Miniaturized Clean-Up and Preconcentration Procedure Based on Deep Eutectic Solvent for the Determination of Melatonin in Pharmaceuticals and Dietary Supplements

A miniaturized clean-up and preconcentration procedure involved deep eutectic solventbased solidified floating organic drop microextraction was developed for the determination of melatonin in pharmaceuticals and dietary supplements by the high-performance liquid chromatography with UV detection. Melatonin is widely used for the treatment of large spectrum of diseases and many studies were focused on its efficacy in reducing COVID-19 severity. Various hydrophobic deep eutectic solvents based on menthol, medium chain fatty acids and long-chain alcohols were studied for the microextraction of melatonin for the first time. Among the studied solvents the deep eutectic solvent based on menthol and heptanoic acid provided highest extraction recovery (90 %). In the developed procedure a flat magnetic stirrer bar was covered by microliter amount of the deep eutectic solvent and sample solution was added under a magnetic stirring. In this case deep eutectic solvent phase was easily dispersed into aqueous phase without the use any organic disperser solvents resulting to 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. Low melting/freezing point and low density of the extraction solvent compared with water allowed one to fast and easily retrieve low volume of extract (25 μL) in microextraction procedure by a solidification. Validation of the procedure showed that limits of detection and quantification, calculated from the blank tests based on 3σ and 10 σ, were 0.03 mg L-1 and 0.1 mg L-1 respectively.