A closed-vessel system was used
in the digestion procedure, with the addition of 5.0 mL of sample,
2.0 mL of 30% H2O2, 1.0 mL of 65% HNO3 and 3.0 mL of ultrapure
water. The UV lamps were inserted in the quartz vessels and the
following optimized program was used: step 1—power of 900 W,
ramp time of 10 min and ventilation of 127 m3 h−1
; step 2—power
of 900 W, hold of 20 min and ventilation of 127 m3 h−1
; and step 3
—power of 0 W and ventilation of 190 m3 h−1 (cooling). After
digesting and cooling, the resulting digests were transferred to
15-mL volumetric flasks, 300 µL of a 500 g L−1 Cs solution was
added, and the volume completed with ultrapure water.