For Cu, Cr, and Mn measurements, 1 g of homogenized sample
was treated with 3 mL of a HNO3–HClO4 (4:1) mixture in Pyrex
tubes placed in the digestion block and then heated at 60 C for
45 min, 90 C for 30 min, and 120 C for 45 min. The solutions
obtained were cooled to room temperature, transferred to a volumetric
flask, and diluted to a final volume of 25 mL with bidistilled
deionized water