Urea

Polydimethylsiloxane (PDMS) prepolymer was purchased from Dow Corning (Sylgard 184Midland, MI, USA). Acetonitrile (ACN) and trifluoroacetic acid (TFA) were purchased from J.T. Baker (Phillipsburg, NJ, USA). Dithiothreitol (DTT), iodoacetamide (IAA), and formic acid (FA) were purchased from Sigma-Aldrich(St Louis, MO, USA). Octadecyl-coated silica particles (C₁₈, 3 μm, 100 Å, Develosil) were purchased from Nomura Chemical Co., Ltd (Seto, Japan). Sinapic acid (SA) was purchased from Bruker Daltonics (Germany). Trypsin (modified, sequencing grade) was obtained from Promega (Madison, WI, USA). Urea, was purchased from Bio Basic Inc. (Toronto, Canada).

Urea (purity ≥ 99%, Bio Basic, Inc.), oleic acid (Acidimetric 197–202 mg, Fisher ${}^{\circledR }$ Chemical), acetic anhydride, AR (purity ≥ 98%, AVONCHEM ${}^{\circledR }$ ), buffer solution (pH7.0 (20 °C), Certipur ${}^{\circledR }$ ), sebacic acid (purity ≥ 99.5%, SAOC ${}^{\circledR }$ ) diethyl ether (purity ≥ 99.8%, Sigma Aldrich ${}^{\circledR }$ ), n-hexane (purity ≥ 99% (GC), Honeywell ${}^{\circledR }$ ), petroleum ether (purity ≥ 95%, Sigma Aldrich ${}^{\circledR }$ ), methanol (purity ≥ 99.8% (GC), Sigma Aldrich ${}^{\circledR }$ ), tetrahydrofuran (purity > 99.5%, Fisher ${}^{\circledR }$ Chemical), hydrochloric acid 37% (Acidimetric 36.5–38.0%, Panreac ${}^{\circledR }$ ), acetone (purity ≥ 99.5%, Sigma Aldrich ${}^{\circledR }$ ), sodium sulfate anhydrous ( purity > 99%, Fisher ${}^{\circledR }$ Chemical), ethanol (purity > 99.8%, CHEM-LAB ${}^{\circledR }$ ), sodium hydroxide (Acidimetric ≥ 98%, Panreac ${}^{\circledR }$ ) and potassium hydroxide (purity as KOH ≥ 85%, Loba Chemie ${}^{\circledR }$ ) all were used as supplied. Deionized water was obtained by instrument (Milli-Q ${}^{\circledR }$ Direct ultrapure water, Millipore ${}^{\circledR }$ System). Olive solid waste samples were provided by local farmers and mills in the northern region of Saudi Arabia.