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.
Second pass at: Counter-ion content and its effect on measured mass Writing it up because I had to work it out twice and would rather nobody else did.
Something about Counter-ion content does not reconcile and I would like a second pair of eyes before I decide which half is wrong.
Two sources, both reputable, giving figures that cannot both be right unless they are measuring different quantities. My suspicion is that they are, and I cannot see how.
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.
Posted with less confidence than the sentence structure implies.
The opening post put the caveat in the right place and I want to underline it.
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.
I have said this before in a thread nobody could find, so it is worth repeating.
Monoisotopic and average mass are different numbers and both appear on documents. At this molecular weight the difference is a few daltons, which is more than the tolerance being claimed.
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Distinguishing a deletion sequence by mass alone — the long version
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2 | 18k | 7mo | |
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+117 | 123 | 6.5k | 8mo |
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+124 | 146 | 15k | 16mo |
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Deconvolution of a multiply charged envelope
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2 | 63 | 10mo | |
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+78 | 91 | 1k | 10h |
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Coming back to: Deconvolution of a multiply charged envelope
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2 | 12k | 22mo | |
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+4 | 8 | 7.2k | 1mo |
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2 | 293 | 3mo |