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.
Post #29 and I disagree about the size of the effect, not about the direction.
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.
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.
That much is documented. The rest is how I have interpreted 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.
Where I would look next, rather than where I would stop.
Narrowing post #33, because the general version has more than one answer.
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.
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.
It reads as pedantry until the day it does not.
Answering the question post #33 raises rather than the one it answers.
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.
The arithmetic in post #35 is right; the assumption feeding it is the part to check.
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.
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.
I am confident about the direction and much less about the magnitude.
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.
None of the above is medical advice and I am not qualified to give any.
Post #40 describes the usual case. This is about the unusual one.
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.
I would call that likely rather than established.
Following this. I have the same question and no better information than the first post.
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.
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.
The general case is well covered; this is the awkward specific one.
Post #46 is the version of this I will quote in future. One addition.
Adding a reference point for counter-ion content. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.
Where I part company with post #44, and it is a narrow parting.
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.
Not a strong opinion, just a consistent 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.
I would not lead a decision with this, but I would not ignore it either.
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.
Worth separating two things that post #47 runs together.
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 checking against a second source before it gets quoted onward.
A request rather than an answer: could whoever has the primary source for counter-ion content post it? I have seen the claim three times this month and each version had lost a qualifier.
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.
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.
Thank you for the correction. I would rather find out here than later.
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 have left out the parts I could not verify.
Confirming post #55 from a second method, which matters more than confirming it from a second person.
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.
Two sources, same conclusion, and I could not rule out that one copied the other.
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.
The short answer was in the first line; everything after is the working.