where Bmax is the maximum amount of solute bound by the polymer particles and Keq the equilibrium constants. pmol digest) and 1:1000 (10 fmol of peptide mix:10 pmol digest). Each solution was prepared in 90% MeCN + 0.1% TFA. The polymer particle (10 mg) was packed in single fritted SPE cartridges (ISOLUTE, Biotage) and was protected with a frit on top. In the enrichment protocol, the polymers were conditioned by using 90% MeCN + 0.1% TFA (3 x 1 mL) followed by loading the spiked solution (1 mL prepared by 90% MeCN + 0.1% TFA). The washing step was performed by using loading solution (2 x 0.5 mL) and 100% H2O + 0.1% TFA (2 x 0.5 mL), lastly the elution step was done by using two solutions, which were 80% MeOH + 0.1% TFA (0.5 mL) and 100% MeOH + 0.1% TFA (0.5 mL). In the regeneration, each SPE column was washed with following solutions: 80% MeOH + 20% HCI (0.1 M) (3 x 1 mL), 100% H2O (3 x 1 mL), and 90% MeCN + 0.1% TFA (3 x 1 mL). Three fractions as loading (L), washing (W), and elution (E) were dried and elution fractions were used to test in LC-MS analysis. Each dried elution fraction for different spiking level with three replicas were tested in LC-MS analysis. Each fraction was firstly dissolved, vortexed and sonicated. The preparation of fraction for testing in LC-MS followed as: 1:100 elution fractions were dissolved in 18.8 µL in 0.1% FA (5.5 fmol/µL) and then 2 µL of the solution was diluted in 9 µL of loading solvent, 1:10 elution fractions were dissolved in 181.1 µL of 0.1% FA (5.5 fmol/µL) and then 2 µL of the solution was diluted in 9 µL of loading solvent (each dilution process was estimated as 1 fmol/µL peptides exist).