HPLC and Mass Spectrometry: What Each Test Proves
Purity and identity are different questions, answered by different instruments. Conflating them is the most common analytical error in evaluating a peptide supplier.
What HPLC answers
Reverse-phase HPLC separates a mixture by hydrophobicity. The sample is injected onto a column packed with a hydrophobic stationary phase and eluted with an increasing gradient of organic solvent; components emerge in order of how strongly they bind. A UV detector — 214 nm for the peptide bond, 280 nm for aromatic residues — records absorbance against time.
The question HPLC answers is: of everything the detector could see, what fraction was the main component? That is a relative measure, bounded by the detector’s sensitivity and the method’s ability to resolve closely related species. Deletion sequences and diastereomers can co-elute with the target under a poorly chosen gradient, inflating the apparent purity.
What mass spectrometry answers
MS ionises the sample and measures mass-to-charge ratio. For peptides, electrospray ionisation produces a series of multiply charged ions from which the molecular mass is deconvoluted and compared to the theoretical mass of the declared sequence.
The question MS answers is: is this the molecule it claims to be? It is exquisitely sensitive to composition — a single amino acid deletion shifts the mass by that residue’s exact weight, unmistakably. It is comparatively poor at quantifying relative abundance, because ionisation efficiency varies between species.
Why a lot needs both
| HPLC alone | Tells you the sample is homogeneous. Does not tell you what it is. |
|---|---|
| MS alone | Tells you the target is present. Does not tell you how much else is. |
| Both | Tells you the sample is predominantly one species, and that species is the declared compound. |
A certificate carrying only one of these leaves a real gap. Ask for both, and ask for the chromatogram and the spectrum rather than only the summary figures — the raw traces show peak shape, baseline quality and resolution, which the numbers alone conceal.
Orthogonal methods
For work where the purity figure is load-bearing, a second orthogonal method is worth requesting: amino acid analysis for absolute peptide content, or a second HPLC method with a different stationary phase or pH. Impurities that co-elute under one set of conditions rarely co-elute under two.