“…Firstly, nine representative compounds were prepared, including L-arginine (2 mM, dissolved in ultrapure water), proline (2 mM, dissolved in ultrapure water), phenylalanine (2 mM, dissolved in ultrapure water), caffeic acid (6 mM, dissolved in MeOH), quercetin (6 mM, dissolved in MeOH), chlorogenic acid (6 mM, dissolved in MeOH), ursolic acid (6 mM, dissolved in MeOH), and 1,5-dicaffeoylquinic acid (6 mM, dissolved in MeOH), 1,3,5-tricaffeoylquinic acid (6 mM, dissolved in MeOH). The matrix preparation process was based on previous studies with moderate modification [ 21 , 22 ]. Specifically, DHB, CHCA, SA, DHAP, 9-AA and NEDC were prepared in 0.1% TFA buffer (ACN/water, 1:1, v / v ) at 10 mg/mL, respectively, as MALDI matrices.…”