Reading an HPLC Chromatogram: Shoulders, Baselines and Deletions

A purity percentage is a summary. The chromatogram it came from carries information the number discards, and reading it is a skill worth having if you buy peptides regularly.

The main peak

Retention time locates the compound under a given method but is not an identity. Two different peptides can elute at the same time under one gradient and separate under another, which is why retention time alone never establishes what a vial contains.

Peak shape matters more than most people assume. A symmetrical peak suggests a well-behaved single species. Tailing often indicates secondary interactions with the stationary phase, common with basic residues. Fronting can indicate column overload — worth noting, because an overloaded column also degrades resolution of nearby impurities, which flatters the purity figure.

Shoulders

A shoulder on the main peak is the single most informative feature to look for. It usually indicates a closely related species that the gradient did not fully resolve — most often a deletion sequence, a peptide missing one residue from an incomplete coupling during synthesis.

Deletion sequences are chemically very similar to the target and elute very close to it. Whether they appear as a separate peak, a shoulder, or nothing at all is a function of the separation, not of the material. A certificate showing a clean, symmetrical peak from a shallow gradient is more informative than a higher percentage from a steep one.

The baseline

A rising baseline is normal in gradient elution as the organic proportion increases. A noisy or drifting baseline makes integration less reliable, and where the analyst sets the baseline directly changes the reported percentage. A chromatogram with a visibly unstable baseline should be read with more caution than the number alone suggests.

Early-eluting peaks

Peaks near the void volume are worth attention. Highly polar impurities, counter-ion artefacts and oxidation products commonly appear there. For peptides containing methionine, tryptophan or cysteine, an early peak that grows across storage is a plausible oxidation signal rather than a synthesis impurity.

What to ask for

A certificate that reproduces the chromatogram is more useful than one reporting a percentage alone, and a method statement — column, gradient, wavelength, run time — is what makes two certificates comparable at all. Ours are published on this site; see how to read a certificate of analysis and why two certificates can disagree.

ExoLabz supplies compounds for laboratory research use only. Nothing on this page is medical advice or a suggestion of human or veterinary use. Certificates of analysis are published on this site.