HPLC and mass spectrometry: what each one proves

Two analyses turn up on nearly every peptide certificate, and they are not alternatives. They measure different properties, and each is blind to things the other catches.

HPLC: how clean, not what

High-performance liquid chromatography pushes the dissolved sample through a packed column. Different molecules travel at different speeds, so they come off at different times, and a detector records what passes. The output is a chromatogram: a baseline with peaks on it.

Purity is the area of the main peak as a share of all peak area. That is all it is. The technique tells you that one thing dominates the sample — it does not tell you what that thing is. A vial of the wrong peptide, cleanly made, gives a beautiful chromatogram.

It has two further limits worth knowing. Impurities that happen to travel at the same speed as the main compound co-elute: they sit underneath the main peak and are counted as part of it. And the number depends on the method — column, gradient, temperature — so two labs can report different purities for the same material without either being wrong.

Mass spectrometry: what, within limits

Mass spectrometry ionises the sample and measures mass-to-charge ratio. Compare the measured mass against the mass calculated from the intended sequence, and agreement rules out a great deal: a different compound, a chain missing residues, a substitution that changes the formula.

What it cannot rule out is anything that leaves the mass unchanged. The same amino acids assembled in a different order weigh exactly the same. Leucine and isoleucine are different amino acids with identical masses. Stereochemistry — the same atoms, mirrored — is invisible to it.

Why both, and in which order

Take them separately and each leaves an obvious hole. Purity without identity: 99% pure, of something. Identity without purity: the right peptide, in a sample that may be a third byproduct. Together they answer the two questions a buyer actually has, which is why a certificate carrying only one has stopped halfway.

What to look for on the certificate

• Both analyses present, not one. Identity and purity are separate questions.
• The chromatogram itself, not only the percentage. The trace shows where the impurities sit and how well separated the peaks are.
• The method conditions. Without column, gradient and wavelength, a purity figure cannot be reproduced or compared.
• Measured mass against theoretical mass, stated as both numbers rather than a bare verdict.
• The batch number on the certificate matching the batch number on the vial in your hand.