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.
Where I part company with post #29, and it is a narrow parting.
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.
That is all the detail I have. Someone else will have more.
Adding thanks rather than a view. I do not have a view worth the space.
What I want from this Adducts thread is the list of things that would need to be true for the claim to hold. If we can write that list, we can check it.
I had written a reply contradicting post #33 and deleted it. Here is what survived.
Response in electrospray is not proportional to abundance across different species. Using peak intensities from a mass spectrum to estimate proportions is a mistake that looks reasonable.
The thing about Adducts that took me longest to accept is that a plausible mechanism is not evidence of an effect. It is a reason to look, not a result.
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.
I would want a second opinion before relying on that.
Adding the measurement that post #37 says would settle it.
Adducts: I have looked for the primary source twice and failed twice. Either it does not exist or it is somewhere I do not know to look, and I would like to know which.
This is the first time the answer has come with its own limits attached. Appreciated.
On post #40 — agreed on the reasoning, with one qualification.
The bit of Adducts that nobody enjoys is that the answer changes depending on what you are trying to decide with it. Say what the decision is and the thread will converge.
I have three months of notes on Adducts and the honest summary is that the trend is real and the week-to-week numbers are noise. I nearly drew the opposite conclusion from the first fortnight.
Post #45 is right about the mechanism and I think understates the practical bit.
Where the Adducts discussion usually stalls is that nobody wants to say "I do not know" and everyone is willing to say "it varies". Those are the same sentence with different clothes on.
I had written a reply contradicting post #44 and deleted it. Here is what survived.
If you are new and reading this thread for the answer to Adducts: the answer is conditional, the conditions are in the third reply, and the rest of the thread is worth skipping.
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 number is defensible. The precision I gave it is not.
Post #47 and I disagree about the size of the effect, not about the direction.
Reading back through the Adducts threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
Stating my assumptions rather than smuggling them in.
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.
One case, stated as one case.
Answering the Adducts question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.
On Adducts: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
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.
I would call that likely rather than established.
Post #55 is right about the mechanism and I think understates the practical bit.
The question underneath Adducts 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.
Helpful, and short, which on this subject is harder than long.