Posting this under the heading it deserves: High-resolution MS on aggregates and what it can see Everything below is what sits behind that.
Something about high-resolution does not reconcile and I would like a second pair of eyes before I decide which half is wrong.
Two sources, both reputable, giving figures that cannot both be right unless they are measuring different quantities. My suspicion is that they are, and I cannot see how.
The arithmetic in the opening post is right; the assumption feeding it is the part to check.
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.
Narrowing post #4, because the general version has more than one answer.
High-resolution is a good example of a question where the honest answer is boring and the interesting answers are unsupported. I would go with boring.
Post #8 is right about the mechanism and I think understates the practical bit.
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.
That is a description of practice, not a recommendation of it.
Practical note on high-resolution: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.
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.
Anyone who has looked at this more carefully, please correct the record.
Coming back to post #9, because the follow-up matters more than the original answer.
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.
That has held every time I have looked, which is not the same as always.
Speaking only to high-resolution 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.
Adding the measurement that post #13 says would settle it.
Mass error in parts per million is (observed minus theoretical) divided by theoretical, times a million. On a high-resolution instrument a low single-digit figure is unremarkable and expected.
I am not the right person to answer the follow-up to this.
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.
Fine by me. I had wanted a stronger conclusion and there is not one available.
Post #17 and I disagree about the size of the effect, not about the direction.
Where I have landed on high-resolution, having got it wrong once in public: the direction is clear, the magnitude is not, and anyone quoting a precise magnitude has borrowed it from somewhere that did not measure it.
Taking post #17 at face value and following it one step further.
The confident answers on high-resolution and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
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.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Everything in post #20 holds. The case it does not cover is the one I have.
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.
Adding a source would improve this post and I do not have one to hand.
Building on post #22 rather than restating it.
The question underneath high-resolution is usually "how would I tell?" rather than "what is true?", and that one has a method attached to it.
Write down what you would expect to see under each hypothesis before you collect anything. If they predict the same observation, collecting it will not help.
I would put moderate confidence on the mainstream reading of high-resolution and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.
Adding thanks rather than a view. I do not have a view worth the space.
I read post #24 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.
Worth separating high-resolution as a question about the compound from high-resolution as a question about the documentation. They get answered by different people and only one of them is answerable here.
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.
For anyone finding this later: the short answer on high-resolution is that it depends on one thing, and the rest of the thread is people identifying which thing.