This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Everything in post #29 holds. The case it does not cover is the one I have.
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.
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.
I have kept the units in throughout, for the obvious reason.
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.
Not the answer, but possibly the question that gets there.
Building on post #33 rather than restating it.
The honest summary of what a mass result buys you: it narrows the field of what the material could be, considerably. It never closes it, and no certificate should be read as though it had.
Where I part company with post #33, and it is a narrow parting.
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.
Post #37 is right about the mechanism and I think understates the practical bit.
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.
If that reads as pedantic, it is, and it has saved me twice.
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.
The evidence for this is thinner than the way I have phrased it suggests.
Picking up post #42: that is the part I would want checked first.
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.
One of those cases where knowing the mechanism does not help the decision.
On post #40 — agreed on the reasoning, with one qualification.
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.
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.
The honest summary of what a mass result buys you: it narrows the field of what the material could be, considerably. It never closes it, and no certificate should be read as though it had.
I would treat that as a working assumption and revisit it.
Post #45 describes the usual case. This is about the unusual one.
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.
It took me longer than it should have to see that.
Building on post #46 rather than restating it.
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.
No disagreement from me. Posting only so the question does not look ignored.
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 right answer here may simply be that it has not been measured.
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.
Stating my assumptions rather than smuggling them in.
On post #51 — agreed on the reasoning, with one qualification.
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.
I have seen it go both ways, which is why I hedge.
Picking up post #51: that is the part I would want checked first.
The honest summary of what a mass result buys you: it narrows the field of what the material could be, considerably. It never closes it, and no certificate should be read as though it had.
The short answer was in the first line; everything after is the working.
Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak.
This is where my knowledge stops and I would rather mark the edge than blur it.
Confirming post #55 from a second method, which matters more than confirming it from a second person.
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.
Anyone with a larger sample, please post it.
Post #55 describes the usual case. This is about the unusual one.
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.
Understood, and I withdraw the assumption I opened with.
Where I part company with post #55, and it is a narrow parting.
The honest summary of what a mass result buys you: it narrows the field of what the material could be, considerably. It never closes it, and no certificate should be read as though it had.
Someone should write this up properly, and it should probably not be me.
Post #59 is the version of this I will quote in future. One addition.
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.