The purity figure on a certificate is a summary of the chromatogram. The trace itself carries information the summary discards, and it takes about thirty seconds to read once you know what to look at.
The axes
Time runs along the bottom — how long each component took to travel through the column and reach the detector. Absorbance runs up the side. A component that interacts more strongly with the column stays longer and appears further right.
The main peak
Tall, roughly symmetrical, and accounting for most of the total area. Two features are worth attention.
Symmetry. A peak that rises sharply and trails off slowly — tailing — can indicate secondary interactions with the column, overloading, or a sample that is not fully dissolved. Mild tailing is routine; pronounced tailing is worth a question.
Shoulders. A bump on the flank of the main peak is a closely related impurity the gradient did not fully resolve. This is the most common finding worth following up, because a shoulder is usually a deletion sequence or an oxidised form — structurally similar enough to co-elute, and therefore sometimes integrated into the main peak, inflating the purity figure.
The rest of the trace
Small peaks elsewhere are related substances, reagent residues, or degradation products. What matters is their distribution. A single 1.5% peak and a dozen 0.15% peaks both leave you at 98.5% purity but describe different materials — the first is one identifiable impurity, the second is general degradation.
Peaks very early in the run, near the solvent front, are usually salts and small molecules that do not interact with the column. They are often excluded from integration.
The baseline
It should be flat and quiet. A drifting baseline is normal in gradient runs as the mobile phase composition changes. A noisy or wandering baseline makes integration unreliable, which makes the purity figure unreliable with it.
Retention time
Consistency across lots is a useful identity signal. The same sequence on the same method should elute at close to the same time. A material that suddenly elutes noticeably earlier or later is worth asking about, even if the purity figure is unchanged.
What the trace cannot tell you
It does not identify anything. A single clean peak confirms that one thing is present in high proportion, not that it is the molecule on the label — that is what mass spectrometry establishes. And it says nothing about how much of the powder is peptide, which is the peptide content figure.
Purity, identity and content are three separate questions. A complete certificate answers all three.