Distinguishing a deletion sequence by mass alone — the long version

Building on post #2 rather than restating it.

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).

Coming back to post #8, because the follow-up matters more than the original answer.

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.

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.

Take it as a starting point and not as a specification.

Answering the question post #22 raises rather than the one it answers.

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.

Seconded. It reads as careful rather than confident, which is the right register.

On post #40 — agreed on the reasoning, with one qualification.

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.

Not a strong opinion, just a consistent one.