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.
Building on post #89 rather than restating it.
Mass error in ppm has a well-known answer and a correct answer, and the interesting work is establishing that they are the same. Nobody has done that here yet.
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.
Quietly grateful for the plain phrasing. Not every thread gets that.
The arithmetic for a doubly charged species is (M + 2 x 1.00728) / 2, and the analogous expression for higher charge states. Working it through once makes the reported values legible.
It is the kind of thing that is obvious once and never again.
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.
The arithmetic in post #95 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.
I checked the source rather than the summary, and they differ.
Answering the question post #96 raises rather than the one it answers.
Two sentences on mass error in ppm and then I will stop, because the rest is speculation and the thread is better without mine.
What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.
I had written a reply contradicting post #96 and deleted it. Here is what survived.
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.
The honest answer is that it depends, and here is what it depends on.
Clear enough that I do not think I have a follow-up, which is unusual.
Adding the measurement that post #100 says would settle it.
Practical note on mass error in ppm: 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.
Post #101 describes the usual case. This is about the unusual one.
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.
Where I part company with post #104, and it is a narrow parting.
For anyone finding this later: the short answer on mass error in ppm is that it depends on one thing, and the rest of the thread is people identifying which thing.
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.
Take the reasoning and check the arithmetic; I do not always get it right.
Sensible. I would want the same detail before I acted on it either.
Post #106 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.
Worth saying I have only my own numbers here, and n is small.
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.
A guess, clearly labelled as one.
Adding the measurement that post #110 says would settle it.
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 would not lead a decision with this, but I would not ignore it either.
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.
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.
Reading rather than contributing, but this is the most useful thread I have found on it.
Coming back to post #113, because the follow-up matters more than the original answer.
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).
If the premise is wrong, everything after it is decoration.
Post #117 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.
Correct me on the arithmetic if it is wrong; I would rather know.
Reading this mass error in ppm thread as someone who came in with a fixed view: the third and seventh replies moved me and the confident ones did not.
The number people quote for mass error in ppm is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.