Briefly describe how a fish sample can be prepared for the analysis of trace metals in the lab.
Answers
Answer:
A new procedure for separation and preconcentration of trace amounts of Cu(II), Ni(II), Fe(III), Zn(II), Cr(III), Cd(II), and Pb(II) in fish samples was proposed. The procedure is based on the adsorption of these metal ions on the column of Amberlite XAD-7 as congo red complexes prior to their determination by flame atomic absorption spectrometry (FAAS). Several factors that can affect the sorption and elution efficiency of the metal ions were investigated and optimized. The sorption was quantitative in the pH range of 6.0–9.0 for Cu(II) and Ni(II), 5.5–8.0 for Fe(III), 6.0–8.5 for Zn(II) and Cd(II), and 7.0–8.5 for Cr(III) and Pb(II). The optimum pH for simultaneous retention was 7.5. The sorption capacity of the resin was found to be 0.89, 0.72, 0.82, 0.61, 0.53, 0.84, and 0.78 mg/g for Cu(II), Ni(II), Fe(III), Zn(II), Cr(III), Cd(II), and Pb(II), respectively. The precision of the method was evaluated as the relative standard deviation obtained by analyzing a series of six replicates and below 6% for all seven elements. The validation of the method was performed by the analysis of certified reference materials.
2. Experimental
2.1. Apparatus
The concentrations of metal ions were determined by FAAS technique according to the standard guideline of the manufacturer (Perkin-Elmer AAnalyst 700, Norwalk, USA). A Sartorius PP-15 model pH meter, with a glass-electrode employed was used for measuring pH values in the aqueous phase. For the sample digestion, Milestone Ethos D microwave system (Sorisole-Bg, Italy) was also used in the experiments.
2.2. Chemicals and Standard Solutions
Analytical reagent-grade chemicals were employed for the preparation of all solutions. Milli-Q water was used to prepare aqueous solutions (Millipore, Milford, MA, USA). Stock metal solutions, 1000 mg/L (Sigma Chemical Co., St. Louis, MO), were used for the preparation of standard and working solutions. Congo red (CR) solutions (1.0 mmol/L) were daily prepared by dissolving the required amounts of CR (Merck, Germany) in water. The pH of the model solution was adjusted to pH 2-3 with hydrochloric acid-potassium chloride, pH 4–6 with sodium acetate-acetic acid, and pH 7–10 with ammonia-ammonium chloride buffer solutions.
Amberlite XAD-7 with surface area and bead size are 450 m2/g and 20–40 mesh was purchased from Aldrich (Milwaukee, USA) and firstly allowed to stand in 1.0 mol/L sodium hydroxide for 1 h and 4.0 mol/L hydrochloride acid for 1 h before use. After separation by filtration, it was rinsed by distilled water and methanol. Finally, the dried XAD-7 resin was ground by means of agate mortar and kept in a desiccator.
2.3. Experimental Procedure
The proposed method was tested with model solutions containing metal ions (30 μg of Pb(II), 30 μg of Cr(III), 20 μg of Fe(III), 15 μg of Cu(II), 15 μg of Ni(II), 10 μg of Cd(II), and 10 μg of Zn(II)) prior to its applications. 10 mL of appropriate buffer solution (in order to adjust the pH between 2 and 10, as given above) and 4.0 mL 1.0 mmol/L CR solution were added to 50–60 mL of model solution. 0.5 g of XAD-7 filled column was pretreated with the relevant buffer solution. Metal-CR complexes were passed through the XAD-7 column at a flow rate of 5–10 mL/min. The adsorbed metal ions on the resin were then eluted with 5 mL of 3.0 mol/L HNO3 in acetone. The solution was evaporated almost to dryness and diluted to 5 mL with 0.1 mol/L HNO3. The eluent was analyzed by FAAS for the determination of metal concentrations.
2.4. Sample Collection and Preparation
Fish samples of Dicentrarchus labrax, Sparus aurata, Mullus barbatus, and Merlangius merlangus euxinus species were purchased from the fish market in Istanbul, Turkey. Fishes were dissected to separate organs (scale, skin, muscle, gill, and liver) according to FAO methods [33]. The collected samples were packed on ice for transport to the laboratory and stored at −4°C until analyzed.
For the fish analysis, about 1 gram of sample (scale, skin, muscle, gill, or liver) was digested with 6 mL of 65% nitric acid and 2 mL of 30% hydrogen peroxide in microwave digestion system and diluted to 10 mL with Milli-Q water. A blank digest was performed in a similar manner (digestion conditions for microwave system; 2 min for 250 W, 2 min for 0 W, 6 min for 250 W, 6 min for 400 W, and 10 min for 500 W, vent: 8 min, resp.). The preconcentration procedure given above was also applied to the real samples. The final volume was 5 mL. In order to assess the accuracy of the developed method, the certified reference material NRCC-DORM-2-Dogfish muscle which was obtained from National Research Council, Canada, was used and digested in accordance with the digestion method described above.