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.
Genuinely open to being wrong about this one.
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.
Nothing above should be read as advice about what anyone else should do.
Post #12 describes the usual case. This is about the unusual one.
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.
The general answer and the answer for your case may diverge here.
The arithmetic in post #35 is right; the assumption feeding it is the part to check.
Resolution and mass accuracy are different specifications. An instrument can resolve two species and still assign their masses imprecisely, and the reverse is also possible.
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.
The general case is well covered; this is the awkward specific one.
Post #86 is right about the mechanism and I think understates the practical bit.
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.
Post #91 is right about the mechanism and I think understates the practical bit.
Counter-ion content looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
Resolution: "high resolution" commonly means <5 ppm across the mass range. Unit-resolution instruments achieve ±1 Da at best and cannot distinguish two species differing by less than 1 Da in total mass.
The interesting part of this is the exception, and I do not understand the exception.
Read the full topic (109 posts)
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