A chromatogram can look intimidating at first glance — a line with a few spikes on it — but the underlying logic is straightforward once you know what each feature represents. This is a plain-language walkthrough for reading one attached to a Certificate of Analysis.

The x-axis: retention time

As a sample moves through the HPLC column, different compounds travel at different speeds depending on their chemical properties. Retention time is simply when a compound exits the column and reaches the detector — expressed in minutes. It's mainly useful as a consistency check between batches run under the same method, rather than a meaningful number on its own.

The y-axis: detector response

The height of the trace reflects how strongly the detector is responding at that moment — generally proportional to how much of a compound is passing through. A tall, narrow spike (a "peak") indicates a concentrated band of a single compound.

Peak area vs. peak height

Purity calculations use peak area (the space under the curve), not peak height. Area is a more reliable measure of quantity because it accounts for peak width — two peaks of the same height but different widths represent different amounts of material.

Baseline and drift

The "baseline" is the flat line the trace returns to between peaks. Gradual upward or downward drift in the baseline (rather than a flat line) is often a sign of instrument or column conditions rather than sample impurity, and a well-run report typically corrects for it before final calculations.

Calculating percentage purity

The standard calculation divides the target compound's peak area by the total area of all detected peaks, then multiplies by 100. A report showing "99.2% purity by HPLC (area %)" means the target peak accounted for 99.2% of everything the detector saw — with the remainder attributed to minor impurities or synthesis byproducts.

What to look for as a reviewer