This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1 · go to the accepted answer.
This follows post #60 rather than contradicting it.
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.
That is the honest state of it as of this week.
The strongest argument against my own position on Adducts, stated as well as I can state it, since nobody else has yet.
That is consistent with mine, for whatever one more account is worth.
Taking post #63 at face value and following it one step further.
Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
I would put the burden of proof on the interesting explanation, not the dull one.
Post #62 and I disagree about the size of the effect, not about the direction.
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.
I think the Adducts question is answerable and has not been answered, which is a more optimistic position than most of this thread.
Where the Adducts reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.
Adducts is worth one more sentence than it usually gets, and the sentence is the one about how the number was arrived at.
Post #69 put the caveat in the right place and I want to underline it.
I would be cautious about generalising from the Adducts example above. It is a good example. It is one example.
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.
Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.
Taking post #73 at face value and following it one step further.
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 uncertainty is in the assumption, not in the calculation.
Two sentences on Adducts and then I will stop, because the rest is speculation and the thread is better without mine.
What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.
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.
A partial answer, offered because a partial answer beats none.
Confirming post #77 from a second method, which matters more than confirming it from a second person.
Adducts 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.
Post #82 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.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
Worth separating two things that post #84 runs together.
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 one more sentence than it usually gets.
Practical answer on Adducts, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.
Genuine question rather than a rhetorical one: has anyone here actually observed Adducts, as opposed to read about it? The thread is long and I cannot tell.
Adding a reference point for Adducts. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.