Digital image determination of copper in food and water after preconcentration and magnetic tip separation for in-cavity desorption/ color development

A new analytical method for measuring copper in food and water was developed and validated, employing a solid -phase extraction (SPE) technique combined with digital -image -based (DIB) detection. A novel magnetic adsorbent of zinc ferrite/Citrullus colocynthis biochar (ZF@C.BC) was used to preconcentrate copper. A magnetic tip was used to separate the copper -loaded adsorbent from the extraction medium and to dispense it to the DIB plate. In -situ desorption and development of the spot color with iodide -starch reagent were carried out, and a digital image of the developed spots was captured using a smartphone and processed using ImageJ software. The copper adsorption capacity was 91.3 mg g-1. Desorption was effected using a 0.3 mol L-1 hydrochloric acid. The preconcentration factor was 300, the limit of detection was 4.8 mu g L-1, the linearity was 16-600 mu g L-1 and the sample throughput was 12 h-1. The developed approach was validated by analyzing food and water samples, confirming recoveries >= 91 % and 88 %, respectively, with RSD <= 8.4 %, n = 3.