2.2. Chemical reagents
Standard solutions of Se, Cu, Cr, and Mn (1000 ± 0.002 mg/l) (Tritisol, Merck, Darmstadt, Germany) were used and diluted as necessary to obtain working standards with acidified water with 0.2% nitric acid. High-quality concentrated nitric acid (65%) and perchloric acid (65%) were used for sample mineralization. Hydrochloric acid (37%) was used to reduce the Se+6 to Se+4 in samples after mineralization for Se determination. Sodium borohydride and sodium hydroxide were employed for hydride generation. Magnesium nitrate was used to prepare the matrix modifier for Cu and Cr determinations. All solutions were prepared from analytical reagent-grade chemicals (Suprapur, Merck, Germany).