The following buffers were used to control the pH of the solutions: hydrochloric acid–glycine
(pH 1–3), sodium acetate–acetic acid (pH 3–6), ammonium acetate–ammonia (pH 6–8), and ammonium chloride–ammonia (pH 8–9). All other reagents were of analytical reagent grade or better. Doubly distilled water was used throughout the entire study. The pipettes and vessels used for trace analysis were kept in 10% nitric acid for at least 24 h and subsequently washed four times with doubly distilled water.