Certified reference material. 0.5 g of the certified reference material DOLT-3 (Dogfish liver) and 0.25 g of Pig Kidney (ERMBB186) were digested in 6 mL of concentrated nitric acid using a microwave mineralizer (time: 10 min, power:100%, max. pressure 45 atm). Microwave-assisted digestion of the sample was performed in closed 100 mL vessels. After cooling, the obtained solution was diluted to a volume of about 50 mL. Next, the sample was prepared using the preconcentration procedure. The same procedure was used for the blank solutions.