Analytics · Mass spectrometry

Second pass at: Counter-ion content and its effect on measured mass

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

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Solved by f.fenwick in post #4

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.

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sa.okonkwoTL2 Moderator5 Sep 2025#1

Second pass at: Counter-ion content and its effect on measured mass Writing it up because I had to work it out twice and would rather nobody else did.

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.

47 likes 11mo

r.scholtenTL2Member13 Nov 2025#2

Calibration matters: a high-resolution instrument out of calibration can report mass with ppm error large enough to be uninformative. Check when the instrument was last calibrated before trusting the reported accuracy.

12 likes 8mo

l.aguirreTL2 Moderator1 Jan 2026#3

I read the opening post twice before replying, because I had assumed the opposite.

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

1 like 7mo

f.fenwickTL3Regular Solution14 Feb 2026#4

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.

7 likes 5mo

o.vogelTL2 Moderator27 Mar 2026#5

f.fenwick, post #4: 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. Go to post

What mass accuracy establishes: the measured mass is consistent with a specific composition. What it does not establish: purity, sequence order, stereochemistry, or the absence of an isobaric species. Every one of those requires something else.

18 likes in reply to #4 4mo

This topic was referenced in

Counter-ion content and its effect on measured massAnalytics › Mass spectrometry · 108 replies
Isotope patterns and why the monoisotopic peak is not the tallest — does this still hold?Analytics › Mass spectrometry · 2 replies

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