HPLC vs. Mass Spectrometry: How Purity Is Verified

Two analytical techniques do most of the work in peptide characterisation: high-performance liquid chromatography (HPLC) and mass spectrometry (MS). They answer different questions, which is why neither alone is sufficient.

What HPLC measures

In reversed-phase HPLC, the sample is driven through a hydrophobic stationary phase under a changing solvent gradient. Components partition between mobile and stationary phases according to hydrophobicity and therefore elute at different times, producing a chromatogram of resolved peaks. Integrating the area under the target peak against total peak area yields the reported purity percentage. HPLC is quantitative about proportions but says nothing definitive about what any peak chemically is.

What mass spectrometry measures

Mass spectrometry ionises the sample and separates ions by mass-to-charge ratio. For peptides, electrospray ionisation typically produces a series of multiply charged states that deconvolute to a single molecular weight. Comparing that observed mass to the theoretical mass of the target sequence confirms identity. Tandem MS can fragment the peptide backbone to read sequence information directly.

Why the combination matters

A sample can be 99% one substance that is not the intended peptide — HPLC would report high purity while the material is wrong. Conversely, MS can confirm the target is present without quantifying how much of the sample is something else, such as deletion sequences missing a single residue. Deletion impurities are chemically similar to the target and are precisely the class of contaminant that co-elutes poorly and misleads a single-method assessment.

Orthogonality as a principle

Analytical chemists refer to this as orthogonal verification: two methods relying on different physical properties are far harder to fool than one. Applied to research material, it means identity and purity are established independently rather than inferred from a single chromatogram.

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