Sample Preparation. Sample preparation was carried out by
following the procedure (27). Green leaves were air-dried followed by
drying at 105°C in a hot air oven for 2 h and then powdered. One
gram aliquots of each sample were taken in Teflon flasks, and 5 mL
of concentrated nitric acid and 5 mL of 30% (v/v) hydrofluoric acid
were added to each flask. After they were left to stand for 8 h, the
samples were digested in a microwave system for 10 min at 106°C.
The clear solutions obtained were transferred to a 100 mL beaker, the
pH was adjusted to 5.0 using acetate buffer, and volumes were adjusted
to 50 mL.