For the combined determination of cadmium and zinc (Table S3), the sequential aspiration of two sample/standard plugs between three NaOH solution plugs was performed (steps BMD-FMD). Then, the stacked plugs were propelled to the flow cell (step GMD) for retaining the metal ions in the NTA resin at a strong alkaline pH (approximately 12). Afterwards, NaOH solution and DTZ reagent were sequentially aspirated to the holding coil (steps HMD-IMD) to attain an alkaline pH, in the range 12–14, for the reaction and a reference scan was performed in order to eliminate the intrinsic sample absorption (step JMD). The stacked plugs were then directed to the flow cell; a stop period of 10 s for absorbance measurement of the coloured product formed was set (steps KMD-LMD). In order to prepare the NTA beads column for the following cycle, in a reusable approach, a washing and a conditioning steps were included. The washing step consisted in the aspiration of HCl solution and propelling it through the NTA beads column in the flow cell (steps NMD-OMD). The conditioning step consisted in aspiration of the NaOH solution with consequent propelling to the flow cell for pH adjustment of the NTA beads column (steps PMD-RMD).