Desenvolvimento de métodos analíticos utilizando laser-induced breakdown spectroscopy (LIBS) para determinação de contaminantes, macro e micronutrientes em fertilizantes

Abstract

In this study analytical procedures were developed using laser-induced breakdown spectroscopy (LIBS) for determination of B, Ca, Cd, Cr, Cu, K, Mg, Mn, Na, Pb, and Zn in solid fertilizers and suspensions. Factorial designs (fractional factorial, central composite, and Doehlert) were used to define the most appropriate LIBS parameters. Emission lines for all of the analytes were collected and were employed 12 signal normalization modes in order to minimize laser fluctuations and sample heterogeneity. In the case of solid fertilizers the best results were obtained using a laser energy of 75 mJ, a spot size of 50 μm, a pellet pressure of 10 t/in., and a delay of 2.0 μs, and for suspension fertilizers were obtained with 80 mJ of laser energy, a 50 µm of spot size and after 0.5 μs of delay time. A novel sample preparation method was developed for liquid samples analysis in LIBS. The suspension fertilizer samples were mixed with 10% m/v polyvinyl alcohol (PVA) and dried for approximately 2 h at 50 ºC, resulting in a solid polymer film forming with the immobilized liquid sample. The results clearly demonstrate a good agreement between the LIBS predicted concentrations and the reference concentrations determined by ICP OES after microwave-assisted acid digestion (AOAC 2006.03 Official Method). The limits of detection (LOD) for the proposed method for solid fertilizers varied from 2 mg/kg (for Cd) to 1% (for Zn) fit within the intended purpose, mainly aimed at the Brazilian regulators to verify the occurrence of the potential contaminants (Cd, Cr, and Pb). The limits of detection of 0.01% for Cu, 0.008% for K, 0.02% for Mg, 0.001% for Mn and 0.02% for Zn for suspension fertilizers, with lower standard errors of prediction (ranging from 0.02 to 0.06%), indicate that the proposed liquid sample preparation procedure exhibits a significant potential for the development and implementation of methods for determining essential nutrients in suspension fertilizers and other samples for which mineralization is difficult. The proposed methods in LIBS showed high analytical frequency and simplicity, compared to existing methods that require acid digestion, as well as its are within the principles of green chemistry.