The mixture was gradually heated up to 130 ◦C and then kept for 2 h. After that, the tubes were left to cool into room temperature. Secondly, 2.5mL of concentrated sulfuric acid was added and the glass tubes were gradually heated up to a temperature of 160 ◦C, keeping that temperature during 30 min. After this period, they were left to cool into room temperature. Thirdly, 3.5mL of distillated nitric acid was added drop by drop to avoid foam formation. Afterwards, the glass tubes were gradually heated up to a temperature of 170 ◦C then kept for 60 min. Finally, 2.5mL of hydrogen peroxide (30% v/v) was added and the tubes were left to rest during an hour to complete the digestion. The digestion products were transferred to a volumetric flasks and the volume was completed up to 25mL with ultrapure water. All samples were digested in triplicate. Standards were prepared by serial dilutions from aqueous stock solutions (1000mgL−1) of calcium, magnesium and zinc in 0.014 mol L−1 nitric acid.