Each figure plots the number of peptides detected as a function of FDR threshold for the E.coli dataset, broken down by whether the precursor had an m/z in (a) the bottom quartile range of 350–467 m/z or (b) the top quartile range of 704–1389 m/z. In each plot, the series correspond to different score functions, and the 1% FDR threshold is highlighted with a red dashed line. Casanovo-DB performs much worse on low m/z precursors compared to those with high m/z, exemplifying the calibration problems which are resolved by Percolator.