Analysis of Heavy Metals in Soil by Laser Induced-Breakdown Spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) is a potential analytical technique for quantitative elemental analysis of various materials, including soils. Soil is a heterogeneous material, which differs significantly in its structure and texture. While in LIBS analytical processes, the sample’s homogeneity is of the most importance for measurement accuracy and precision. The objectives of this experiment are: (1) Optimize the equipment parameters (td, tb, and repetition rate) in order to obtain the stronger and the more stable signal intensity; (2) Study the matrix effect of the different kinds of soils (sandy soil, sandy loam, and clay loam); (3) Present a common homogeneous substrate for the different kinds of soils and then use LIBS technique to do simultaneous multi-elemental analysis. In this experiment, heavy metals (Cu, Cd, Cr, Mn, Ni, Pb, Zn, and Hg) are proportionally mixed with the tested soil samples, respectively, to concentrations of 0 ppm (blank sample), 10 ppm, 50 ppm, 100 ppm, 500 ppm, 1000 ppm, and 5000 ppm. Part of the mixed soil is ground and pressed into soil pellets, and the other part is microwave digested with concentrated nitric acid and hydrofluoric acid, filtered, centrifuged, decanted, and diluted to a certain volume. The solution is dropwise added on a filter paper, and then determine the elemental concentration after about 5 minutes. The limits of detection of heavy metals are determined and compared with those determined by the conventional spectroscopy; the measurement accuracy and precision are analyzed with statistical method, and finally the potential applicability of the LIBS technique to the measurement of heavy metals in soils is discussed.