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.
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 have said this before in a thread nobody could find, so it is worth repeating.
Where I part company with post #32, and it is a narrow parting.
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.
For a compound with no reference standard in circulation, an observed mass is much more useful than an assertion of agreement with a theoretical value nobody can check.
I had written a reply contradicting post #35 and deleted it. Here is what survived.
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 have deliberately not rounded that, because the rounding is where the argument starts.
Adding a data point of agreement rather than a data point.
Worth separating two things that post #35 runs together.
Purity and identity are different questions: LC-MS establishes that the species at a retention time has the expected mass. It does not establish how much of the sample is that species (that is what LC-UV purity answers).
This is the sort of thing that ought to be settled and apparently is not.
This follows post #39 rather than contradicting it.
Tandem mass spectrometry: MS/MS fragments the molecular ion and uses fragment masses to confirm identity and detect modifications. A simple identity confirmation by LC-MS does not address post-translational modifications or impurities with the same or very close mass.
It cost nothing to check and would have cost something not to.
This is the sort of exchange that makes the archive worth searching.
I had written a reply contradicting post #39 and deleted it. Here is what survived.
What mass accuracy establishes: the measured mass is consistent with a specific composition. What it does not establish: purity, sequence order, stereochemistry, or the absence of an isobaric species. Every one of those requires something else.
Filing this under things that are true until someone shows me otherwise.
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.
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.
Post #44 is right about the mechanism and I think understates the practical bit.
Sample preparation for mass spectrometry can itself introduce modifications, particularly oxidation. A finding at trace level may be telling you about the preparation rather than the material.
I would treat that as a working assumption and revisit it.
Coming back to post #42, because the follow-up matters more than the original answer.
Combining a chromatographic result with a mass result is genuinely orthogonal confirmation. Either alone leaves a specific class of problem invisible, and the two classes do not overlap much.
Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
Two people can read the same figure differently here and both be reasonable.
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.
Noting that the question and the thing people usually mean by it are different.
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 back through, this was answered upthread and I missed it. My fault.
A mass match establishes that the measured mass is consistent with the proposed composition. It does not establish purity, sequence order, or the absence of an isomer, and all three are frequently claimed from it.
Someone should write this up properly, and it should probably not be me.
Following, with nothing to contribute beyond having asked the same thing elsewhere.
Monoisotopic and average mass are different numbers and both appear on documents. At this molecular weight the difference is a few daltons, which is more than the tolerance being claimed.
Tandem mass spectrometry with fragmentation gives sequence information that intact mass cannot. It is the analysis that would actually confirm identity, and it is rarely supplied.
I keep a log of this specifically because memory is unreliable about it.
Mass accuracy is expressed in parts per million. It is the difference between observed and theoretical mass divided by theoretical mass, multiplied by a million. A high-resolution instrument in good calibration achieves low single-digit ppm on a peptide of this size.
If anyone has run this properly I would rather read that than my own guess.
Worth separating two things that post #54 runs together.
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.
Reporting rather than recommending, on oxidation and deamidation mass shifts. What happened is above. Whether it should have is a different question and not one I am qualified to answer.
Answering the question post #57 raises rather than the one it answers.
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.
The short answer was in the first line; everything after is the working.
Summarising the oxidation and deamidation mass shifts thread so far, since it is long and the answer is buried: the first reply has the method, the fourth has the correction to it, and the rest is people agreeing at length.