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 #118 rather than restating it.
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 is the sort of thing that seems obvious in retrospect and was not at the time.
Post #120 put the caveat in the right place and I want to underline 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.
That has been true for the cases I have seen and I have not seen many.
Having read the whole distinguishing a deletion sequence 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.
Trying to state the distinguishing a deletion sequence 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.
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.
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.
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.
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