Natural thymol-based ternary deep eutectic solvents: Application in air-bubble assisted-dispersive liquid-liquid microextraction for the analysis of tetracyclines in water.

Four new thymol-based ternary deep eutectic solvents were prepared and evaluated as the extractive phase in air-bubbles assisted dispersive liquid-liquid microextraction for extraction of tetracycline, doxycycline, and oxytetracycline from the water before high-performance liquid chromatography. The maximum extraction efficiencies were obtained using 400 μL of [choline chloride]:[thymol]:[nonanoic acid] in the molar ratio of 1:2:2 at pH = 5. The solvent was characterized by FTIR and NMR spectroscopy. The hydrophobicity of the deep eutectic solvent and its effect on the pH of water samples after mixing was also studied. Besides, the extraction efficiency of the ternary deep eutectic solvent was compared with that of two binary thymol-based deep eutectic solvents, including [choline chloride]:[thymol] and [thymol]:[nonanoic acid] at the same conditions. Under optimal conditions, limits of detection and quantification were 1.2-8.0 and 3.8-26.6 μg/L, respectively. The linear ranges were 18.2-500 μg/L for oxytetracycline, 26.6-500 μg/L for tetracycline, and 3.8-500 μg/L for doxycycline with the determination coefficients > 0.9912. Intra- and inter-day relative standard deviations were 1.2-3.8 and 7.7-11.2%, respectively. The developed method was applied to the analysis of tetracyclines in unspiked and spiked environmental water samples, and the obtained recoveries were 74.5-95.4% with relative standard deviations of 1.2-4.0%.