RESUMO
An on-line system employing switchable-hydrophilicity solvent-based liquid-phase microextraction (SHS-LPME) is described in this work. The method is based on the preconcentration of the species formed between cobalt and the reagent 1-nitroso-2-naphthol (NN), with subsequent detection by digital image colorimetry. The system's operation begins with the on-line mixture of sample, switchable solvent, and an alkaline agent in a reaction coil. Then the mixture is transported to an extraction chamber. The introduction of a proton donor leads to the passage of the solvent to its hydrophobic form, which allows phase separation. The rich phase is then directed to a glass tube, where detection is performed. Octanoic acid, sodium carbonate, and sulfuric acid were used as the extraction solvent, the alkaline agent, and the proton donor, respectively. Under optimized conditions, the method presented a detection limit of 0.8 µg L-1 and an enrichment factor of 41. The precision obtained was 4.8% (20 µg L-1). The accuracy of the method was tested by the analysis of Tomato Leaves certified reference material (NIST 1573a). The method was applied to the determination of cobalt in food, dietary supplements, and water samples. The method is presented as a green alternative and very accessible to the determination of cobalt in the analyzed samples.