This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Post #27 describes the usual case. This is about the unusual one.
Having read the whole high-resolution thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.
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.
It took me longer than it should have to see that.
Thank you for taking the time. That was more work than a reply usually is.
Answering the question post #31 raises rather than the one it answers.
I would keep high-resolution and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
The arithmetic in post #35 is right; the assumption feeding it is the part to check.
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.
If this contradicts something upthread, the upthread version may well be the better one.
The practical version of high-resolution is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
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.
Worth reading the earlier posts in this thread before acting on mine.
On high-resolution: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
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.
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.
Following, with nothing to contribute beyond having asked the same thing elsewhere.
Taking post #44 at face value and following it one step further.
Two things can be true about high-resolution at once: the mechanism is plausible and the evidence for the size of the effect is thin. Most of the argument here is people defending the first against attacks on the second.
Post #45 and I disagree about the size of the effect, not about the direction.
Before the thread moves on from high-resolution — 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.
Picking up post #48: that is the part I would want checked first.
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.
The variance between people here is larger than the effect being discussed.
High-resolution 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.
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.
A partial answer, offered because a partial answer beats none.
Appreciated. The plain phrasing does more work here than a longer post would.
Coming back to post #49, because the follow-up matters more than the original answer.
Trying to state the high-resolution position in a way that someone who disagrees would recognise as fair, because I do not think the version in this thread passes that test.
High-resolution is worth one more sentence than it usually gets, and the sentence is the one about how the number was arrived at.
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 sort of thing that ought to be settled and apparently is not.
Post #57 answers the question as asked. The question underneath it is different.
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.
Answering the question post #57 raises rather than the one it answers.
Small methodological point on high-resolution: repeating a measurement is cheap and resolves most of what is being argued about here at no cost to anyone.