Topic summary

MS/MS sequence confirmation: what it costs and when it is worth it

Analytics › Mass spectrometry LC-MS MALDI-TOF charge states Solved Pinned

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s.vanheckeTL220 May 2026#1

MS/MS sequence confirmation: what it costs and when it is worth it — setting out what I have, and where I think it stops being reliable.

I would like to understand what this number means before I repeat it anywhere.

A PeptideMeter report on a retatrutide lot gives 98.4% purity. The supplier certificate for the same lot states 99%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

24 likes 2mo

s.lindqvistTL26 Jun 2026#2

This follows the opening post rather than contradicting it.

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.

4 likes 2mo

sterile_tableTL3Regular18 Jun 2026#3

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.

Reporting the observation and leaving the explanation open deliberately.

0 likes 1mo

lyophil_marginTL3Regular8 Jul 2026#5

g.bakken, post #4: Isotope pattern is a check that costs nothing to look at. A species at this molecular weight has a characteristic envelope, and something with the wrong envelope is not what it says it is. A qualification I should have led with rather than closed on. Go to post

Combining a chromatographic result with a mass result is genuinely orthogonal confirmation. Either alone leaves a specific class of problem invisible, and the two classes do not overlap much.

Noting that I have skin in this question and have tried to discount for it.

8 likes in reply to #4 19d

r.marsdenTL3Regular Solution27 Jul 2026#7

Everything in post #3 holds. The case it does not cover is the one I have.

Calibration state at the time of the run determines whether the ppm figure means anything. A report that states when the instrument was last calibrated is unusual and is worth more than one that does not.

7 likes 1d

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This topic was referenced in

What a 2 ppm match establishes, and what it does notAnalytics › Mass spectrometry · 60 replies
Reading an MS report that only gives you a single numberAnalytics › Mass spectrometry · 113 replies
Second pass at: Counter-ion content and its effect on measured massAnalytics › Mass spectrometry · 4 replies

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