Iminodiacetate chelating disks and cation-exchange disks (Empore™ Chelating and Cation-SR, respectively; 8.0 nm pore size, 47 mm diameter, and 0.5 mm thickness; 3 M) and hydrophilic PTFE membrane filters (0.5 μm pore size, 47 mm diameter, and 35 μm thickness; Advantec ) were used.