Nondestructive determination for concentration of Pb and Cd in navel orange peel by laser induced breakdown spectroscopy

Abstract: Toxic heavy metal cadmium (Cd) and lead (Pb) are found in water and soil primarily due to the effluents of industrial wastewater. Primary products may be contaminated by heavy metal Cd and Pb, which are not essential elements in the human body, and may bring harm to human. In this case, it is necessary to determine Cd and Pb concentration in primary products for the evaluation of nutritional status and the optimization of the growing conditions. The traditional techniques in determining heavy metals require consumption of time and/or reagent, sample preparation as well as laboratory analysis environment. Laser-induced breakdown spectroscopy (LIBS) is a form of atomic emission spectroscopy based on plasma induced by focusing a laser pulse on the sample surface. LIBS has the characteristics such as multi-element analysis, direct measurement without contact with the material to be analyzed, high measuring speed, and no or less sample preparation, which make it a useful tool for rapid, real-time and in-situ measurements. Based on the huge potential of LIBS in elemental analysis, more and more researchers have begun to study the application of LIBS for analysis of agricultural products. In this experiment, the fruits of a common orange (Gannan navel orange) from Ganzhou municipality, Jiangxi Province in China were used. Because the area has better ecological and environmental quality, there is lower Cd and Pb concentration in natural oranges. To prove the feasibility of quantitatively determining Cd and Pb in Gannan navel oranges, the oranges need to be contaminated before the analysis. A LIBS experimental platform was set up in the laboratory . The laser source was a Q-switched Nd:YAG operating at 1064 nm with pulse duration of 10 ns. The spectrometer provided high spectral resolution (full wave at half maximum, FWHM 0.05 nm) in the region from 200 nm to 1100 nm. The charged-coupled device(CCD) detector was triggered with suitable delay time between the laser pulse and the acquisition of plasma emission using a digital pulse delay generator DG535. The spectra of Cd and Pb in navel oranges peel were detected by LIBS. The characteristic spectral lines of Cd and Pb were analyzed based on NIST (National Institute of Standards and Technology) database of USA.The optimal characteristic spectral line were 226.502 nm and 405.783 nm for Cd and Pb, respectively. The variation of signal noise ratio and intensity of spectrum with delay time and laser energy were observed. The results demonstrated that the delay time and laser energy optimized were 1300ns and 130mJ, respectively. At the same time, the real concentrations of Cd and Pb in sample peel were determined by atomic absorption spectroscopy (AAS).The relationships between spectral intensity and concentrations of Cd and Pb were constructed .The results showed that the correlation coefficient of calibration curves were 0.94669 and 0.96059 for Cd and Pb, respectively. The limit of detection (LOD) were acquired by the models, and the results showed that the LOD were 17.375 and 22.782 μg/g for Cd and Pb, respectively. The models were verified by three predictive samples. The results presented that the relative error was near to 10%.The whole experiment displayed the possibility of LIBS in analyzing heavy metals in agricultural products.