Analytics · Mass spectrometry

MALDI versus ESI for a quick identity check

Analytics › Mass spectrometry high-resolution MS charge states mass error (ppm) MALDI-TOF

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r.marsdenTL3Regular5 Aug 2025#1

On the subject in the title: MALDI versus ESI for a quick identity check Working notes rather than a conclusion.

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

tirzepatide has a monoisotopic mass close to 4813.5 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 (4813.5 + 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.

2 likes 12mo

to.vargaTL2 Moderator21 Aug 2025#2

Resolution: "high resolution" commonly means <5 ppm across the mass range. Unit-resolution instruments achieve ±1 Da at best and cannot distinguish two species differing by less than 1 Da in total mass.

6 likes 11mo

OTeixeiraTL3Regular2 Sep 2025#3

to.varga, post #2: Resolution: "high resolution" commonly means Go to post

post #2 is right about the mechanism and I think understates the practical bit.

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.

16 likes in reply to #2 11mo

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Counter-ion content and its effect on measured massAnalytics › Mass spectrometry · 108 replies

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