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.
Confirming post #58 from a second method, which matters more than confirming it from a second person.
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.
I had written a reply contradicting post #60 and deleted it. Here is what survived.
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).
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.
I have left out the parts I could not verify.
Post #62 is the version of this I will quote in future. One addition.
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.
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.
Not disagreeing with anyone above, just adding the bit I keep having to look up.
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.
It is a small point and it changes the answer, which is an awkward combination.
Quietly grateful for the plain phrasing. Not every thread gets that.
Building on post #66 rather than restating it.
Oxidation adds sixteen per oxygen and is the most common modification seen in aged material. Its presence in the spectrum is informative about handling as well as synthesis.
Worth checking against a second source before it gets quoted onward.
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.
Happy to expand any of that if it is the useful part.
Post #69 answers the question as asked. The question underneath it is different.
I changed my mind about oxidation and deamidation mass shifts after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.
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 would rather be precise about what I do not know than vague about what I do.
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.
Post #75 is right about the mechanism and I think understates the practical bit.
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.
On post #75 — agreed on the reasoning, with one qualification.
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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