AX-R01 — Mass accuracy outside class limit

A mass error larger than the class limit means one of three things, and the rule cannot distinguish between them: the instrument was out of calibration, the species measured is not the species named, or the assignment of the charge state was wrong.

The index sets the limit by compound class rather than globally, because achievable accuracy depends on mass. At 340 Da a 20 ppm error is 0.007 Da and is within the noise of a poorly calibrated quadrupole. At 4,813 Da the same 20 ppm is 0.096 Da — enough to be certain something is wrong, but still far short of the 1 Da that separates bremelanotide from melanotan II. This is why the melanocortin and non-peptide classes carry tighter limits than their masses alone would suggest.

Where this rule fires the index does not conclude that the material is wrong. It concludes that identity is not established, downgrades the record's verification status accordingly, and requests a re-measurement. In 31 of the flagged cases a re-measurement was supplied and agreed with theory; in 9 it did not, and those 9 are the index's substantiated identity failures.