Analytics · Mass spectrometry

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Analytics › Mass spectrometry LC-MS high-resolution MS mass error (ppm)

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dr_okonkwoTL4 Moderator24 May 2026#1

Community wiki post. Any member at trust level 3 or above can edit this post; every edit is recorded. Last edited by dr_okonkwo on 25 May 2026.

25 May 2026 — dr_okonkwo: Initial wiki conversion.

Editors: dr_okonkwo, mira.patel, hana.sato

Staff post. Actions described here are recorded in the public moderation log and may be challenged in Meta.

LC-MS and high-resolution MS, charge states, mass accuracy in ppm, and what identity confirmation does and does not establish.

This post is a community wiki: any member at trust level 3 or above can edit it, and every edit is recorded with its author and a note. If the scope described here has drifted from what actually gets posted, edit it rather than starting a topic about it.

What belongs here

Questions specific to mass spectrometry, with the units and the context stated.
Reports and datasets, with the collection method stated.
Disagreement about the substance, conducted in public.

What belongs elsewhere

Anything urgent belongs in Safety, immediately, and not here.
Anything about how this site works belongs in Meta.
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Before posting, it is worth checking the documentation commons and this category's tag pages. A great deal of what gets asked here has a maintained answer with a review date on it, which is more reliable than a two-year-old reply.

3 likes 2mo

owen.bradyTL4 Moderator5 Jun 2026#2

Staff post. Actions described here are recorded in the public moderation log and may be challenged in Meta.

Moderation notes for this category, so that nothing about how it is run is a surprise.

Topics get moved into and out of here regularly. A move is not a judgement — the category tree is not obvious from outside and putting a question in the wrong place is expected. When I move something I say so in the topic and the action appears in the public log with the guideline cited.

The two guidelines that come up most often in this category are R2, on supplying a source when asked, and R7, on category placement. Neither is used punitively.

0 likes 2mo

l.dialloTL213 Jun 2026#3

As a member rather than staff: the most useful thing I did when I started reading this category was to work through the tag pages rather than the topic list. The topic list shows you what is recent. The tags show you what is settled.

23 likes 1mo

b.demirTL220 Jun 2026#4

Suggestion for the wiki post above: it would be worth linking the two or three most-referenced topics in this category directly, since new members reliably ask for exactly those. Happy to make the edit myself if nobody objects, with a revision note.

10 likes 1mo

r.villalobosTL227 Jun 2026#5

l.diallo, post #3: As a member rather than staff: the most useful thing I did when I started reading this category was to work through the tag pages rather than the topic list. The topic list shows you what is recent. The tags show you what is settled. Go to post

Deamidation adds approximately one dalton and produces a species that frequently elutes very close to the parent. It is the hardest common impurity to see chromatographically and the easiest to see by mass.

I would be glad to be shown a cleaner way of putting this.

3 likes in reply to #3 1mo

o.vogelTL24 Jul 2026#6

A mass match establishes that the measured mass is consistent with the proposed composition. It does not establish purity, sequence order, or the absence of an isomer, and all three are frequently claimed from it.

The confident version of this sentence would be wrong, so here is the hedged one.

0 likes 24d

m.duarteTL210 Jul 2026 · edited#7

Post #3 and I disagree about the size of the effect, not about the direction.

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.

31 likes 18d

l.aguirreTL216 Jul 2026#8

Taking post #7 at face value and following it one step further.

Sample preparation for mass spectrometry can itself introduce modifications, particularly oxidation. A finding at trace level may be telling you about the preparation rather than the material.

16 likes 12d

f.fenwickTL3Regular22 Jul 2026#9

Second this, and I would have said it less carefully.

0 likes 6d

ar.petrovTL228 Jul 2026#10

Post #7 is right about the mechanism and I think understates the practical bit.

Positional isomers and epimers are mass-identical. Any argument that a mass result rules them out is wrong, and it is the commonest overclaim in this subcategory.

24 likes 6h

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