*2.1.4 Method D: F (solid-phase extraction)*

 This method is based on the study of fluoroquinolones in milk [12] and determination of 20 pharmacologically active substances in various milk samples [13]. After 10 min of ultrasonication, 7.0 mL 1% trichloroacetic acid (TCA) and 3.0 mL acetonitrile were added. The resulting solution was vigorously shaken for 2 min and centrifuged at 4000 rpm for 10 min. The supernatant obtained extracted by SPE. Three kinds of cartridges (CNWBOND LC-C18 SPE, Poly-Sery HLB SPE, and Poly-Sery MAX SPE) were examined. The SPE protocol on three types of cartridges was consisted of the following steps: (1) activation of the cartridges with 3 mL methanol first and then conditioned with 3 mL deionized water, (2) sample loading, and (3) sample elution with 3 mL 20 mmol L<sup>−</sup><sup>1</sup> ammonium sulfate and 3 mL 80% acetonitrile. After the eluates were filtered, the steps of evaporation and residues dissolving were the same as method A (liquid-liquid extraction).
