Coming back to: Oxidation and deamidation mass shifts, tabulated posts 31–39

Narrowing post #30, because the general version has more than one answer.

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.

I would treat the number as indicative rather than as a measurement.

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 rule of thumb is fine; the edge cases are where it earns its keep.

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.

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.

I am aware this is the third time this month I have made this point.

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.

This is the version I would want a new member to read first.

This follows post #35 rather than contradicting 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.

A qualification I should have led with rather than closed on.

That matches what I have seen, for whatever a single anecdote is worth.

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.