2.2. Reagents and standards
Analytical grade nitric acid (65%, Sigma Aldrich) and perchloric acid (70–72%, Sigma Aldrich) were used for sample preparation. Calibration standards for each heavy metal were prepared each day from the certified standard stock solution (1000 ppm-manufactured under ISO 9001 Quality Assurance system-Perkin Elmer) in the range from 0.5 to 10 ppm. All the solutions were prepared in double distilled water. Dilution correction was applied for samples diluted or concentrated during analysis.