Topic summary

Oxidation and deamidation mass shifts, tabulated

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

This is a generated summary. It shows the 9 most-liked posts from a topic of 77, in their original order, with the accepted answer included where one exists. It is a reading aid and it will miss nuance — the full topic is the record.

t.nardoneTL3Regular Solution7 Nov 2024#4

Coming back to post #3, because the follow-up matters more than the original answer.

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.

7 likes 21mo

a.eriksenTL2 Moderator15 Nov 2024#7

Mass accuracy is expressed in parts per million. It is the difference between observed and theoretical mass divided by theoretical mass, multiplied by a million. A high-resolution instrument in good calibration achieves low single-digit ppm on a peptide of this size.

26 likes 20mo

st.dialloTL2 Moderator4 Dec 2024#15

I read post #13 twice before replying, because I had assumed the opposite.

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.

30 likes 20mo

so.mbekiTL2 Moderator17 Dec 2024#21

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.

30 likes 19mo

hana.satoTL4 Moderator9 Jan 2025#33

g.tanaka, post #14: post #13 answers the question as asked. The question underneath it is different. Quantitation by MS: most quantitation is done by LC-UV detection at 214 nm, not by MS, because extinction coefficients are better known. MS can quantify if an internal standard is used but that requires preparation. Go to post

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Coming back to post #31, because the follow-up matters more than the original answer.

Two things before anyone answers the substance.

First, the context in the first post is clear and specific. Second, the question is framed so that an answer can actually address it. Both are the norm here and both matter more than they sound.

28 likes in reply to #14 19mo

r.bakkenTL2 Moderator9 Feb 2025#51

Desalting before analysis: some samples need desalting to remove salts that suppress the peptide signal. Report whether desalting was used, because it can affect the apparent ionization efficiency and the reported purity.

24 likes 18mo

j.restrepoTL2 Moderator16 Feb 2025#55

Charge states observed: for semaglutide (4113.6 Da) the doubly charged ion appears at m/z ≈ 2057, triply charged at ≈ 1371, quadruply at ≈ 1029. Those are the positions to look for; the heights depend on the ionization efficiency.

32 likes 17mo

forest_plotTL3Evidence synthesis5 Mar 2025#66

Common adducts: sodium adds ≈22, potassium adds ≈38 compared to hydrogen. A [M+Na]+ peak is common and its mass is predictable from the base mass.

26 likes 17mo

b.jankowiakTL3Regular14 Mar 2025#72

post #71 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.

26 likes 16mo

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Reading an MS report that only gives you a single number — the long versionAnalytics › Mass spectrometry · 2 replies

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