All calibration standard solutions were prepared from 100 μgmL−1 multi-element standard solution (SCP Science, Paris, France) by dilution with the suitable percentage of HNO3 in ultrapure water. In solution (1000 μgmL−1) was obtained from SCP Science. Analytical reagent grade HNO3 65% (Sigma-Aldrich, Madrid, Spain) was additionally cleaned by sub-boiling distillation. The sub-boiling still was built up with components obtained from Savillex (www.savillex.com). The certified reference materials, skim milk powder BCR-151 and skim milk powder BCR-063R were obtained from Sigma-Aldrich (Madrid,Spain).