This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
I read post #89 twice before replying, because I had assumed the opposite.
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 confident version of this sentence would be wrong, so here is the hedged one.
My experience of high-resolution contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Coming back to post #93, because the follow-up matters more than the original answer.
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.
Marking that as an opinion rather than a finding.
High-resolution is one of those subjects where the general answer and the answer for a specific case diverge, and the thread will go in circles until someone says which one is being asked for.
Adding the boring version of high-resolution, because the interesting version keeps getting posted and the boring one is usually right.
Check the ordinary explanations, in order, and stop when one of them accounts for what you are seeing. Most of the time the second one does.
This follows post #97 rather than contradicting 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.
I would be glad to be shown a cleaner way of putting this.
A request rather than an answer: could whoever has the primary source for high-resolution post it? I have seen the claim three times this month and each version had lost a qualifier.
Post #97 put the caveat in the right place and I want to underline it.
One more thing on high-resolution that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.
Adding a note of thanks rather than an opinion. I did not know most of that.
Genuine question rather than a rhetorical one: has anyone here actually observed high-resolution, as opposed to read about it? The thread is long and I cannot tell.
Deamidation adds approximately one dalton and produces a species that frequently elutes very close to the parent. It is the hardest common impurity to see chromatographically and the easiest to see by mass.
Not the whole picture, but the part of it I can speak to.
Taking post #104 at face value and following it one step further.
I have no financial interest in anything named in this thread and I want to say so before I comment on high-resolution, because it is the sort of subject where it matters.
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.
A weak preference rather than a position.
Two people in this thread mean different things by high-resolution and are disagreeing about the definition while believing they are disagreeing about the facts. Worth pausing to define 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 treat the number as indicative rather than as a measurement.
Agreed on high-resolution, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part nobody states.
Practical experience of high-resolution, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.
Narrowing post #111, because the general version has more than one 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.
If that is already documented somewhere, ignore me and link it.
Everything in post #110 holds. The case it does not cover is the one I have.
What I would check first on high-resolution is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.
This is the answer, and the reason it is the answer is the more useful part.
I read post #114 twice before replying, because I had assumed the opposite.
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 part I am sure of is shorter than the part I have written.
Picking up post #117: that is the part I would want checked first.
High-resolution has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.
On post #118 — agreed on the reasoning, with one qualification.
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.