Reversed-phase switchable-hydrophilicity solvent liquid-liquid microextraction of copper prior to its determination by smartphone digital image colorimetry

Abstract

Reversed-phase switchable-hydrophilicity solvent liquid-liquid microextraction (RP-SHS-LLME) was used prior to smartphone digital image colorimetry (SDIC) for the determination of copper as its metal chelate with N,N-diethyl-N’-benzoylthiourea (DEBT). Images of the extract were captured inside a colorimetric box, which were split into their red-green-blue channels and the intensity of the blue channel was linearly correlated to the concentration of copper. The optimum SDIC conditions were achieved at a wavelength of 400 nm, a distance of 7.0 cm between the sample holder and the detection camera, a region of interest of 2500 px2 and light source brightness of 30.0 %. Optimum RP-SHS-LLME performance was realized with 80.0 μL of triethylamine as a switchable-hydrophilicity solvent, 140.0 μL of 7.5 mol L−1 nitric acid as a hydrophilicity-switching trigger, at a complexation pH of 5.5, 200 μL of DEBT and 200 μL of 1-dodecanol as the complexation solvent within 4.0 min. The limit of detection (LOD) was found as 0.3 μg mL−1 with coefficients of determination of the calibration graphs higher than 0.9963 and relative standard deviations lower than 6.9 %. The proposed method was applied for the determination of copper in edible oil samples (i.e., black seed, corn, olive and sunflower oil), with percentage relative recoveries obtained in the range of 92.5–105.0 %.