After sonication, the vessels were then sealed and the digestion
was performed in a microwave oven using the following
procedure: step 1–room temperature to 85 C for 3 min; step 2–
heat 85–145 C for 9 min; step 3–heat 145–180 C for 4 min; step 4–
hold time at this temperature for 15 min; step 5–temperature
decrease to 30 C for 25 min. After digestion, reactors were cooled
to room temperature overnight, opened and the solutions
transferred to a volumetric
flask and diluted to 25 mL with
ultrapure water.