My experience of adducts 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.
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Analytics · Mass spectrometry · continued
Adducts: sodium, potassium, and the peak you did not expect posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
r.scholten, post #19: 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. Go to post
One caution on adducts: everything above assumes the underlying documentation is what it claims to be. That assumption is doing real work and is rarely stated.
Ziegler, post #17: 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 qualification I should have led with rather than closed on. Go to post
I would put moderate confidence on the mainstream reading of adducts and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.
j.delacroix, post #25: 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. Go to post
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 interested in a counterexample if anyone has one.
steady_state, post #6: 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. Go to post
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.
I would put the burden of proof on the interesting explanation, not the dull one.
Everything in post #39 holds. The case it does not cover is the one I have.
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.
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Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
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dietitian_hollis, post #2: 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. Go to post
Adducts sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.
h.espinoza, post #36: 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 interested in a counterexample if anyone has one. Go to post
Adding the measurement that post #43 says would settle 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 rather be precise about what I do not know than vague about what I do.
Following, with nothing to contribute beyond having asked the same thing elsewhere.
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Confirming post #43 from a second method, which matters more than confirming it from a second person.
An honest declaration on adducts: I have a prior here and it is strong enough that you should weight what I say downward. Stating it rather than hiding it.
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j.falk, post #34: I had written a reply contradicting post #30 and deleted it. Here is what survived. Nobody has said the unglamorous part of adducts yet, so: most of the variation is explained by things that are boring to write about and easy to check. Go to post
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.
Genuinely open to being wrong about this one.
n.cabrera, post #3: Coming back to the opening post, because the follow-up matters more than the original answer. The useful distinction on adducts is between what was measured and what was inferred from it. Both end up in the same sentence and only one of them has error bars. Go to post
Taking post #47 at face value and following it one step further.
Adducts looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour.
Post #48 and I disagree about the size of the effect, not about the direction.
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.
It is a small point and it changes the answer, which is an awkward combination.
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Collapsed as off-topic by two members at trust level 3 or above
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.
Reading it again, the caveat matters more than the finding.
s.leclerc, post #52: Post #48 and I disagree about the size of the effect, not about the direction. 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. It is a small point and it changes the answer, which is an awkward… Go to post
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.
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.
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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.
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w.novak, post #37: Post #36 is the version of this I will quote in future. One addition. Small correction to my own earlier position on adducts. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely. Go to post
Post #54 is right about the mechanism and I think understates the practical bit.
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.
physio_marchetti, post #50: Taking post #47 at face value and following it one step further. Adducts looks different depending on whether you are reading the primary literature or the summaries of it, and the difference is not in our favour. Go to post
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.
It is worth stating the boring hypothesis before the interesting one.
steady_state, post #6: 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. Go to post
Post #58 is the version of this I will quote in future. One addition.
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 a description of practice, not a recommendation of it.