Two hundred mg of the sample was placed in the
vessel and 5 mL of concentrated HNO3
was added
and maintained overnight. Then, 1mL of H2
O2 was
added. The digestion program was as follows: the
temperature started at 100o
C for 10 min and then was
increased to 150o
C for 10 min and then to 250o
C for
30 min. The digestion completed in about 60 minutes
as indicated by the appearance of transparent liquid
mixture.