Reading the Chromatogram Behind Your Result
Peak shape, baseline drift and shoulders: what the raw trace shows that a purity percentage cannot.
How do you read an HPLC chromatogram?
Look at three things in order. Peak symmetry: a fronting or tailing main peak signals column overload or a chemistry problem and distorts integration. Baseline stability: drift or noise inflates or deflates small impurity peaks depending on how integration was drawn. Shoulders: a bulge on the main peak usually means a closely related impurity that integration may have folded into the main result.
The purity percentage is a summary of this trace. The trace is the evidence, and it carries information the summary discards.
Retention time is a weak identifier
Retention time tells you when something eluted, not what it was. Two different compounds can co-elute under one gradient and separate cleanly under another.
This is precisely why chromatographic purity is paired with mass-spectrometric identity rather than identity being inferred from where a peak appeared. A peak in the expected place is consistent with the expected molecule; it is not proof of it.
Frequently asked questions
- What does a shoulder on the main peak mean?
- Usually a closely related impurity co-eluting with the target, most often a deamidation product, an oxidation product, or a deletion sequence missing one residue. These are chemically similar enough to the target that they bind the column almost identically. The practical concern is that integration software may fold the shoulder into the main peak, inflating the reported purity figure, which is why the trace should be reviewed rather than only the number.
About the author
REPLACE: Analytical Lead
Head of Analytical Chemistry · PhD Analytical Chemistry
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