Analytics · Mass spectrometry

Second pass at: MALDI versus ESI for a quick identity check

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

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Solved by k.okafor in post #3

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.

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m.almeidaTL2 Moderator28 Aug 2025#1

Second pass at: MALDI versus ESI for a quick identity check 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.

tirzepatide has a monoisotopic mass close to 4813.5 Da. On an electrospray instrument I would expect to see the multiply charged series rather than the intact singly charged ion, so for the doubly charged species I calculate (4813.5 + 2 x 1.00728) / 2, and for the triply charged the analogous expression.

The observed values in the report sit within a few ppm of those. My question is what that actually establishes, because I have seen people treat a mass match as a purity result and I do not think it is one.

3 likes 11mo

LeitermanTL3Regular14 Nov 2025#2

the opening post answers the question as asked. The question underneath it is different.

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.

0 likes 8mo

k.okaforTL2 Moderator Solution9 Jan 2026#3

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.

16 likes 7mo

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