Fig. 1.

Graphical representation of the steps of the optimized protocol for particle preparation, including digestion and dilution: 20/50 μL of the sample were pipetted into a 20/50 mL volumetric flask. Then, 300 μL HCl 37% were added. The mixtures were heated to approximately 80 °C for one hour. After this time, the solutions were allowed to cool down to RT and the flasks were subsequently filled up to the mark with MilliQ water resulting in two individual dilutions for each sample, which were measured three times either by ICP-OES or by photometry. As a result, we obtained six iron concentration values for each sample which have been used for the calculation of the average iron concentration