This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
I had written a reply contradicting post #57 and deleted it. Here is what survived.
Mass error in parts per million is (observed minus theoretical) divided by theoretical, times a million. On a high-resolution instrument a low single-digit figure is unremarkable and expected.
I have left out the parts I could not verify.
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.
It reads as pedantry until the day it does not.
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.
Posting it because the silence on this was starting to look like agreement.
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.
Second-hand, so weight it accordingly.
Agreed on all of that, and I have nothing to add to it.
For anyone finding this later: the short answer on mass spectrometry is that it depends on one thing, and the rest of the thread is people identifying which thing.
Picking up post #65: that is the part I would want checked first.
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.
I would put moderate confidence on the mainstream reading of mass spectrometry and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.
Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
The most useful reply I ever got about mass spectrometry was a request to state my units. It sounds like pedantry and it has saved me twice.
Post #71 put the caveat in the right place and I want to underline it.
Counterpoint on mass spectrometry, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.
Reading rather than contributing, but this is the most useful thread I have found on it.
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.
A guess, clearly labelled as one.
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.
Happy to be corrected if someone holds better data than mine.
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.
That is the practical version. The rigorous version is longer and says the same thing.
Adding the measurement that post #76 says would settle it.
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.
Post #74 describes the usual case. This is about the unusual one.
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.
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.
Nothing to add, except that this is the answer I would give if asked.
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.
Flagging that the sources on this are thinner than the confidence in the thread suggests.
Where I part company with post #79, and it is a narrow parting.
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.
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