This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Everything in post #28 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.
Not the answer, but possibly the question that gets there.
I would rather this thread reach "we do not know" about 2 ppm match than reach a confident answer that nobody can support when asked.
Post #35 answers the question as asked. The question underneath it is different.
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 is a small point and it changes the answer, which is an awkward combination.
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.
This is the answer, and the reason it is the answer is the more useful part.
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.
Someone should write this up properly, and it should probably not be me.
Adding the measurement that post #40 says would settle it.
Speaking only to 2 ppm match as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.
I changed my mind about 2 ppm match after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.
That is a cleaner way of putting what I was circling around.
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.
Building on post #48 rather than restating it.
On 2 ppm match, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
Post #46 put the caveat in the right place and I want to underline it.
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).
The right answer here may simply be that it has not been measured.
I had written a reply contradicting post #49 and deleted it. Here is what survived.
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.
Worth reading the earlier posts in this thread before acting on mine.
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.
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.
The mechanism is plausible, which is not the same as established.
Coming back to post #53, because the follow-up matters more than the original answer.
An observation about 2 ppm match that I cannot explain and am posting anyway, on the principle that unexplained observations are more useful public than private.
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.
This is the first time the answer has come with its own limits attached. Appreciated.
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 checked the source rather than the summary, and they differ.