Smartphone digital image colorimetry combined with deep eutectic solvent-liquid–liquid microextraction for the determination of cobalt in milk and dairy products

Deep eutectic solvent-liquid–liquid microextraction (DES-LLME) was combined with smartphone digital image colorimetry (SDIC) for the determination of cobalt as its chelate with 1-(2-pyridylazo)-2-naphthol. The final extract was placed in a quartz micro-cuvette inside a laboratory-designed colorimetric box from which images of the extract were captured and split into their red-green-blue channels. The intensity of the red channel, found to give the highest intensity, was used to calculate the concentration of cobalt. Optimum SDIC performance was obtained at a distance of 8.0 cm between the cuvette and the detection camera with a 60.0% brightness of the light source at a wavelength of 560 nm and 1600 px² as the region of interest. Optimum DES-LLME conditions were found as follows: complexation pH of 5.00, 300 µL of DES (choline chloride/phenol, 1:4 molar ratio), 900 µL of tetrahydrofuran within 2.0 min complexation and 2.0 min extraction time. The limits of detection (3S_b/m) and quantitation (10S_b/m) were found as 0.03 and 0.16 µg g⁻¹, respectively. The coefficient of determination (R²) was found to be higher than 0.9966 and the relative standard deviation was lower than 7.8%. The proposed method was applied for the determination of cobalt in milk and dairy products with percentage relative recoveries ranging between 95.0% and 107.5%.