any change in the underlying sample or measurement.
Likewise, a crop covering only the main peak omits the rest of the run. A zoom is useful for examining a shoulder, but it cannot show what lies outside its horizontal limits. Compare the full trace and the enlarged region for their different purposes.
When traces are overlaid, read whether their scales are linked, individually normalised or vertically offset. A figure that rescales each run to its own maximum can help compare timing and shape while hiding an absolute response difference. Record what the display allows you to compare.
Describe the observation before assigning its meaning
A precise first description might be: one dominant UV feature appears in the displayed interval, with a smaller feature on its later side. Naming the smaller feature as a particular impurity requires additional evidence. The picture supplies its appearance and location, not its molecular name.
Heath and Giordani's primary peptide study used UV, fluorescence and mass-spectrometric detection together. The different channels supplied complementary information about the digest. It illustrates why the label above a chromatogram matters: changing what is detected changes what a trace can tell you.Heath and Giordani — Complementary detection of peptide separations (opens in a new tab)
- Read the time range and response units.
- Identify the detector and channel.
- Check zooming, offsets and normalisation.
- Separate visible features from chemical assignments.
Sources and further detail
- IUPAC Gold Book — Chromatogram (opens in a new tab)
Definition C01071. The time/volume axis and measured detector response are distinct quantities.
- Heath and Giordani — Complementary detection of peptide separations (opens in a new tab)
J Chromatography 638, 9–19 (1993), DOI 10.1016/0021-9673(93)85002-o. Primary abstract describing UV, fluorescence and electrospray-MS channels; no new sample findings implied.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.