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Marcel Maeder Institute of Inorganic Chemistry University of Basel Spitalstrasse 51 4056 Basel, Switzerland RECEIVED for review August 21, 1986. Accepted October 6, 1986.
AIDS FOR ANALYTICAL CHEMISTS Determination of Tin in Foods by Hydride Generation-Atomic Absorption Spectrometry George
H.AIvarez* a n d Stephen G. C a p a r
Food and Drug Administration, Division of Contaminants Chemistry, 200 C Street, S. W., Washington, D.C. 20204 The World Health Organization has recommended a provisional maximum tolerable daily intake of 2 mg of Sn/ kg of body weight, based on the fact that levels of Sn >200 mg/kg in food may cause acute gastric irritation (1). Sn is present in foods at levels usually lo0 Mg/g, further dilution was required. The final acid concentration of the sample was 0.9 M H,SO,. Standard reference materials and reagent blanks were treated in the same manner. Hydride Generation. The reductant reservoir (SB-4, Figure 1) was pressurized with nitrogen to 4.8 psi; then the carrier flow was adjusted to 650 mL/min and maintained for the duration of the analysis. The carrier flow was measured with a soap film flowmeter connected to the '/,-in. glass tubing (IV, Figure 1) with the atomic absorption cell removed and with the reaction vessel ffled with 20 mL of 0.12 M HCI. The pressurized system provided a reductant flow rate of 86.5 mL/min. For sample measurement, an aliquot (0.14.5 mL) of the sample was pipeted into the reaction vessel containing 20 mL of 0.12 M HCI. The contents were mixed by swirling and the reaction vessel was secured to the inner T joint of the reducing adapter. After the solution was purged with nitrogen for 10 s, the reductant flow valve was opened. The valve was turned to the closed position when the absorption peak tracing had descended at least past mid-peak height. When the tracing showed that the peak had returned to the base line, the vessel was removed and the outsides of the capillary tubes were rinsed with distilled water. The next test portion was then analybed. Quantitation. Working standard solutions of 0.0.5, 1.0,1.5, 2.0, 2.5, and 3.0 pg of Sn/mL were prepared in 0.24 M HCI from
RESULTS AND DISCUSSION Optimization of Hydride Generation. The parameters and conditions affecting absorption of Sn were evaluated and optimized by measuring 100 ng of Sn standard. The parameters studied included type of acid, acid volume, acid and reductant concentrations, nitrogen flow rate, NaBH, flow rate, and uniformity of flow rates. Each parameter was considered optimum when the absorbance signal showed good peak symmetry and good precision of replicate measurements. The effect of acid and reductant concentrations is shown in Figure 2. Optimum concentrations of acid and reductant were determined to be 0.12 M HCI and 2% NaBH,. Stannane could not be generated reproducibly when using 0.184.9 M H,SOI. However, we determined that up to 1 mL of the 0.9 M H&04 diluted digests added to 20 mL of 0.12 M HCI in the reaction flask did not adversely affect the atomic absorption response for Su. Higher signal responses were obtained hy using 2-4% NaBH, rather than 2% NaBH, solutions; however, peak symmetry and precision were affected. Peak distortion occurred when the nitrogen flow rate was decreased from 650 mL/min, which was selected as being the optimum flow rate for nitrogen. Nitrogen flow rates >650 mL/min caused excessive aerosol carryover into the quartz cell. Constant flow rate of the reductant was also important. The hydrochloric acid volume influenced the absorption of Sn to a lesser extent than other parameters. The optimum volume was 20 mL of 0.12 M HCI. There was a 5% reduction in absorbance when using 15 and 25 mL of 0.12 M HCI and a 21% reduction in the response signal when using 5 or 10 mL of 0.12 M HCI. Digestion Methods. Various wet digestion procedures were evaluated to select a method that minimized losses of Sn during the digestion. Nitric-hydrochloric acid digestion of foods to which Sn had been added resulted in excessive losses of Sn, particularly in those instances where the total Sn content of the food was
