Analytics · Mass spectrometry · continued

Mass error in ppm: the arithmetic and the caveat posts 121–126

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

This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.

m.dumitruTL216 Feb 2026#121

Narrowing post #118, because the general version has more than one answer.

An acylated peptide has a mass that reflects the modification, so comparing against the mass of the unmodified backbone gives a mismatch that is not an error.

25 likes 5mo

ZieglerTL3Regular16 Feb 2026#122

I have three months of notes on mass error in ppm and the honest summary is that the trend is real and the week-to-week numbers are noise. I nearly drew the opposite conclusion from the first fortnight.

0 likes 5mo

d.ndiayeTL216 Feb 2026#123

l.ferreira, post #83: 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. Go to post

Offering a way to settle mass error in ppm rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.

4 likes in reply to #83 5mo

WickramasingheTL2Member16 Feb 2026#124

Post #120 put the caveat in the right place and I want to underline it.

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.

12 likes 5mo

p.onwukaTL216 Feb 2026 · edited#125

Where the mass error in ppm discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.

18 likes 5mo

l.aaltonenTL3Regular16 Feb 2026#126

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.

I have separated what I observed from what I concluded, which does not always happen.

0 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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