Fast and simple determination of estrogens in milk powders by magnetic solid-phase extraction using carbon nitride composites prior to HPLC

A graphitic carbon nitride (g-C 3 N 4 /Fe 3 O 4 )–based magnetic solid-phase extraction (MSPE) approach was established for fast and simple analysis of estrogens in milk powders. The composites were characterized by X-ray diffractometer, scanning electron microscope, and Brunauer-Emmett-Teller surface area and pore size distribution analyzer. Compared with the bulk g-C 3 N 4 , g-C 3 N 4 /Fe 3 O 4 gave a narrower distribution of mesopores and provided an enhanced surface area from 77.1 to 113.7 m 2 /g. Polar analytes of estrogens were selected as model compounds and the extraction of four estrogens was achieved in n-hexane using 15 mg of adsorbent within only 2 min. Possible extraction mechanism of g-C 3 N 4 /Fe 3 O 4 for these estrogens was explored in terms of the polarity of the analytes and the adsorption performance of the adsorbent. The hydrophobicity and the hydrogen-bond interaction between the estrogens and g-C 3 N 4 were responsible for the efficient adsorption. Combined with HPLC, MSPE with the prepared adsorbent gave the enhancement factors of 20 to 24 and the linear ranges of 2–200 μg/kg for 17 β -estradiol and 17 α -ethinylestradiol, 1.5–150 μg/kg for estrone, and 3–300 μg/kg for hexestrol. The detection limits and quantification limits for the estrogens in milk powders were 0.5–0.9 μg/kg and 1.5–3.0 μg/kg, respectively. The recoveries varied from 75.1 to 97.2%, with the intra-day and inter-day precisions ≤ 14.2%. Furthermore, the enrichment of the analytes and the clean-up of fat and protein interferences were achieved simultaneously with one-step g-C 3 N 4 -based MSPE. The present method was convenient, fast, and sensitive, and therefore could be successfully applied for the determination of estrogens in milk powders. Graphical abstract