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Isotope patterns and why the monoisotopic peak is not the tallest — does this still hold?

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

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Solved by t.duarte in post #3

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pe.onwukaTL2 Moderator13 Jun 2026#1

Isotope patterns and why the monoisotopic peak is not the tallest — does this still hold? — 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.

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dr_bhattacharyaTL3Physician9 Jul 2026 · edited#2

Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak.

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t.duarteTL2 Moderator Solution28 Jul 2026#3

For anyone arriving from a search: the marked solution above is the direct answer, and the replies underneath it add the caveats that make it safe to use.

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