Alginate-Encapsulated HDES as a Green Sorbent for Elemental Preconcentration Prior to Determination by FAAS

This study presents the development, characterization, and application of a new solid-phase extraction (SPE) sorbent for the preconcentration of Cd, Co, Cu, Fe, Mn, Ni, and Zn in water samples prior to analysis by flame atomic absorption spectrometry (FAAS). The sorbent was prepared by encapsulating hydrophobic deep eutectic solvents (HDES) in calcium alginate, a strategy that prevents HDES accumulation in FAAS and avoids nebulization and transport interferences. The developed method allows the analyte to be eluted into a clean aqueous phase (HNO₃ 3.5% v/v), containing negligible amounts of HDES. This is demonstrated by the excellent analytical figures of merit obtained, such as the correlation coefficient (R ² > 0.9982) and precision (RSD <4.2%), and is further confirmed by the absence of HDES accumulation in the FAAS nebulization system. The encapsulation process involves Methol/Thymol (1:1 molar ratio) emulsified in a sodium alginate solution (10% v v^-1 HDES content) droplet into a cross-linking agent (CaCl₂ solution). Under optimized conditions, a 36 mL sample volume (pH 6.0 ± 0.1), 1 g of sorbent, 25 min extraction time, and desorption with 4 mL of 3.5% v v^-1 HNO₃ for 5 min were used. In these conditions, the method provided a 9-fold preconcentration factor, with recoveries between 81% and 109% for the seven monitored analytes. The accuracy was confirmed by analysis of real water samples, where concentrations of Cu and Zn ranged from 38 to 74 μg L^-1 and 68 to 297 μg L^-1, respectively, showing good agreement (91-109% recovery) with the independent ICP-OES procedure. Additionally, the sorbent demonstrated reusability, highlighting its potential as a low-cost, sustainable, and effective alternative for the trace elements determination in water samples by FAAS.