The digested samples were heated by increasing the temperature to 180 5 C for 10 min and kept at that temperature for an additional 10 min. Sample digests were filtered through Whatman no. 541 filter paper into a 100 mL volumetric flask and diluted to 100 mL with DDW. In order to determine the accuracy of the digestion procedure, a standard material (BCR-701) was digested using the same protocol.