Fast and simple determination of estrogens in milk powders by magnetic solid-phase extraction using carbon nitride...

A graphitic carbon nitride (g-C₃N₄/Fe₃O₄)-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-C₃N₄, g-C₃N₄/Fe₃O₄ gave a narrower distribution of mesopores and provided an enhanced surface area from 77.1 to 113.7 m²/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-C₃N₄/Fe₃O₄ 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-C₃N₄ 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 17β-estradiol and 17α-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-C₃N₄-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.