[2026 update] Why mass spectrometry cannot tell you about an epimer

Why mass spectrometry cannot tell you about an epimer — that is the question, and I have not found it answered plainly anywhere I have looked.

Working through the identity arithmetic and I would like it checked.

retatrutide has a monoisotopic mass close to 4731.3 Da. On an electrospray instrument I would expect to see the multiply charged series rather than the intact singly charged ion, so for the doubly charged species I calculate (4731.3 + 2 x 1.00728) / 2, and for the triply charged the analogous expression.

The observed values in the report sit within a few ppm of those. My question is what that actually establishes, because I have seen people treat a mass match as a purity result and I do not think it is one.

Where I part company with the opening post, and it is a narrow parting.

The honest summary of what a mass result buys you: it narrows the field of what the material could be, considerably. It never closes it, and no certificate should be read as though it had.

If it helps: the failure mode here is usually boring rather than dramatic.

Tandem mass spectrometry: MS/MS fragments the molecular ion and uses fragment masses to confirm identity and detect modifications. A simple identity confirmation by LC-MS does not address post-translational modifications or impurities with the same or very close mass.