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.
That is one dataset and I would not build a rule on it.
Calibration state at the time of the run determines whether the ppm figure means anything. A report that states when the instrument was last calibrated is unusual and is worth more than one that does not.
The answer changed when I changed how I was measuring, which was informative.
Following this. I have the same question and no better information than the first post.
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).
Post #65 describes the usual case. This is about the unusual one.
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.
Adding the measurement that post #65 says would settle it.
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.
Worth reading the earlier posts in this thread before acting on mine.
Post #65 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.
Taking post #69 at face value and following it one step further.
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.
Taking post #68 at face value and following it one step further.
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.
The general answer and the answer for your case may diverge here.
Post #71 and I disagree about the size of the effect, not about the direction.
Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
I have no interest in any supplier named above.
Mass error in ppm is well covered in the tag pages, and the older discussions are better than the recent ones because they were argued out properly. Worth twenty minutes before adding to this one.
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.
A methods point on mass error in ppm rather than a substantive one: if the comparison is not like for like, the difference you are measuring is the difference in method.
Post #74 put the caveat in the right place and I want to underline it.
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.
I would treat the number as indicative rather than as a measurement.
The arithmetic in post #76 is right; the assumption feeding it is the part to check.
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.
The rule of thumb is fine; the edge cases are where it earns its keep.
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.
A qualification I should have led with rather than closed on.
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 #79 and deleted it. Here is what survived.
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.
Posted with less confidence than the sentence structure implies.
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.
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.
Where I part company with post #83, and it is a narrow parting.
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 have kept the units in throughout, for the obvious reason.
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.
Not the answer, but possibly the question that gets there.
I would call the community position on mass error in ppm likely rather than established, and I would be comfortable defending that hedge.
Adding the measurement that post #87 says would settle 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.
That is a description of practice, not a recommendation of it.
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).
Worth one more sentence than it usually gets.
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.