The calibration of the spectrometer
for each metal was performed according to its wavelength and
standard solutions. For Cu, a 5-ppm standard solution was prepared,
for Ni two standard solutions (5 and 25 ppm), for Pb a 10-
ppm solution, for Cd and Cr two solutions (1 and 2 ppm, 2 and
5 ppm, respectively) and for Zn a 0.5 ppm standard solution. The
spectrometer re-calibrated after each group of 10 successive measurements