In the
first procedure 4 mL conc. HNO3, 2 mL H2O2 and 1 mL
conc. HCl were added separately, and after sonication, 3 mL of H2O
were added to the mixture. In the second procedure 8 mL conc.
HNO3 were added followed by 2 mL H2O2. For the third procedure
used 4 mL conc. HNO3 were added followed of 3 mL of conc. HCl
and 3 mL of H2O. In the fourth procedure 10 mL conc. HNO3 were
added.