Post #2 put the caveat in the right place and I want to underline it. 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.
Posting this under the heading it deserves: Oxidation and deamidation mass shifts, tabulated Everything below is what sits behind that.
Oxidation and deamidation mass shifts, and specifically the version of it that the documentation does not cover. The maintained page handles the general case well and stops exactly where my question starts.
Setting out the gap in case it is a gap in the page rather than a gap in what is known.
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.
Reading it back, the second half matters more than the first.
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 #2 put the caveat in the right place and I want to underline it.
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.
Adding the measurement that post #4 says would settle it.
Since oxidation and deamidation mass shifts keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.
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.
Two sources, same conclusion, and I could not rule out that one copied the other.
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.
That is the practical version. The rigorous version is longer and says the same thing.
Grateful for the specificity. Vague answers to this question are what sent me looking.
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 request rather than an answer: could whoever has the primary source for oxidation and deamidation mass shifts post it? I have seen the claim three times this month and each version had lost a qualifier.
Building on post #13 rather than restating it.
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.
Worth reading the earlier posts in this thread before acting on mine.
Answering the question post #13 raises rather than the one it answers.
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 one dataset and I would not build a rule on 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.
I would not lead a decision with this, but I would not ignore it either.
Practical note on oxidation and deamidation mass shifts: write down what you expect before you look. The number of times I have found what I went looking for is higher than chance would allow.
Coming back to post #17, because the follow-up matters more than the original answer.
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.
For what it is worth, the same held on the two occasions I checked.
Appreciated. The plain phrasing does more work here than a longer post would.
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 would be glad to be shown a cleaner way of putting this.
Post #22 put the caveat in the right place and I want to underline 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.
A single observation, in a thread that deserves better than single observations.
The arithmetic in post #22 is right; the assumption feeding it is the part to check.
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.
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).
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.
The answer changed when I changed how I was measuring, which was informative.
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.
Worth saying I have only my own numbers here, and n is small.
Adding the measurement that post #26 says would settle 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.
Where I would look next, rather than where I would stop.