An ultrasonic water bath of 350 mL with 50 W power and 50 Hz
frequency from Selecta (Barcelona, Spain) was employed for
sample sonication. For sample mineralization, a microwave system
Ethos SEL from Milestone (Sorisole, Italy) equipped with an
thermocouple for automatic temperature control, an automatic gas
detector and ten high pressure closed digestion vessels made of
high purity TFM (a thermally resistant form of Teflon) with 100 mL
inner volume was used. The system was operated at a maximum
exit power of 1000 W.