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

Adding the measurement that the opening post says would settle it.

Two people in this thread mean different things by 2 ppm match and are disagreeing about the definition while believing they are disagreeing about the facts. Worth pausing to define it.

Adding the measurement that post #4 says would settle it.

A mass match establishes that the measured mass is consistent with the proposed composition. It does not establish purity, sequence order, or the absence of an isomer, and all three are frequently claimed from it.

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

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.

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.

Answering the question post #46 raises rather than the one it answers.

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.

Written in the hope of being told what I have missed.

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.

That is one dataset and I would not build a rule on it.

Post #57 is right about the mechanism and I think understates the practical bit.

My understanding of 2 ppm match is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it.

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.

Written from notes rather than memory, which is why the numbers are specific.