For analyses using TXRF, the calibration and internal standards were prepared by serial dilutions of a solution containing
10,000 mg/L (Certiprep solutions) in 5% HNO3. An aliquot of a cobalt (Co) solution was added as an internal standard to achieve a final concentration of 10 mg/L. The samples were then homogenized with a vortex mixer and analyzed after pipetting 10 mL onto a quartz sample carrier/reflector and evaporating the solvent on a hot plate at low power. The samples were irradiated under vacuum for 1000 s at 50 kV and 40 mA in a Wobistrax TXRF spectrometer (Streli et al., 2004). Blanks were treated identically to the samples.