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

•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.