Reading an HPLC Chromatogram: Peaks, Retention Times, and Purity

The chromatogram is the heart of the COA

A certificate of analysis reports purity as a percentage, but that number comes from a single source: the HPLC chromatogram. Learning to read that chromatogram turns a certificate from a number you take on trust into evidence you can evaluate.

High-performance liquid chromatography separates the components of a mixture based on how strongly each one interacts with the stationary phase of the column versus the mobile phase flowing through it. The output is a plot of detector response (usually UV absorbance) against time, with each compound appearing as a peak at its characteristic retention time.

The main peak

On a peptide COA, the dominant peak is the target peptide. Its retention time — the point on the time axis where it appears — should match the reference standard for that compound. A peptide's retention time is reproducible under fixed conditions (column, solvent, flow rate, gradient), so a main peak at the expected retention time is the first confirmation of identity.

The area under the main peak, expressed as a percentage of the total area of all peaks, is the purity figure. A single sharp peak taking up 99% of the total area is the signature of a high-purity product.

Impurity peaks

Smaller peaks elsewhere on the chromatogram represent impurities. Their origin and significance vary:

• Truncated sequences and deletion peptides. Incomplete coupling during synthesis can produce shorter chains. These often elute at different retention times and show up as small pre- or post-main peaks.

• Protecting-group remnants. Incompletely deprotected side chains can produce related impurities that chromatograph near the main peak.

• Adducts and oxidised forms. Oxidation of sensitive residues can create slightly different species that appear as shoulders on the main peak or as separate small peaks.

• Solvent or system peaks. Some small peaks are artefacts of the mobile phase or injection, not the sample. A reputable COA distinguishes these from genuine sample impurities.

Peak shape

The shape of the main peak matters as much as its size. A sharp, symmetrical, narrow peak indicates a well-behaved compound under the method used. A broad, tailing, or fronting peak can indicate column overload, a poorly optimised method, or a heterogeneous sample — all reasons to look more closely.

What 99%+ purity actually means

When a COA reports 99%+ purity by HPLC, it means the integrated area of all impurity peaks totals less than 1% of the main peak's area. This is an area-percent measurement, not a mass measurement, and it is specific to the wavelength and method used. A different method might reveal impurities invisible at the original wavelength, which is why rigorous QC uses orthogonal methods alongside HPLC.

Practical checks

1. Confirm the main peak's retention time matches the reference.
2. Confirm the purity percentage matches the integrated area of the main peak.
3. Scan for any impurity peak above roughly 0.1% area and check whether it is annotated.
4. Check that the method (column, gradient, wavelength) is stated so the result is reproducible.

A chromatogram you can read is a certificate you can trust. The percentage is the summary; the peaks are the proof.