*2.5. Preparation of Fe3O4@C<sup>18</sup> Nanoparticle Composite*

The adsorbent (magnetic nanoparticles, Fe3O4@C18) used for magnetic solid-phase extraction was synthesized according to the literature [23,24] with minor modification. Briefly, ferric chloride hexahydrate (2.7 g) and sodium acetate (7.2 g) were dissolved entirely in ethylene glycol (100 mL) before being poured into a Teflon-lined stainless steel autoclave heated at 200 ◦C for 8 h. The resultant Fe3O<sup>4</sup> nanoparticles were washed with ethanol and dried. Afterward, the Fe3O<sup>4</sup> nanoparticles (10 mg) were dispersed in a mixture of 970 µL ethanol, 10 µL water, and 20 µL octadecyltrimethoxysilane through sonication for 30 s, followed by shaking for 8 h at 45 ◦C. The derivatized Fe3O4@C18 was washed with ethanol, water, and acetonitrile three times, respectively. The final product was resuspended in 1 mL of acetonitrile (10 mg/mL).
