Reading an MS report that only gives you a single number

Post #12 describes the usual case. This is about the unusual one.

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.

That is the honest state of it as of this week.

Adding the measurement that post #15 says would settle it.

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 reasoning is more useful than the number, which is why I have shown 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.

I would put a moderate confidence on that and no more.

Electrospray on a peptide of this size gives a multiply charged series rather than a single ion. Seeing only one charge state usually means the deconvolution has already been done for you, which is worth knowing.

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 will take the caveat as seriously as the claim, which is the point of putting it there.

Useful. I had the fact and not the reason, which turns out to be the important half.

Coming back to post #18, because the follow-up matters more than the original answer.

Electrospray on a peptide of this size gives a multiply charged series rather than a single ion. Seeing only one charge state usually means the deconvolution has already been done for you, which is worth knowing.

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.

A partial answer, offered because a partial answer beats none.

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.

Posted with less confidence than the sentence structure implies.

Confirming post #24 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.

I had written a reply contradicting post #22 and deleted it. Here is what survived.

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.

The uncertainty is in the assumption, not in the calculation.

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.

I have said this before in a thread nobody could find, so it is worth repeating.

Adducts — sodium and potassium especially — shift the observed mass in predictable increments. A report showing an unexplained plus twenty-two is usually showing you a sodium adduct.

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.

I would put this at better than even and not much better.

Noted, and I have changed what I was going to do on the strength of it.