Microextraction of melamine from dairy products by thymol-nonanoic acid deep eutectic solvent for high-performance liquid chromatography-ultraviolet determination

In this work, it was shown for the first time that deep eutectic solvents provide effective liquid-phase microextraction of melamine from dairy products. In accordance with the developed procedure, the milk sample is mixed with salt (sodium sulfate) to precipitate proteins and, after that, melamine is extracted into the phase of a hydrophobic eutectic solution based on thymol and nonanoic acid. After phase separation, the analyte is detected in the eutectic solvent phase. In the developed analytical procedure, a medium-chain fatty acid as a precursor of the extraction solvent promotes mass-transfer of melamine due to strong interactions between carboxyl and amino groups. The terpenoid as a precursor of the extraction solvent has a synergistic effect in melamine extraction due to possible 7C-7C interaction. The estimated phenomenon was applied to the effective separation of highly hydrophilic analyte from aqueous sample phase followed by its determination by high-performance liquid chromatography. Under optimal conditions, a range of determined concentration was from 0.1 to 30 mg L-1 with a limit of detection of 0.03 mg L-1. The procedure is fast (7 min), miniaturized (100 ILL of the extraction solvent) and repeatable (RSD % < 5 %). The developed procedure was optimized and used for the determination of melamine in real samples of powdered and whole milk.

Automated summary

This study demonstrated the effective liquid-phase microextraction of melamine from dairy products using deep eutectic solvents for the first time. The developed procedure involves precipitation of proteins from milk samples using salt (sodium sulfate) and subsequent extraction of melamine into a hydrophobic eutectic solution phase based on thymol and nonanoic acid. The use of a medium-chain fatty acid and a terpenoid as precursors of the extraction solvent promotes the mass transfer of melamine through strong interactions between carboxyl and amino groups. The optimized procedure allows for the fast, miniaturized, and repeatable determination of melamine in real samples of powdered and whole milk, with a wide range of determined concentrations and low detection limit.