What a 2 ppm match establishes, and what it does not

2 ppm match would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.

Post #13 is right about the mechanism and I think understates the practical bit.

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 put this at better than even and not much better.

I think the 2 ppm match question is answerable and has not been answered, which is a more optimistic position than most of this thread.

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.

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

I had written a reply contradicting post #17 and deleted it. Here is what survived.

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.

It cost nothing to check and would have cost something not to.

2 ppm match sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.

A methods point on 2 ppm match rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.

I read post #20 twice before replying, because I had assumed the opposite.

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.

This is the sort of thing the wiki should carry and currently does not.

Taking post #22 at face value and following it one step further.

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.

Two sources, same conclusion, and I could not rule out that one copied the other.

Purity and identity are different questions: LC-MS establishes that the species at a retention time has the expected mass. It does not establish how much of the sample is that species (that is what LC-UV purity answers).

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

Post #22 is right about the mechanism and I think understates the practical bit.

Mass accuracy is expressed in parts per million. It is the difference between observed and theoretical mass divided by theoretical mass, multiplied by a million. A high-resolution instrument in good calibration achieves low single-digit ppm on a peptide of this size.

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

Coming back to post #24, because the follow-up matters more than the original answer.

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.

Posted with less confidence than the sentence structure implies.

Marking my uncertainty on 2 ppm match explicitly. I am confident about the direction, much less confident about the size, and not confident at all that it generalises past the case in the first post.

The arithmetic in post #26 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.

Where I would look next, rather than where I would stop.

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.