HPLC and mass spectrometry answer different questions

01 · Two questions, not one

There are two separate things worth knowing about material in a vial. How much of it is the main component, and what that main component actually is. Chromatography answers the first. Mass spectrometry answers the second. Neither answers the other.

A certificate carrying only a purity figure has answered half the question, and it is the half that can be satisfied by any sufficiently pure wrong molecule.

02 · What HPLC establishes

High-performance liquid chromatography separates a mixture by pushing it through a column. Different species move at different speeds, emerge at different times, and register as peaks. Integrating the area under each peak gives the relative proportions.

That yields a purity figure and, more importantly, a shape: how many other things are present and how close they sit to the main peak. What it does not yield is identity. A peak is a arrival time, not a name.

03 · What mass spectrometry establishes

Mass spectrometry ionises the sample and measures mass-to-charge ratio. For a peptide of known sequence the expected mass is arithmetic — sum the residues, subtract water for each bond. A measured mass matching the calculation to within a small tolerance is strong evidence the molecule is what it is claimed to be.

It is not conclusive on its own. Different sequences can share a mass, which is why sequence confirmation for a novel or disputed compound uses tandem MS, fragmenting the peptide and reading the pieces.

04 · Why publishing only one is a choice

HPLC alone

Establishes proportion. Silent on identity.

MS alone

Establishes identity of the main species. Silent on how much else is present.

Both, same lot

Establishes that the intended molecule is present and is the dominant component.

Both, different lots

Establishes very little, and is more common than it should be.

The last row is the one to watch. A certificate pairing an HPLC trace from one lot with an MS result from another has documented two batches and characterised neither.

05 · The failure modes each method misses

A deletion sequence — the peptide minus one residue — differs in mass and often elutes close to the target. MS catches it. A coarse HPLC method may not resolve it.

A correctly-identified but heavily contaminated batch passes MS and fails HPLC.

An aggregated peptide can pass both while behaving differently in use, which is why aggregation is called out separately in the literature for compounds where it is a known problem.

The methods are complements, not alternatives, and a supplier choosing one is choosing which failure to remain silent about.