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

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 px(2) and light source brightness of 30.0 %. Optimum RP-SHS-LLME performance was realized with 80.0 mu L of triethylamine as a switchable-hydrophilicity solvent, 140.0 mu L-1 of 7.5 mol L-1 nitric acid as a hydrophilicity-switching trigger, at a complexation pH of 5.5, 200 mu L of DEBT and 200 mu L of 1-dodecanol as the complexation solvent within 4.0 min. The limit of detection (LOD) was found as 0.3 mu g 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 %.

Automated summary

The combination of RP-SHS-LLME and SDIC was successfully used for the determination of copper concentration, with images of the extract captured inside a colorimetric box for analysis. Optimal testing conditions included specific wavelength, light source brightness, distance, etc., and optimal performance was achieved by controlling parameters such as the amount of extraction solvent and pH value.