A rapid on-site analysis method for the simultaneous extraction and determination of Pb2+ and Cd2+ in cereals

Abstract

In order to achieve rapid on-site screening and solve the problem of rapid pretreatment for the determination of lead (Pb²⁺) and cadmium (Cd²⁺) in cereals by a portable electrochemical analyzer with disposable screen-printed electrodes (SPEs), a new reliable and simple extraction method for Pb²⁺ and Cd²⁺ in cereals was developed. The Pb²⁺ and Cd²⁺ in cereals were purified by a mixed solution of 1 mol L^-1 potassium iodide (KI)/5% vitamin C (VC)/ethyl acetate after being extracted by 10% HNO₃, which transfers the Pb²⁺ and Cd²⁺ into ethyl acetate after a reaction with KI-VC. Then, the Pb²⁺ and Cd²⁺ were eluted from ethyl acetate with 5% HNO₃ and were determined by an electrochemical analyzer with screen printed electrodes. Under the optimized conditions, the matrix calibration curves of Pb²⁺ and Cd²⁺ in rice and wheat showed good linear relationships with R ² > 0.996. The method shows a detection limit (LOD) for Cd²⁺ in rice and wheat of 6.7 μg kg^-1 and 11.5 μg kg^-1, and the corresponding values for Pb²⁺ were 34.9 and 31.1 μg kg^-1, respectively. The relative standard deviation (RSD) was less than 8.7% for Cd²⁺ and Pb²⁺. In addition, the recoveries of the tested reference materials using this method were between 80% and 120%. From sample pretreatment to testing results, the whole process took no more than 25 min, and the operation was simple for operators, green to the environment, cheap in terms of instruments, and above all suitable for on-site detection. The results implied that this portable electrochemical method with new pretreatment may be a good choice for screening Pb²⁺ and Cd²⁺ in cereal samples on-site.

Fig. 1. The principle of the portable electrochemical method.

Fig. 2. The effect of the concentration of nitric acid.

Fig. 3. The effect of the solid to liquid ratio for extraction.

Fig. 4. The effect of the extraction time.

Fig. 5. The effect of the concentration of KI.

Fig. 6. The effect of the concentration of VC.

Fig. 7. Comparison and analysis of calibration curves of the standard solution and sample solution. Inset: square-wave stripping voltammograms at the SPEs for concentration levels of Cd²⁺ and Pb²⁺ at 12.5 μg L⁻¹ and 6.5 μg L⁻¹ in 0.5 M acetate buffer; deposition E: −1.3 V; deposition time 200 s.

Fig. 8. The matrix-matched standard calibration curves of Cd²⁺ and Pb²⁺.