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What is retention time co-elution?
The short answer
Co-elution is when two different compounds emerge from the column at the same time and appear as a single peak. It matters because chromatographic identity assumes one peak equals one compound. Co-eluting material can inflate a purity figure or mask an impurity, and only orthogonal evidence such as mass spectrometry or a different method resolves it.
In detail
Closely related molecules co-elute most readily — a degradation product differing by one residue, or an analogue from the same family. Those are precisely the impurities most likely to appear in synthetic peptide material.
Laboratories reduce the risk with gradient design, peak-purity checks using diode-array detection, and where necessary a second method with different selectivity.
It is also the honest reason retention-time identity is called presumptive. The technique's main limitation is stated rather than glossed over.
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