Natural deep eutectic solvent as extractor and support for determination of metals by laser induced breakdown...

Natural deep eutectic solvents (NADES) are a subclass of DES consisting of natural compounds that generally are green and biodegradable solvents. In this study, hydrophobic NADES composed of palmitic acid and DL-menthol, with a melting point slightly above room temperature, was pioneeringly used for analyte extraction and as a solid support for the analysis of aqueous samples by Laser-Induced Breakdown Spectroscopy (LIBS). Dispersive liquid-liquid microextraction of metal ions in the form of metal - [1-(2-pyridylazo)-2-naphthol] (metal- PAN) complexes was conducted at a temperature above the melting point of the NADES and, after cooling, metal ions were determined in the solid NADES extract by LIBS.

NADES discs were stable and yielded reduced background signals. Optimization using a disc containing the metal - PAN complexes yielded the optimal conditions: lens-to-sample distance of 18.5 cm, laser energy of 100 mJ, delay time of 4.0 μs, integration time of 13.0 μs, and 180 laser pulses. Linear responses were obtained between 0.1 and 3.0 mg L^-1 (Mg, Mn, and Cd), 0.2-3.0 mg L^-1 (Ca and Zn), and 0.4-3.0 mg L^-1 (Pb). The coefficients of variation (n = 5) ranged from 5 to 20 %, and the limits of detection were estimated at 0.1 mg L^-1 (Mg, Mn, and Cd), 0.2 mg L^-1 (Ca, Zn, and Cu), and 0.4 mg L^-1 (Pb). Recoveries were within 78-128 % (water samples) and 94-122 % (plant materials). The procedure was successfully applied to the determination of metal ions in digests of vegetable materials.

The method requires only 5.4 mg of PAN and 2 mL of a biodegradable NADES per determination. The results indicate the potential use of NADES for simultaneous preconcentration of metal ions and solid support, being an alternative for the analysis of liquid samples by LIBS.