Fig. 4

Enrichment and separation of MPPs in model protein samples. a Schematic illustration of PP enrichment strategy based on a dSPE mode. b, c MS of PPs enriched with PNI-co-ATBA_0.2@SiO₂ from tryptic digests of casein and BSA at molar ratios of 1:100 (b) and 1:500 (c). Peptides are labelled with their m/z values, mono-, di-, tri-, tetra-, penta-PPs are marked with blue, red, green, purple and dark blue characters, respectively. The y-axis in panel b or c represents relative abundance in MS. The base peak (the tallest peak) of a particular ion is normalized to 100%. The other peaks in MS appear between 0 and 100% abundance. Detailed information about peptide sequences and phosphorylation sites is shown in Supplementary Table 5