AA (≥99%, CAS 79-06-1), F (≥98.5%, CAS 98-01-1), MF (≥98.5%, CAS 620-02-0) and HMF (≥98%, CAS 67-47-0) was purchased from Sigma–Aldrich (St. Louis, MO, USA). Stock standard solutions (1000 mg/L) were prepared by diluting 10 mg of each analyte in 10 mL of methanol (MeOH) and stored at −20 °C in darkness. Working standard solutions (0.05–250 mg/L) were prepared by appropriate dilution of the stock solutions with water-MeOH (90:10, v/v) and were stored at 2–8 °C. LC-MS grade MeOH, acetonitrile (ACN) and hexane (Hex) were obtained from Mervilab (Madrid, Spain). Formic acid LC-MS grade was acquired from Thermo Fisher Scientific (Leicestershire, UK). Hydrochloric acid (HCl, 37%, 36.45 g/mol, 1.19 g/mL), toluene, ethanol and diethyl ether were purchased from Scharlau (Barcelona, Spain). Tetra-ethyl-orthosilicate (TEOS, 98%, 208.3 g/mol, 0.93 g/mL), poly (ethylene-glycol)—block-poly (propylene-glycol)—block-poly (ethylene-glycol) (PEG-PPG-PEG or Pluronic 123®, 5800 g/mol, 1.019 g/mL), 3-aminopropyl-triethoxysilane (≥98%, 221.37 g/mol), decane (≥95%), potassium hexacyanoferrate and zinc sulfate analytical grade were obtained from Sigma-Aldrich (St. Louis, MO, USA). Ammonium fluoride (NH4F) was acquired from Panreac (Castellar del Vallès, Barcelona, Spain). All water used in this work was ultra-pure water (18.2 MΩ cm quality) obtained from a Millipore Milli-Q-System (Billerica, MA, USA). Polyethylene frits, nylon filter membranes (0.45 µm), empty SPE cartridges (3 mL) and nylon syringe filters (0.45 µm) were purchased from Scharlau (Barcelona, Spain). Carrez solutions used during sample preparation were prepared by dissolving 1.5 g of potassium hexacyanoferrate in 10 mL of water (Carrez I) and 3 g of zinc sulfate in 10 mL of water (Carrez II).

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