Peak area: how a chromatogram becomes a number

What is peak area?

As each compound leaves the column, the detector signal rises and falls, drawing a peak on the chromatogram. Peak area is the area enclosed by that peak above the baseline. Software calculates it by integration: it decides where the peak starts and ends, draws the baseline beneath it, and adds up the signal in between.

Peak height is sometimes used instead, but area is the usual measure for peptides because it is less sensitive to small changes in peak shape.

How does peak area become a purity figure?

For HPLC purity, the analyst divides the main peak's area by the total area of every integrated peak. This is area percent. It is a ratio inside one chromatogram, so it needs no reference standard, which is why it is cheap and common.

Area percent assumes that each compound gives the same signal for the same amount of material. That is roughly true for closely related peptides, but not exactly. In a study of synthetic glucagon, the authors concluded that UV purity results were not accurate because some impurities do not absorb UV light while others overlap with the main component (Wang et al., 2020).

How does peak area become an amount in milligrams?

By comparison with a known quantity. The laboratory runs a reference standard of known content, records its peak area, and builds a relationship between area and concentration. The sample's main peak area is then read against that relationship to give milligrams. This is an assay, and it is how an amount is checked against a label claim.

Validation guidance expects a laboratory to show that the response, here the peak area, is proportional to concentration across the range the method is used for, usually with a regression line (ICH Q2(R2)). That property is linearity. Pharmacopeial work on peptide reference materials uses this same approach, assigning a content to a bulk standard and then using it in an HPLC assay to give the content of vialled material (McCarthy et al., 2023).

What can make a peak area wrong?

• Integration choices. Where the software or analyst sets the start, end and baseline of each peak changes the area, especially for small or overlapping peaks.
• Co-elution. If an impurity sits under the main peak, its area is counted as main peak. See co-elution.
• Small peaks ignored. Peaks below a reporting threshold may not be integrated, which raises the main peak's share.
• Detector saturation. A very concentrated sample can flatten the top of a peak and understate its area.
• Different response. Compounds that absorb UV weakly produce small areas even when present in quantity.

What should a reader look for on a certificate?

A peak table listing each peak's retention time, area and area percent is a good sign, because it shows what was integrated. An attached chromatogram lets you see whether the baseline is flat and whether small peaks were left out. A purity percentage with no peak table and no trace cannot be checked against anything.

If the certificate reports an amount, look for the words assay or quantified against a reference standard. An amount derived from area percent alone is not an amount at all.

The lesson Reading a chromatogram shows integrated traces in detail, and Amount versus label explains how an assay turns area into milligrams. The method itself is on HPLC-UV.