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

[Display omitted] •Novel approach of digital-image based method combined to SPE.•In-situ Cu(II) desorption and spot color development using iodide-starch reagent for DIB imaging.•Development of a new magnetic SPE adsorbent for copper preconcentration.•The method provided low detection limit sufficient for trace copper analysis in food and water.•An adsorbent with a more than fifteen-fold cycle life. 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 μg L-1, the linearity was 16–600 μg 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.