Reading an MS report that only gives you a single number posts 91–114

Tandem mass spectrometry with fragmentation gives sequence information that intact mass cannot. It is the analysis that would actually confirm identity, and it is rarely supplied.

That is what the documentation says. What happens in practice is usually close.

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.

Marking that as an opinion rather than a finding.

Following this. I have the same question and no better information than the first post.

Adding the measurement that post #91 says would settle 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.

Coming back to post #91, because the follow-up matters more than the original answer.

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.

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.

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 the burden of proof on the interesting explanation, not the dull one.

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 single observation, in a thread that deserves better than single observations.

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 step people skip is the one I have spelled out.

This is the sort of exchange that makes the archive worth searching.

Clear enough that I do not think I have a follow-up, which is unusual.

Taking post #99 at face value and following it one step further.

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.

If anyone can point at the primary source I would be grateful.

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.

That has held every time I have looked, which is not the same as always.

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 would call that likely rather than established.