Adducts: sodium, potassium, and the peak you did not expect Writing it up because I had to work it out twice and would rather nobody else did.
Adducts keeps being re-asked here and I think that is because the answer is conditional and the conditions never travel with it.
Attempting to write the conditional version down properly. Corrections welcome and expected — I would like this to end up as a maintained page rather than a fifth thread.
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.
It is one reading of the data and not the only reasonable one.
The opening post is right about the mechanism and I think understates the practical bit.
What would change my mind on adducts is a second dataset collected by someone with no stake in the first. Until then I hold it loosely and I would rather say so than pretend to more.
On the opening post — agreed on the reasoning, with one qualification.
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.
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.
I disagree with the framing of adducts above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.
The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.
Confirming post #5 from a second method, which matters more than confirming it from a second person.
Source for the adducts figure, since it was asked for. It is in the discussion rather than the abstract, which is why the version circulating is stronger than the paper is.
Reading the surrounding paragraph is worth the two minutes. The authors are more careful than their summarisers.
Post #5 put the caveat in the right place and I want to underline 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.
It is the sort of thing that seems obvious in retrospect and was not at the time.
That is clearer than the version I had in my head. Thank you.
This follows post #9 rather than contradicting 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.
Worth separating two things that post #8 runs together.
Common adducts: sodium adds ≈22, potassium adds ≈38 compared to hydrogen. A [M+Na]+ peak is common and its mass is predictable from the base mass.
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 where I would start, not where I would stop.
On adducts the community has more anecdote than the confidence in this thread implies, and I include my own contribution in that.
Understood, and I withdraw the assumption I opened with.
I read post #16 twice before replying, because I had assumed the opposite.
Small methodological point on adducts: repeating a measurement is cheap and resolves most of what is being argued about here at no cost to anyone.
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.
Noted, and thank you for writing it out rather than summarising it.
This follows post #19 rather than contradicting it.
Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak.
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.
Adducts — sodium and potassium especially — shift the observed mass in predictable increments. A report showing an unexplained plus twenty-two is usually showing you a sodium adduct.
That is the practical version. The rigorous version is longer and says the same thing.
Picking up post #25: that is the part I would want checked first.
Offering a way to settle adducts rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
I had written a reply contradicting post #25 and deleted it. Here is what survived.
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.
Reporting rather than recommending, on adducts. What happened is above. Whether it should have is a different question and not one I am qualified to 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.
The uncertainty is in the assumption, not in the calculation.