Analysis of estrogens in milk samples using ionic liquid-modified covalent organic framework and stable isotope labeling technique

Estrogens in milk samples were extracted by a new ionic liquid (IL)-modified covalent organic framework (COF), and then analyzed by stable isotope labeling (SIL) method using dansyl chloride (DNS-Cl) and d 6 -dansyl chloride (d 6 -DNS-Cl) as SIL agents. The IL-modified COF (TpPa-Ct@Mi) was synthesized by chemically bonding methylimidazole on to the surface of normal COF to make full use of the high surface area and big conjugated structure of COF, and the abundant charge of IL, which are of crucial importance for adsorption. The synthesized COF can form π–π interactions and hydrophobic effect with estrogens, while the IL can form electrostatic interactions with the analytes. It was applied to the dispersive solid-phase extraction (DSPE) of estrogens in milk samples. The extracted estrogen samples were heavy labeled by d 6 -DNS-Cl, and then mixed with the light labeled standard. Samples and standards were mixed and analyzed in a single run, and thus the matrix effect was greatly minimized. The optimized extraction conditions were adsorbent amount, 20 mg; desorption solvent and volume of desorption solvent, 2 mL acetonitrile; NaCl concentration, 0 g/L; sample pH 7. The limits of detection were in the range of 9–13 ng/L, and the limits of quantitation were in the range of 29–40 ng/L. Recoveries and matrix effect were in the range of 89.6–95.5% and 96.1–98.9%, respectively, with relative standard deviations of less than 6.2%.