Solidified floating organic drop microextraction procedure based on deep eutectic solvent for the determination of melatonin in pharmaceuticals and dietary supplements

A miniaturized clean-up and preconcentration procedure involving deep eutectic solvent-based solidified floating organic drop microextraction was developed for the determination of melatonin in pharmaceuticals and dietary supplements by high-performance liquid chromatography with UV detection. Melatonin is widely used for the treatment of a large spectrum of diseases, and many studies have focused on its efficacy in reducing COVID-19 severity. For the first time, various hydrophobic deep eutectic solvents based on menthol, medium-chain fatty acids, and long-chain alcohols were studied for the microextraction of melatonin. Among the studied solvents, the deep eutectic solvent based on menthol and heptanoic acid provided the highest extraction recovery (90 %). In the developed procedure, a flat magnetic stirrer bar was covered by a microliter amount of the deep eutectic solvent and the sample solution was added under magnetic stirring. In this case, the deep eutectic solvent phase was easily dispersed into the aqueous phase without the use of any organic disperser solvents, resulting in fast analyte extraction (1 min). In the absence of stirring, the aggregation of extract as a floating drop on the surface of the aqueous phase was observed immediately. The low melting/freezing point and low density of the extraction solvent compared with water allowed one to quickly and easily retrieve a low volume of extract (25 μL) in a microextraction procedure by solidification. Validation of the procedure showed that limits of detection and quantification, calculated from the blank tests based on 3σ and 10 σ, were 0.003 mg g−1 and 0.01 mg g−1, respectively.