Figure 4.
The intensity of base-peak-normalized immonium ion peaks plotted against the percentage of the population that contains the residue of interest for triply-charged ions. The intensity of the normalized immonium ion peak is grouped in five quintiles plus a zero value on the x-axis. The percentage of the population having the residue of interest is given on the y-axis. The horizontal line represents the proportion of sequences in the database (1300−2900 Da) containing the residue. Each chart represents a single amino acid as indicated at the top; W cor indicates a plot corrected for confounding peaks.