Improving the LIBS analysis of heavy metals in heterogeneous agricultural samples utilizing large laser spotting

Abstract

The impact of the laser spot dimensions on the laser-induced breakdown spectroscopy (LIBS) analysis of heterogeneous samples was studied in detail. The study was stimulated by comparing the analytical performance of three LIBS instrument types that have potential utilization at different stages of agricultural material control: a hand-held analyzer, laboratory LIBS system, and an online industrial system installed on a production line (conveyer belt, etc.). Laser spot diameters (120 to 1100 μm) with the same averaged energy density were utilized for heterogeneous sample analysis of materials with particle sizes from 20 to 300 μm; thus, every spot could probe from one to a few hundred particles. Few-hundred-point LIBS maps were generated to construct calibration curves with different numbers of averaged spots (from 4 to 400). The analytical capabilities were compared in terms of the linearity and accuracy of determination. For large spot sampling, a better linearity and better accuracy of zinc determination in soybean grist were achieved compared to the small spot sampling. It was demonstrated for the first time that a few dozen LIBS spots should be probed to achieve a representative analysis in the case of heterogeneous material LIBS analysis (such as plants or its products) while only a few spots are generally sampled in the published LIBS studies to date.