1 ppm of the metal to be analyzed was added to each sample so that the sample concentration was in the optimum detection level of the employed spectrophotometer (atomic absorption with flame detection from Varian trademark, model 240FS). The standard addition technique was taken into account since the result contained the amount added (fixed concentration) plus the amount of the original sample. The relation of nitric acid (HNO3) and perchloric acid (HClO4) was 6:1 during the digestion of the samples and the concentration of Zn and Cu was determined by applying the method for airacetylene flame atomic absorption spectrophotometry according to Standard Methods [23].