This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
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.
That is what I would do. It may not be what is correct.
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.
Same conclusion as the reply above, reached differently, which is mildly reassuring.
Coming back to post #31, because the follow-up matters more than the original answer.
An acylated peptide has a mass that reflects the modification, so comparing against the mass of the unmodified backbone gives a mismatch that is not an error.
One caution on mass spectrometry: everything above assumes the underlying documentation is what it claims to be. That assumption is doing real work and is rarely stated.
Confirming post #35 from a second method, which matters more than confirming it from a second person.
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).
I had written a reply contradicting post #34 and deleted it. Here is what survived.
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.
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.
Answering the question post #38 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.
Same experience here, different supplier, so it is at least not unique to one of them.
Building on post #39 rather than restating it.
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.
Adding the caveat now so it does not have to be extracted later.
Quantitation by MS: most quantitation is done by LC-UV detection at 214 nm, not by MS, because extinction coefficients are better known. MS can quantify if an internal standard is used but that requires preparation.
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.
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.
That has been true for the cases I have seen and I have not seen many.
Post #43 describes the usual case. This is about the unusual one.
An update on my earlier mass spectrometry post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.
Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.
Bookmarking this. I will come back when I have something worth adding.
This follows post #49 rather than contradicting 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.
The rule of thumb is fine; the edge cases are where it earns its keep.
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.
A qualification I should have led with rather than closed on.
Picking up post #54: that is the part I would want checked first.
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.
This is the version I would want a new member to read first.
On post #52 — agreed on the reasoning, with one qualification.
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.
That is where I would start, not where I would stop.
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.
Before the thread moves on from mass spectrometry — what is the sample size behind the claim? I am not being difficult; I have seen the same figure quoted from an n of four and from an n of four hundred.
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.
A weak preference rather than a position.