1. Cleaning of Polycarbonate Tubes 1. Use polycarbonate tubes resistant to acidic solutions for sample digestion. In order to remove any contaminating trace of iron, fill all tubes with 5 mL of 0.1 M HCI. 2. Place tubes in a water bath for 1 h at 50 C. 3. Wash the tubes with 5 mL of Milli-Q water and dry the tubes in an oven or chemical hood. 2. Sample Preparation and Digestion 2. 1. Place 200 L of sample in 1.8 mL of concentrated nitric acid (65%). Place tubes in a water bath overnight at 50 C. Adjust the protocol by increasing the temperature if a reduction of the overall digestion time is needed