The MIP sample processing protocol effectively removed background signals at both spiking levels as judged from the LC-UV chromatograms (light pink) of the elution fractions (E) compared to reference (R) (Fig. ). Meanwhile highly specific extractions of the target peptides were evident but with slight variations depending on the phosphorylation pattern and spiking level. Tests using the YpY (YY, YpY, and pYpY peptides spiked in enrichment) and pYY (YY, pYY, and pYpY peptides spiked in enrichment) MIPs, targeting the monophosphorylated forms, cleanly extracted their targets while completely (Fig. ) or partially (Fig. ) rejecting the nonphosphorylated and diphosphorylated peptides, albeit with breakthrough noted for pYY. The relative abundances of the spiked peptides (Fig. ) confirm the results in Fig. and Fig. (Fig. ) while revealing other more subtle effects. First considering the SPE results using the digest spiked with pYpY, YpY and YY (left graphs in Fig. ). The enrichments of the target peptides (YpY and pYpY) on their MIP complements, reflected in their relative abundance in the elution fractions, decreases with increasing dilution but still with a noticeable target selectivity. This contrasts with the experiments using the digest spiked with pYpY, pYY and YY (right graphs in Fig. ). In this case, only pYY is selectively enriched whereas pYpY enrichment is noted only at the 1/10 spiking level. This suggests that pYY, in contrast to YpY, effectively competes with pYpY for interacting with the pYpY imprinted sites.