Each sample (about 3 g) was homogenized, and added with ten
mL of nitric acid in a screw cap polypropylene 50 mL sample tube
(Digi-Tubes SCP Science); samples were placed in a Digi-Prep system
(SCP Science) at 75 C overnight. Then, after cooling, samples
were diluted to 200 mL with an acid solution and analyzed by ICP/
MS (7700 Agilent Technologies USA) with an ASX-500 CETAC
Autosampler (Cetac Technologies Omaha NE, USA). All procedures
were done according to UNI EN 13804, 2002 (1), UNI EN 13805,
2002 (2), UNI EN 15763, 2010 (3) and UNI EN 13806, 2002 (4).