The real sample analysis procedure was as follows: the 0.5 g
sample, 5 mL HNO3 and 1 mL H2O2 were placed into a microwave
digestion tank over night and then digested according to a different
microwave digestion procedure (3 min at 120 ◦C or 30 min at
140 ◦C). When the microwave digestion finished, the digestion tank
was cooled down naturally. Then, the digestion tank was heated to
empty it of HNO3. In order to make sure Se(VI) reduced to Se(IV),
1 mL of 6 mol/L HCl was added into the digestion tank, and the