Figures 4(a) and 4(b) shows precursor mass error of PSMs in filtered result. They help examine whether the mass spectrometer is properly calibrated.

Precursor mass error is calculated in ppm as 106 × (precursor mass – peptide mass) / peptide mass. For a well-calibrated instrument, precursor mass errors should center at 0 ppm across the range of m/z.

Figure 4(a) shows the distribution of precursor mass error in histogram

Figure 4(b) shows precursor m/z versus precursor mass error in scatterplot.