For total digestion samples must be dried and ground (minimum mass: 500 mg; if less material is available please discuss in advance). We suggest to ground larger masses to guarantee highest possible representativity of the sample.

(modified from Grawunder et al., 2014):

100-150 mg of each ground sample were digested in a pressure digestion system (DAS, PicoTrace). The ground material was put in TFM vessels and 2 mL 65% HNO₃ (subboiled), 3 mL 40% HF and 3 mL 70% HClO₄ (both suprapur, Merck) were added. This mixture was then heated up to 180°C within 6 h. After maintaining the temperature for 12 h the samples were cooled down. For evaporating the acids, the system was heated up again to 180 °C for 4-5 h in a special evaporating hood maintaining the temperature again for 14 h. The remaining solids were dissolved again after adding 2 mL HNO₃ (p.A. Merck, subboiled), 0.6 mL HCl (suprapur, Roth) and 7 mL pure water (GenPure UV-TOC, Thermo Fisher Scientific) at 150°C within 8 h. The cooled samples were transferred to calibrated 25 mL flasks (PMP, Vitlab) and replenished to 25 mL by addition of pure water.