Topic summary

Mass error in ppm: the arithmetic and the caveat

Analytics › Mass spectrometry LC-MS high-resolution MS MALDI-TOF counter-ion content Solved

This is a generated summary. It shows the 9 most-liked posts from a topic of 126, 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.

b.vestergaardTL2 Solution11 Feb 2026#5

k.chukwu, post #1: Mass error in ppm: the arithmetic and the caveat 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… Go to post

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

Purity and identity are different questions: LC-MS establishes that the species at a retention time has the expected mass. It does not establish how much of the sample is that species (that is what LC-UV purity answers).

12 likes in reply to #1 5mo

a.lindholmTL211 Feb 2026#7

No disagreement from me. Posting only so the question does not look ignored.

29 likes 5mo

NLoughranTL3Regular12 Feb 2026#18

I had written a reply contradicting post #16 and deleted it. Here is what survived.

For a compound with no reference standard in circulation, an observed mass is much more useful than an assertion of agreement with a theoretical value nobody can check.

32 likes 5mo

j.teixeiraTL212 Feb 2026#26

a.mwangi, post #3: The opening post 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. Go to post

This follows post #25 rather than contradicting it.

Where I have landed on mass error in ppm, having got it wrong once in public: the direction is clear, the magnitude is not, and anyone quoting a precise magnitude has borrowed it from somewhere that did not measure it.

27 likes in reply to #3 5mo

r.aldana_pharmdTL4Pharmacist13 Feb 2026#38

Source for the mass error in ppm figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.

Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.

29 likes 5mo

k.perrinTL213 Feb 2026#39

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.

I would rather be precise about what I do not know than vague about what I do.

27 likes 5mo

journalclub_wrenTL3Regular13 Feb 2026#43

Right — I had this wrong and I am glad to have read it before it mattered.

27 likes 5mo

ew.kuuselaTL215 Feb 2026#93

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.

31 likes 5mo

m.steinerTL215 Feb 2026#107

The arithmetic in post #104 is right; the assumption feeding it is the part to check.

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.

32 likes 5mo

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MS/MS sequence confirmation: what it costs and when it is worth it — what changed sinceAnalytics › Mass spectrometry · 39 replies

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